ANALGESIA FOR NEWBORN INFANTS DURING MECHANICAL VENTILATIONethesis.helsinki.fi/julkaisut/laa/kliin/vk/saarenmaa/analgesi.pdf · ANALGESIA FOR NEWBORN INFANTS DURING MECHANICAL VENTILATION

Hospital for Children and Adolescents,and Department of Clinical Pharmacology

University of Helsinki, Finland

ANALGESIA FOR NEWBORN INFANTS DURINGMECHANICAL VENTILATION

- a clinical and pharmacokinetic study

by

Elina Saarenmaa

ACADEMIC DISSERTATION

To be publicly discussed by permission of the Medical Faculty of the University of Helsinki, in the Niilo Hallman

Auditorium of the Hospital for Children and Adolescents,on March 23rd, at 12 noon.

HELSINKI 2001

Supervised by

Docent Vineta Fellman, MDHospital for Children and AdolescentsUniversity of HelsinkiHelsinki, Finland

Professor Pertti Neuvonen, MDDepartment of Clinical PharmacologyUniversity of HelsinkiHelsinki, Finland

Reviewed by

Professor Mikko Hallman, MDDepartment of PediatricsUniversity of OuluOulu, Finland

Docent Kalle Hoppu, MDPoison Information CentreHelsinki University Central HospitalHelsinki, Finland

Official opponent

Professor Neil McIntosh, MDDepartment of Child Life and HealthUniversity of EdinburghEdinburgh, United Kingdom

ISBN 952-91-3245-X (nid.)ISBN 951-45-9882-2 (PDF)Helsinki 2001Yliopistopaino

To Ari,Sara, Miro and Ilari

4

CONTENTS

1 ABSTRACT ............................................................................................... 62 LIST OF ORIGINAL PUBLICATIONS ................................................... 83 ABBREVIATIONS ................................................................................... 94 INTRODUCTION ...................................................................................... 105 REVIEW OF THE LITERATURE ............................................................ 11

5.1 Pain in newborn infants ................................................................. 11Perception of pain ..................................................................... 11Physiological changes associated with pain .............................12Behavioral changes associated with pain ................................. 13

5.2 Painful situations in the postnatal period ...................................... 145.3 Pain assessment ............................................................................. 15

Theoretical basis ...................................................................... 15Unidimensional instruments ..................................................... 16Multidimensional instruments .................................................. 17

5.4 Developmental aspects of pharmacokinetics in newborn infants . 19Distribution and protein binding .............................................. 19Metabolism ............................................................................... 20Excretion ................................................................................... 21

5.5 Pain management .......................................................................... 22Decreasing noxious stimuli .......................................................... 22Environmental and behavioral interventions ........................... 22Pharmacologic interventions ................................................... 23

Morphine ................................................................. 24Fentanyl ................................................................... 25Alfentanil ................................................................. 27Ketamine ..................................................................28

6 AIMS OF THE STUDY ............................................................................. 307 SUBJECTS AND METHODS ................................................................... 31

7.1 Patients .......................................................................................... 317.2 Study design .................................................................................. 33

Fentanyl or morphine for ventilated newborn infants (I, II, III) . 33Alfentanil for procedural pain relief in neonates (IV) ................. 34Ketamine for procedural pain relief in neonates (V) ................... 34

7.3 Blood sampling ............................................................................. 35

5

7.4 Outcome measures ........................................................................ 35Behavioral pain assessment ...................................................... 35Physiological parameters ..........................................................36Hormonal assays .......................................................................36Determination of drug concentrations ...................................... 36Pharmacokinetic calculations .................................................. 37

7.5 Adverse events .............................................................................. 377.6 Statistical analysis ......................................................................... 387.7 Ethical considerations ................................................................... 39

8 RESULTS ................................................................................................... 408.1 Comparison of analgesic effects of fentanyl and morphine (I) ........ 408.2 Serum concentrations, clearance, and effects of fentanyl (II) .......... 428.3 Serum concentrations, clearance, and effects of morphine (III) ...... 448.4 Alfentanil as procedural pain relief (IV) .......................................... 478.5 Ketamine as procedural pain relief (V) ............................................ 48

9 DISCUSSION ............................................................................................ 499.1 Methodological considerations ..................................................... 499.2 Analgesia for persistent pain ......................................................... 51

Comparison of fentanyl and morphine pharmacodynamics (I). 51Pharmacokinetic studies (II, III) ............................................... 53

9.3 Procedural pain management (IV, V) ........................................... 5610 CONCLUSIONS ........................................................................................ 5811 ACKNOWLEDGMENTS .......................................................................... 5912 REFERENCES ........................................................................................... 61

ABSTRACT

6

1 ABSTRACT

The perception of pain in newborn infants has been underestimated until recently,since the infant´s ability to verbalize pain is limited. Newborn infants undergoingintensive care are exposed to repeated painful procedures as well as persistentpain. Thus, they are in need of pain relief. The objectives of this study were todesign clinically applicable, safe, and effective analgesia for persistent pain anddistress in the immediate postnatal period as well as short-acting pain relief forprocedures.

The pharmacodynamics and pharmacokinetics of fentanyl and morphine werestudied in newborn infants on mechanical ventilation. In a randomized double-blind trial, 163 infants were allocated to receive a continuous infusion of fentanyl(10.5 µg/kg over a 1-hour period followed by 1.5 µg/kg/h) or morphine (140µg/kg over a 1-hour period followed by 20 µg/kg/h) for at least 24 hours. Theseverity of pain was assessed using a behavioral pain scale, physiologicalparameters, and stress hormone concentrations. As side effects, decreasedgastrointestinal motility and urinary retention were assessed. Serumconcentrations of fentanyl or morphine and metabolites of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), were assessed insubgroups of 38 fentanyl-treated and 31 morphine-treated infants. Usefulness ofalfentanil and ketamine as procedural pain relief was studied in two separaterandomized, double-blind, crossover trials. Two different doses of alfentanil (10and 20 µg/kg) and placebo were infused in a random order 2 minutes before threeseparate endotracheal suctions to 10 infants, and three different doses ofketamine (0.5, 1, and 2 mg/kg) and placebo were infused 5 minutes before fourseparate endotracheal suctions to 16 infants, respectively. Behavioral pain score,physiological parameters, and stress hormone concentrations were measured.

Fentanyl and morphine infusions provided effective analgesia, as judged by thepain scale. Plasma catecholamine concentrations decreased in both treatmentgroups at 24 hours, but ß-endorphin decreased only in the fentanyl group.Decreased gastrointestinal motility occurred significantly less frequently in thefentanyl-treated (23%) than in the morphine-treated (47%, p<0.01) infants. Meanserum fentanyl steady-state concentration of 2.5 ± 1.0 ng/ml and mean serum

ABSTRACT

7

morphine steady-state concentration of 167 ± 77 ng/ml were achieved between24 and 48 hours of constant infusion. The fentanyl steady-state concentrationcorrelated with the concomitant pain score value (r= -0.57, p<0.01). Meanclearances of fentanyl (11.5 ± 4.0 ml/min/kg) and morphine (2.4 ± 1.1ml/min/kg) correlated with gestational age (p<0.01) as well as birthweight(p<0.01). Significantly higher morphine concentrations were measured in theinfants with decreased gastrointestinal motility (187 ± 82 ng/ml) than in thosewith normal motility (128 ± 51 ng/ml, p<0.05). In both alfentanil and ketaminetrials, heart rate and arterial blood pressure increased in association withendotracheal suction after placebo. Attenuation of the pain score change inresponse to endotracheal suction was noticed with 20 µg/kg alfentanil and 1mg/kg ketamine. Rigidity appeared in five of eight infants receiving 20 µg/kgalfentanil.

In conclusion, when comparing fentanyl and morphine infusions for persistentpain, both proved to be effective. The lower incidence of decreasedgastrointestinal motility in the fentanyl group may, however, render it superior tomorphine, especially for premature infants with a high risk for decreasedgastrointestinal motility. The metabolism of fentanyl and morphine during thepostnatal period showed a large interindividual variability. However, since theclearances of both fentanyl and morphine were correlated with gestational ageand birthweight, the degree of immaturity should be taken into account whenadministering these drugs to newborn infants. As procedural pain relief, whileusing endotracheal suction as a pain stimulus, only alfentanil at a dose of 20µg/kg appeared effective. Nonetheless, because it caused a high rate of rigidity, itshould only be used in conjunction with a muscle relaxant. Ketamine used aloneappeared to be a poor pretreatment for endotracheal suction in the postnatalperiod.

LIST OF ORIGINAL PUBLICATIONS

8

2 LIST OF ORIGINAL PUBLICATIONS

This thesis is based on the following original publications referred to in the textby Roman numerals:

I. Saarenmaa E, Huttunen P, Leppäluoto J, Meretoja O, Fellman V.Advantages of fentanyl over morphine in analgesia for ventilated newborninfants after birth: A randomized trial. J Pediatr 134:144-50, 1999.

II. Saarenmaa E, Neuvonen PJ, Fellman V. Gestational age and birth weighteffects on plasma clearance of fentanyl in newborn infants. J Pediatr136:767-70, 2000.

III. Saarenmaa E, Neuvonen PJ, Rosenberg P, Fellman V. Morphine clearanceand effects in newborn infants in relation to gestational age. Clin PharmacolTher 68:160-6, 2000.

IV. Saarenmaa E, Huttunen P, Leppäluoto J, Fellman V. Alfentanil asprocedural pain relief in newborn infants. Arch Dis Child 75:F103-7, 1996.

V. Saarenmaa E, Neuvonen PJ, Huttunen P, Fellman V. Ketamine asprocedural pain relief in newborn infants. Arch Dis Child. In press.

Some previously unpublished data are also presented.

ABBREVIATIONS

9

3 ABBREVIATIONS

CHEOPS Children’s Hospital of Eastern Ontario Pain ScaleCI confidence intervalCRIES Crying, Requires oxygen to maintain saturation greater than 95%,

Increased vital signs, Expression, SleeplessnessCV coefficient of variationCYP cytochrome P-450 enzymeDSVNI Distress Scale for Ventilated Newborn InfantsHPLC high-performance liquid chromatographyIBCS Infant Body Coding SystemIQR interquartile rangeLIDS Liverpool Infant Distress ScaleM3G morphine-3-glucuronideM6G morphine-6-glucuronideNFCS Neonatal Facial Coding SystemNICU neonatal intensive care unitNIPS Neonatal Infant Pain ScaleNMDA N-methyl-D-aspartateNSAID nonsteroidal anti-inflammatory drugPAT Pain Assessment ToolPID pain intensity differencePIPP Premature Infant Pain ProfileSD standard deviation SUN Scale for Use in Newborns

INTRODUCTION

10

4 INTRODUCTION

During the last two decades, while immense development has occured in neonatalintensive care, few reports have been published suggesting that neonates feel pain,suffer from pain, and should be treated for pain (Levine and Gordon 1982,Williamson and Williamson 1983). The initiation of pain research in the newborninfant can be ascribed to the first randomized controlled trial by Anand et al.(1985), who demonstrated accentuated hormonal responses in neonates undergoingsurgery without anesthesia. Debate arose in the media as to whether these trialswere unethical or inhuman, although the control group received the standardclinical practice at that time, which was no pain relief (Anand et al. 1987).

It has been conclusively documented that the newborn, including the prematureinfant, is capable of perceiving pain (Anand and Hickey 1987, Truog and Anand1989). Low birthweight infants show hormonal and metabolic responses to painfulstimuli, and these responses can be attenuated by efficient analgesia (Anand andHickey 1987). Use of analgesics during surgery and postoperative care reducescomplications (Anand and Hickey 1992), while the deleterious and long-lastingeffects on behavior due to untreated pain are evident from recent publications(Anand and Hickey 1987, Lloyd-Thomas and Fitzgerald 1996, Taddio et al. 1997).However, the most important issue is whether the benefits of treatment outweighthe side effects and potential hazards.

The rationale for the present study was the clinical need for effective and safecontinuous analgesia for infants exposed to repeated procedures and nociceptivestimuli while treated with mechanical ventilation during intensive care. Further, aclinical need exists for a short-acting drug with little respiratory depressing sideeffects, which could be used as an analgesic for minor painful procedures.

REVIEW OF THE LITERATURE

11

5 REVIEW OF THE LITERATURE

5.1 Pain in newborn infants

Pain is defined as “an unpleasant sensory and emotional experience associated withactual or potential tissue damage or described in terms of such damage” by theInternational Association for the Study of Pain (Merskey et al. 1979). According tothis definition, pain is always a subjective phenomenon modified by context,biological variation, and a variety of psychological factors and experiences relatedto injury in early life.

The requirement for subjective reporting conceptualizes pain as an adultexperience, excluding preverbal or nonverbal individuals. An alternative systembased on empirical evidence purports that physiological and behavioral responsesare valid indicators of pain. This perspective suggests that pain is an inherentquality of life that appears early in ontogeny to serve as a signaling system fortissue damage (Anand and Craig 1996). This reconceptualization of pain is alsoapplicable to fetal life and infancy.

Nociception describes neurobehavioral and metabolic effects of a noxious stimulusindependent of higher consciousness, memory, emotional effects, or suffering.Stress applies to adaptive responses generated by external stimuli or internal cues.Distress is the suffering resulting from the emotional effects of excessive stress.

Perception of pain

Research of pain in children accelerated in the 1980’s, and since then, it has beenscientifically proven that newborn infants, even ones born prematurely, are capableof feeling pain (Anand and Hickey 1987, Truog and Anand 1989). Providingeffective analgesia during neonatal intensive care has been neglected until recently,however, because of uncertainty about its necessity and fear of possible side effects.This has been the case despite studies indicating that in premature infants afferentpain pathways and cortical centers are developed, and thus, the physiologicalrequirements for feeling pain are present (Anand and Carr 1989).


12

Cutaneous pain receptors are present over the entire body at 20 weekspostconception (Valman and Pearson 1980). Myelinization of nerve tracts in thespinal cord, brain stem, and thalamus is complete by 30 weeks of gestation, andthalamocortical pain fibers in the posterior limb of the internal capsule and coronaradiata by 37 weeks of gestation (Gilles et al. 1983). In addition, pain threshold islower in neonates than in adults because of immature organization of connectionswithin the dorsal horn of the spinal cord and not yet fully developed inhibitingprocesses (Fitzgerald and Anand 1993, Chiswick 2000). This pain threshold islower still in preterm infants and is presumed to increase with postnatal age andafter painful procedures (Johnston and Stevens 1996). However, in contrast to theseassumptions, a recent study showed that thresholds remain low after injury even interm infants (Andrews and Fitzgerald 1999). This suggests that exposure tomultiple procedures in the neonatal intensive care unit (NICU) increases painawareness (Chiswick 2000). Thus, newborns have all the anatomical and functionalcomponents required for perception of pain and seem to experience more pain thanolder children and adults.

Physiological changes associated with pain

Pain and stress in newborn infants causes changes in cardiovascular variables,regional blood flow, respiratory patterns, oxygenation, and temperature. Increasesin heart rate and blood pressure have been noted in preterm and term infants withheel lancing (Field and Goldson 1984, Owens and Todt 1984, Johnston and Strada1986, McIntosh et al. 1993) and circumcision (Holve et al. 1983, Williamson andWilliamson 1983). Increased palmar sweating (Harpin and Rutter 1983) andelectrical conductance of the skin (Gladman and Chiswick 1990) have also beenobserved in association with heel-prick in neonates.

Hormonal responses associated with painful stimuli include release ofcatecholamines (Anand et al. 1985, Anand et al. 1990, Quinn et al. 1993), beta-endorphin (Anand et al. 1985, Ionides et al. 1994), growth hormone, glucagon(Anand et al. 1985), cortisol (Obara et al. 1984), aldosterone, and renin (Fiselier etal. 1983), and suppression of insulin secretion (Anand et al. 1985). These responsesstimulate metabolic changes resulting in the breakdown of carbohydrate, protein,and fat stores, leading to hyperglycemia, marked increases in levels of blood


13

lactate, pyruvate, total ketone bodies, and fatty acids and enhanced nitrogenexcretion (Elphick and Wilkinson 1981, Anand et al. 1985). These changes havebeen measured primarily in neonates undergoing surgery (Anand et al. 1987, Anandet al. 1988, Anand et al. 1990, Anand and Hickey 1992) and circumcision withoutanesthesia (Talbert et al. 1976, Masciello 1990) but also in relation to minorprocedures (Greisen et al. 1985, Lagercrantz et al. 1986) and postnatal mechanicalventilation (Greenough et al. 1987, Greenough et al. 1990).

Other physiological changes in response to pain- and injury- associated stressinclude alteration of the immunological system (Ward-Platt et al. 1989) and effectson coagulation and hemostasis (Anand and Hickey 1992).

Behavioral changes associated with pain

Motor movements, facial expression, crying, and changes in behavior patterns, suchas sleep-wake cycles, are associated with pain experience. Noxious stimulus to thesole of the foot stimulates pain receptors, which pass the impulse via C- and A-deltafibers to the dorsal horn of the spinal cord, and causes the reflex withdrawal of thelimb known as the cutaneous flexion reflex. This reflex is obvious and measurablein preterm infants born at 26 weeks of gestation (Andrews and Fitzgerald 1994).Infants respond to pain caused by pinprick with diffuse body movements or reflexwithdrawal of the limb in association with grimacing, crying, or both (McGrath1987).

Distinct facial expressions associated with pain in infants have been validated andclassified (Johnston et al. 1993). These expressions include brows lowered andfurrowed, eyes squeezed shut, deepening of the nasolabial furrow, open lips, mouthstretched vertically and horizontally, and a taut, cupped tongue.

The primary method of communication in newborns, that of crying, is also causedby stimuli other than pain. Crying due to pain can be distinguished byspectrographic analysis or by subjective evaluation by trained observers from criescaused by hunger and fear (Levine and Gordon 1982, Wasz-Höckert et al. 1985,Zeskind et al. 1985, Zeskind and Barr 1997). The spectrographic properties of paincry of healthy full-term neonates differs from those of preterm or sick neonates


14

(Wasz-Höckert et al. 1971, Michelsson et al. 1977, Michelsson et al. 1983, Wasz-Höckert et al. 1985).

Alterations in sleep-wake cycles, increased wakefulness, and irritability have beenfound in neonates after heel-stick procedure (Field and Goldson 1984). A controlledstudy showed that an intervention designed to reduce the amount of sensory outputand stressful stimuli during intensive care of preterm neonates was associated withimproved clinical and developmental outcomes (Als et al. 1986).

5.2 Painful situations in the postnatal period

Newborn infants undergoing intensive care are repeatedly exposed to eventsconsidered noxious by older children and adults. Environmental stress factorsinclude bright light, loud noise, and frequent handling during nursing procedures.The need for continuous monitoring and mechanical ventilation may also causepain and stress. Many procedures commonly performed in the NICU result in acute,immediate pain lasting for seconds or minutes. Tissue-damaging proceduresinclude heel lance, insertion of intravenous and arterial lines, suprapubic aspiration,chest drain insertion, and lumbar puncture, among others. Intubation, endotrachealsuctioning, insertion of nasogastric tube, and handling for radiograph areconsidered nontissue-damaging procedures.

In one study of 54 infants consecutively admitted to NICU, over 3000 procedureswere recorded (Barker and Rutter 1995). In another study of 124 infants born at 27to 31 weeks of gestation, an average of 134 painful procedures per infant wereperformed during the first two weeks of life (Stevens et al. 1999).

Surgery and subsequent postoperative course lead to subacute, longer lasting (hoursor days) pain. Acute painful stimulus primarily excites sensory receptors andneurons (Pasternak 1988), whereas with subacute pain there is often aninflammatory cytochemical or paracrine basis initiating or continuing thesensorineural phenomenon (Fitzgerald 1995). Chronic persistent pain, defined aslasting more than 3 months, is thus not a perfectly applicable term in the neonatalperiod (Merskey 1986). However, the feature of inflammatory pain associated withskin lesions, including massive edema and trauma, and certain neonatal illnesses,such as necrotizing enterocolitis, meningitis, sepsis, and intracranial hemorrhages,may more closely resemble severe chronic pain than acute or subacute pain.


15

5.3 Pain assessment

Theoretical basis

Management of pain is based on pain assessment, which describes thephenomenon and influencing factors, and evaluates the need and efficacy ofinterventions. Quality and quantity of pain should be comparable within andbetween infants using these assessment measures. Measurement is thequantifiable aspect of assessment and applies to intensity of pain. The Children’sHospital of Eastern Ontario Pain Scale (CHEOPS) is a behavioral pain scaledeveloped to measure postoperative pain in 1- to 7-year-old children with goodreliability, validity, clinical utility, and feasibility (McGrath et al. 1985). It has alsobeen well validated for procedural pain. Different scaling procedures have beendeveloped to measure pain in the neonatal period (Craig et al. 1984, Grunau andCraig 1987, Ambuel et al. 1992, Lawrence et al. 1993, Hodgkinson et al. 1994,Pokela 1994, Krechel and Bildner 1995, Horgan and Choonara 1996, Sparshott1996, Stevens et al. 1996, Blauer and Gerstmann 1998). The nominal orcategorical scale is a classification rather than a measurement (pain, no pain).The ordinal scale reflects pain intensity, but the extent of increase or decrease isnot specified. With interval scales, the unit change in scale value represents aconstant change across the scale range, and calculation of average values ispossible.

Assessing pain and the efficacy of analgesia is a more comprehensive procedurethan pure measurement. It is generally difficult in the intensive care, because thebehavioral and physiological indicators of pain can be modified by extrinsic andintrinsic factors. It is even more difficult in newborn infants, whose cognitiveresponse to pain is limited, and with whom self-report and instruments such asvisual analog scales cannot be used. In addition, measurement based on anobserver’s judgement is prone to bias due to the respondent’s own experience,personality, and grasp of the situational context. Thus, a maximally objectivemeasure of newborn infants’ behavioral responses to pain, including visualobservation of facial expression and gross movement with or withoutphysiological and contextual indicators, has been used, and scoring systems havebeen developed (Table 1).


17

Gross motor activity is assessed by the Infant Body Coding System (IBCS, Craig etal. 1984) with good reliability and validity. Motor activity of the hands, feet, arms,legs, head, and torso is coded as either being present or absent.

The Behavioral Pain Score (Pokela 1994) consists of several types of behavioralindicators including facial expressions, body movements, response tohandling/consolability, and rigidity of the limbs and body. It was developed fromCHEOPS (McGrath et al. 1985). The score of each indicator varies from 0 to 3,with a total score ranging from 0 to 12.

The Liverpool Infant Distress Scale (LIDS, Horgan and Choonara 1996) wasdeveloped to assess postoperative distress and pain in infants. It is reliable andvalid, but has not established clinical utility. Spontaneous movements, excitability,flexion of fingers and toes, tone, facial expression, quantity and quality of crying,and sleep are scored from 0 to 5.

Multidimensional instruments

Multidimensional instruments for infants are composed of behavioral,physiological, and contextual indicators. The Neonatal Infant Pain Scale (NIPS,Lawrence et al. 1993) includes five behavioral (facial expression, crying,movement of arms and/or legs, state of arousal) and one physiological (breathingpattern) indicator of pain. Indicators are scored from 0 to 1, except for crying (from0 to 2), and the total score ranges from 0 to 7. The NIPS, developed for preterm andterm infants from CHEOPS (McGrath et al. 1985), is reliable and valid, but clinicalutility has not been established.

The Pain Assessment Tool (PAT, Hodgkinson et al. 1994) is designed to measurepostoperative pain in infants. Ten indicators (posture/tone, sleep pattern,expression, color, crying, respiration, heart rate, oxygen saturation, blood pressure,nurse’s perception of infant’s pain) are measured from 0 to 2, with total scoreranging from 0 to 20.

The CRIES (Crying, Requires 02 for saturation above 95%, Increased vital signs(heart rate and blood pressure), Expression, Sleepless, Krechel and Bildner 1995),


18

developed to measure postoperative pain in preterm and term infants, hasestablished validity and inter-rater reliability. Each of the five items is scored from0 to 2, the total score ranging from 0 to 10.

The Premature Infant Pain Profile (PIPP, Stevens et al. 1996), composed of sevenbehavioral, physiological, and contextual indicators, has established validity andreliability (Ballantyne et al. 1999). It is designed to measure acute pain. Theindicators are gestational age, behavioral state, heart rate, oxygen saturation, browbulge, eye squeeze, and nasolabial furrow, each measured from 0 to 3, with the totalscore ranging from 0 to 21. A total score of 6 or under indicates no pain; 12 or morepoints indicate moderate to severe pain.

The Distress Scale for Ventilated Newborn Infants (DSVNI, Sparshott 1996) wasdeveloped from several previous scoring systems to assess procedural pain inventilated infants. Four physiological indicators (heart rate, blood pressure, oxygensaturation, and temperature differential, i.e. the gap between core and peripheraltemperatures) are measured as monitor readings. Three behavioral indicators arescored (facial expressions and body movements from 0 to 3, and color from 0 to 2).It has minimally established validity, but reliability and clinical utility have notbeen established.

The Scale for Use in Newborns (SUN, Blauer and Gerstmann 1998), composed offour physiological (CNS state, breathing, heart rate, mean blood pressure) and threebehavioral (movement, tone, facial expression) indicators, is valid and clinicallyfeasible. Each indicator is scored from 0 to 4 on a symmetric scale, with 2 being theneutral level.

The Comfort Scale (Ambuel et al. 1992) is the only scale developed specificially tomeasure distress in newborn infants in intensive care. It is a reliable and validmeasure composed of five behavioral (alertness, calmness/agitation, physicalmovement, muscle tone, facial tension) and three physiological (respiratoryresponse, blood pressure, heart rate) indicators, each scored from 1 to 5, with thetotal score ranging from 5 to 40.


19

5.4 Developmental aspects of pharmacokinetics in newborn infants

Differences exist in drug absorption, distribution, protein binding, metabolism, andexcretion, not only between neonates and adults, but also between pretermneonates, full-term neonates, infants, and children (Morselli 1989). Absorption isnot dealt with in this context since oral, intramuscular, rectal, subcutaneous, orpercutaneous routes of drug administration are rarely used in newborn infants inintensive care environment. Administration of drug intravenously principallyassures nearly complete bioavailability, although there are some concerns. The drugmay be adsorbed into the infusion system or lost when the tubing is changed.Moreover, a delay may occure before the drug reaches the systemic circulation,because the infusion is given slowly, and this may lead to misinterpretation of theinfant’s response to the medication (Jacobson and Koren 1992). Despite theseconcerns, intravenous infusion is the most reliable and feasible way of givinganalgesics to infants.

Distribution and protein binding

The amount of total body water and the relative volume of extracellular waterdecrease rapidly during the first year of life (Friis-Hansen 1961). In pretermneonates, water comprises up to 90% of total body weight; extracellular fluidvolume being 65%, intracellular fluid volume 25%, and fat less than 1% (Brans1986). In full-term newborns, water comprises 70-75% of total body weight;extracellular volume accounting for 40%, fat 15%, and skeletal muscle mass 20-25%. By six months of age, total body water and extracellular water account for60% and 30% of body weight, respectively, and the amount of fat increases to 25%(Friis-Hansen 1971, Friis-Hansen 1983). In contrast, the brain and liver of theneonate are larger in relation to body weight, and the myelin content of brain tissueis lower than in adults (Morselli 1989).

The alterations in body composition and the qualitative and quantitative changes inplasma proteins are accompanied by a reduction in plasma protein binding capacityin the neonatal period. The concentration of albumin, which generally binds acidicdrugs, is lower in newborn infants, and especially in preterm infants, than in adults.Further, fetal albumin has a low affinity for drugs, and endogenous compounds


20

such as bilirubin compete for protein binding. Basic drugs bind to other proteins,such as gamma-globulins, lipoproteins, and acid-alpha-1-glycoprotein, the latter ofwhich, as an acute-phase protein, may increase in response to surgery and stress(Edwards et al. 1982).

Metabolism

Drugs are eliminated from the body by metabolic degradation in the liver and byrenal excretion. Hepatic clearance of a drug depends on liver blood flow and thehepatic extraction ratio. Elimination of a drug with a high hepatic extraction ratio(>0.7) is mainly dependent on hepatic blood flow, thus postnatal changes in thisflow are important with regard to pharmacokinetics. For drugs with a low (<0.3) tointermediate (0.3-0.7) hepatic extraction ratio, hepatic clearance depends primarilyon hepatic enzyme activity and the degree of plasma protein binding (Rowland andTozer 1995, Scholz et al. 1996). Most drugs are lipophilic and must be transformedinto more water-soluble compounds before being excreted via the bile or kidneys.Biotransformation occurs mostly in the liver and involves series of reactionsdefined as phase I (oxidation, reduction, hydrolysis, and hydroxylation) and phaseII (conjugation with sulfate, acetate, glucuronic acid, glutathione, and glycine)reactions.

Generally, phase I reactions lead to compounds with lower, equal, or higherpharmacologic activity than the parent compound, and phase II reactions give riseto inactive products. Concomitant or previous drug treatment or variouspathophysiological conditions may induce or inhibit these reactions. Thecytochrome P450 (CYP) superfamily, especially the CYP 3A subfamily, is themost important hepatic enzyme system catalyzing phase I reactions. Glucuronosyltransferases, sulfotransferases, and methyltransferases are the predominant phase IIenzymes. Traditionally, the developmental pattern of drug-metabolizing enzymeactivity is viewed as being very low in fetal organs, limited in the newborn infant,rapidly increasing in the first year of life to children’s level, which may exceedadult capacity, and finally declining to adult level at puberty. Recently, theimportance of genetic polymorphism and developmental differences in the activityof drug-metabolizing enzymes during the perinatal and neonatal periods, throughinfancy and childhood, and into adolescence have been recognized (deWildt et al.1999a, deWildt et al. 1999b, Gow et al. 2001). Significant sulfotransferase activity


21

is found as early as 14 weeks gestation in contrast to glucuronosyl transferaseactivity, which is less than 10-30% of adult values in the neonatal period (Gow etal. 2001).

Many drugs are metabolized mainly in the liver by the CYP3A subfamily, whichconsists of at least three isoforms, CYP3A4, CYP3A5, and CYP3A7 (Nelson et al.1996), located on chromosome 7 (Inoue et al. 1992). The most abundantlyexpressed isoform, CYP3A4, accounts for approximately 30% to 40% of the totalCYP content in the human adult liver and small intestine (Watkins et al. 1987,Kolars et al. 1994). CYP3A5, being 83% homologous to CYP3A4, is the mainCYP3A isoform in the kidney (Haehner et al. 1996). CYP3A7 is the prevailingisoform detected in the human embryonic, fetal, and newborn liver. It is alsodetected in the adult liver, but at a much lower level than CYP3A4 (Kitada et al.1985, Yang et al. 1994, Tateishi et al. 1997). CYP3A7 activity is high duringembryonic and fetal life, decreasing rapidly during the first postnatal week. Incontrast, CYP3A4 activity is low before birth, but increases rapidly thereafter,reaching 50% of adult levels between 6 and 12 months of age (Hakkola et al. 1994,Lacroix et al. 1997). Profound changes occur in the activity of CYP3A isoformsduring all stages of development. These changes are clinically important whentreatment involves drugs that are substrates, inhibitors, or inducers of CYP3A.

Expression of CYP2D6 is low during the fetal period, but increases rapidly afterbirth (Treluyer et al. 1991). Activity of CYP1A2 is minimal in fetal liver and seemsto reach adult levels by 120 days of postnatal age (Cazeneuve et al. 1994, Hakkolaet al. 1994). CYP2C9 activity in infants more than one week of age may exceedadult enzyme activity (Leff et al. 1986). Ontogeny of CYP2C19 or of phase IIenzymes during the neonatal period has not been thoroughly studied. However,important differences exist between children and adults, and the developmentalpattern of these enzymes varies (Leeder and Kearns 1997).

Excretion

Renal excretion, the principle route of drug excretion, depends on glomerularfiltration, tubular reabsorption, and tubular secretion. Renal function is limited atbirth because the kidney is anatomically and functionally immature (Strauss et al.1981, Robillard et al. 1992). Gestational age and hemodynamic changes in the


22

postnatal period are the main factors influencing development of renal function(Aperia et al. 1981, Vanpee et al. 1993). At birth, glomerular filtration rate isapproximately 20 ml/min per 1.73 m2 in full-term infants and lower in preterminfants, at only 10.2 to 13.0 ml/min per 1.73 m2 at 28 to 30 weeks of gestation(Guignard and John 1986). A rapid increase occurs after birth because of increasedcardiac output. The increase is greater in full-term than in preterm infants (Arant1978, Arant 1984, Schwartz et al. 1984). Adult values are reached and evenexceeded within one month after birth (Robillard et al. 1992). The renal tubularsecretory function is also low at birth because of the small size of tubules, thelimited number of tubular cells, and reduced blood flow (Guignard and Drukker1999). The tubular function matures more slowly than glomerular filtration(Besunder et al. 1988a, Besunder et al 1988b).

5.5 Pain management

Decreasing noxious stimuli

Environmental and behavioral pain management strategies includenonpharmacologic interventions that should be considered the basis for all painmanagement. Pain can be reduced indirectly by reducing the amount of noxiousstimuli to which infants are exposed, and directly by blocking nociceptive pathwaysor by activating descending pain modulating systems. The most effective strategyfor decreasing pain in the neonatal intensive care unit is to strictly limit thefrequency and amount of painful caregiving and diagnostic procedures to those thatcan be documented to positively affect outcome (Franck and Lawhon 2000). Bloodsamples should be grouped to minimize the number of venipunctures per day,noninvasive monitoring devices should be used when possible, and an indwellingarterial line should be inserted to minimize the number of vein and artery punctures.

Environmental and behavioral interventions

Environmental stress can be decreased by reducing lighting levels, alternating day-night conditions, and reducing noxious noise. Positioning infant to maintain flexedposition and providing a “nest” with blanket rolls stimulates proprioceptive,thermal, and tactile sensory systems and facilitates self-regulation. The pain-


23

relieving effect of nonnutritive sucking has been studied (Field and Goldson 1984,Miller and Anderson 1993, Stevens et al. 1999). The administration of sweet-tastingsubstances (sucrose, glucose, or saccharin) prior to a minor procedure, e.g. bloodsampling, is effective in relieving pain (Blass and Hoffmeyer 1991, Rushforth andLevene 1994, Bucher et al. 1995, Haouari et al. 1995, Abad et al. 1996, Ramenghiet al. 1996, Johnston et al. 1997, Lindahl 1997, Skogsdal et al. 1997). Theeffectiveness of analgesia depends on the concentration of the solution (Skogsdal etal. 1997), and the most appropriate time to give the solution is two minutes prior tosampling (Stevens et al. 1997). The analgesic effect seems to be transmittedthrough the endogenous opioid system (Blass 1994, Lindahl 1997, Skogsdal et al.1997, Stevens et al. 1997).

Pharmacologic interventions

Pharmacologic analgesia and sedation are usually appropriate for infants receivingintensive care. Opioids, which also have a sedative effect, are the most commonlyused analgesics in the newborn period (Johnston et al. 1997, Anand et al. 1999).Morphine and its synthetic derivative fentanyl are the preferred opioids. Pethidinehas a pharmacologic profile close to morphine, but its metabolite norpethidine isneurotoxic. Alfentanil is a fentanyl derivative with rapid onset and brief duration ofaction. Sufentanil is 5 to 10 times more potent than fentanyl, being the most potentopioid in use. Its pharmacokinetic properties are intermediate to those of alfentaniland fentanyl (Jacqz-Aigrain and Burtin 1996). Along with pharmacokineticproperties, opioids vary in their side effects and potential for producing toleranceand dependence.

Analgesic potency without a ceiling effect, maintenance of hemodynamic stability,reversibility of side effects by antagonist drugs, and long-lasting clinical use in termand preterm infants are clear advantages of opioids. Respiratory depression is acommon side effect. Neonates are believed to be more sensitive to opioids thanolder children and adults; however, this claim has been questioned with regard tomorphine (Olkkola et al. 1988, Lynn et al. 1993, Nichols et al. 1993). In addition,sensitivity to respiratory depression has even been reported to be reduced withfentanyl and sufentanil (Greely et al. 1987, Gauntlett et al. 1988). Histamine releasemay cause vascular dilatation, which may lead to hemodynamic instability andbronchospasms, especially when the intravenous bolus is given rapidly. The


24

histamine-releasing effect of morphine is greater than that of fentanyl (Rosow et al.1982). On the other hand, fentanyl may stimulate muscle contractions causing chestwall rigidity, particularly when given as a rapid intravenous bolus (Lindemann1998). Other side effects of opioids are decreased gastrointestinal motility andperistalsis, urinary retention, pruritus, and nausea and vomiting.

Tolerance and physical dependence develop with prolonged or repeated use of allopioids. Signs of withdrawal including irritability, restlessness, insomnia, muscletwitches and movement disorders (Norton 1988, Lane et al. 1991), arise mainlyfrom a pathological excitation of the central nervous system. Objective methodshave been developed to assess these withdrawal symptoms (Suresh and Anand1998), which may occur as soon as 48 hours after initiation of morphine infusion,but clinically significant withdrawal usually occurs after 5 days (Arnold et al.1990). Gradual weaning diminishes the risk of withdrawal syndrome, which ifoccurring, can be treated with pharmacologic and nonpharmacologic methods(Suresh and Anand 1998).

Anesthetic agents, such as ketamine, have been used to provide analgesia andsedation in NICU. Weak analgesics, such as salicylates and nonsteroidal anti-inflammatory drugs (NSAIDs), are not recommended for use in neonates becauseof side effects and the lack of sufficient scientifical studies or evidence-basedclinical guidelines or safety reports. Paracetamol is also extensively used fortreatment of mild pain in term neonates, but its efficacy and safety have not yetbeen proven in preterm infants. Benzodiazepines are pure sedatives withoutanalgesic effect; midazolam is presently the most widely used sedative in neonates.Chloral hydrate is also used to sedate newborn infants, but because of adverseeffects and a risk of hyperbilirubinemia, repeated doses should be avoided.

Morphine

Morphine is a µ receptor agonist and the most widely used opioid in neonatalintensive care, the standard against which all other opioids are compared.Pharmacokinetic studies of opioids show great differences between preterm andterm neonates, older infants (Table 2), children (Vandenberghe et al. 1983), andadults (Stanski et al. 1978, Owen et al. 1983). The postnatal age of the infantsincluded in these studies varies greatly. The total plasma clearance and half-life


25

vary with age, the clearance being slowest and the half-life longest in preterminfants, with a considerable interindividual variation. Concurrent illness and surgeryaffect the pharmacokinetics. Further, renal failure leads to accumulation ofmorphine metabolites (Shelly et al. 1986, Faura et al. 1998), but liver failure hasonly minor effects on morphine pharmacokinetics.

Morphine is metabolized in the liver via glucuronidation. The major metabolites aremorphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). Sulfation is aminor metabolic pathway. Glucuronidation is impaired in the neonatal period(Choonara et al. 1989, Bhat et al. 1992, Chay et al. 1992, Choonara et al. 1992,McRorie et al. 1992, Hartley et al. 1993a, Hartley et al. 1993b). M6G has a morepotent analgesic effect than morphine (Osborne et al. 1992, Barrett et al. 1996);however, the clinical significance of M6G analgesia during morphine therapy isunclear. Morphine metabolites are excreted in the urine and bile, but acutely illpreterm infants also excrete unmetabolized morphine (Bhat et al. 1990, Bhat et al.1992).

Plasma concentration of morphine required for analgesia or sedation seems to varygreatly. Dahlström et al. (1979) reported concentrations of 65 ng/ml sufficient forintra-operative analgesia, and Lynn et al. (1984) concentrations of 12 ng/ml forpostoperative analgesia in children. In infants, the mean analgesic concentrationpostoperatively was 26 ng/ml as compared with 4 ng/ml in children aged 2-6 years(Olkkola et al. 1988). Adequate sedation and analgesia was reported in 50% ofnewborn infants receiving a mean morphine concentration of 125 ng/ml (Chay et al.1992). In a recent study, however, the steady-state concentration (mean 210 ng/ml)did not correlate with analgesia (Scott et al. 1999).

Fentanyl

The use of the synthetic opioid fentanyl for analgesia in newborn infants hasincreased during the last decades. Fentanyl, as a highly lipophilic agent, is widelydistributed in tissues. It crosses the blood-brain barrier more rapidly than morphine,having therefore a faster onset and a shorter duration of action.


27

The pharmacokinetics of fentanyl in newborn infants has not been thoroughlyinvestigated (Table 2). The number of infants in each study is small and thepostnatal age varies greatly. Fentanyl has a high clearance rate in comparison withmorphine. It has a high extraction ratio (0.8 to 1.0) and is eliminated mainly byhepatic metabolism. Liver blood flow and the activity of metabolizing enzyme CYP3A4 affect the rate of elimination. Only small amounts of unmetabolized drug areexcreted in urine, and renal failure during cardiac surgery does not affect fentanylpharmacokinetics in children and adolescents (Koren et al. 1984). The eliminationhalf-life is significantly prolonged if hepatic blood flow is decreased or abdominalpressure increased (Koehntop et al. 1986, Gauntlett et al. 1988).

Scarce data is available on the relationship of fentanyl concentration and itsanalgesic effect in newborn infants. When using continuous infusion of fentanyl toprovide adequate sedation for NICU infants, those delivered at a gestational age ofless than 34 weeks were sedated with a mean plasma concentration of 1.7 ng/mland those delivered after 34 weeks with 2.1 ng/ml, respectively (Roth et al. 1991).

Alfentanil

Alfentanil, a structural analog of fentanyl, has been used as an analgesic for infantsreceiving muscle relaxants (Marlow et al. 1990). It is less lipid-soluble, has a morerapid onset, and a shorter duration of action than fentanyl (Jacqz-Aigrain and Burtin1996). Plasma protein binding increases from 65% in preterm infants to 79% interm infants (Wilson et al. 1997). The concentration of alfentanil declines rapidlyafter intravenous administration. Pharmacokinetic properties vary between newbornand older infants (Table 2), children (Meistelman et al. 1987, Roure et al. 1987),and adults (Camu et al. 1982, Schuttler and Stoeckel 1982).

Alfentanil has intermediate extraction ratio (0.3 to 0.5), it is eliminated mainly byhepatic metabolism, and only a small fraction is excreted unchanged in the urine.Neither renal failure nor hepatic disease (Davis et al. 1989b) has an effect on thepharmacokinetics of alfentanil.

No data appears to exist on the analgesic concentration of alfentanil in newborninfants. Rautiainen (1991) estimated a steady-state concentration ranging from 50to 220 ng/ml (mean 79 ng/ml) to be adequate for providing effective analgesia and


28

sedation during cardiac catheterization in infants aged 1 to 17 months. In anotherstudy with preterm infants, where a larger alfentanil dose (20 µg/kg initial bolus,thereafter infusion of 3 µg/kg/h or 5 µg/kg/h) was used, median steady stateconcentrations of 29 ng/ml and 55 ng/ml, respectively, were achieved (Marlow etal. 1990). Intravenous doses of 10-20 µg/kg alfentanil provided analgesia anddiminished the physiological stress response caused by tracheal intubation ortreatment procedures such as tracheal suctions, blood samplings, and radiographicexaminations (Pokela 1993, Pokela and Koivisto 1994).

As side effects, central nervous system excitation and chest wall rigidity were notedin 4 of 17 children receiving 50 µg/kg, 1 of 17 receiving 25 µg/kg, but none of 17receiving 10 µg/kg alfentanil before the induction of intravenous anesthesia,respectively (Lindgren et al. 1991). Severe rigidity was observed in 4 of 20newborn infants in association with the administration of 9-15 µg/kg alfentanil(Pokela et al. 1992).

Ketamine

Ketamine is an anesthetic agent chemically related to phencyclidine andcyclohexamine, with analgesic properties at subanesthetic plasma concentrations(Reich and Silvay 1989). It produces a blockade of N-methyl-D-aspartate (NMDA)receptors and has been used for dissociative anesthesia in infants. The molecularstructure contains a chiral center at the C-2 carbon of the cyclohexanone ring,permitting the existence of two optical isomers, s(+)ketamine and r(-)ketamine.These two enantiomers differ in their anesthetic and analgesic potency, physicalside effects, and incidence of emergence reactions. The s(+)ketamine producedmore effective analgesia than racemate or r(-)ketamine, and this correlated withdegree of electroencephalogram changes and higher affinity for opiate receptors(White et al. 1980, White et al. 1985). More psychic emergence reactions occurredwith r(-)ketamine treatment than after racemate or s(+)ketamine (White et al. 1980).Previously, the commercially available racemic ketamine preparation containedequal concentrations of both enantiomers, but s(+)ketamine alone has recently alsobecome available. The pharmacokinetic profiles for the individual isomers did notdiffer from that of the racemic mixture (White et al. 1985).

In addition to effective analgesia, ketamine has been used to provide sedation andamnesia for pediatric patients undergoing invasive procedures (Cotsen et al. 1997,


29

Lowrie et al. 1998) and in the NICU in association with procedures (Tashiro et al.1991, Betremieux et al. 1993). Advantages of ketamine include hemodynamicstability and maintenance of respiratory function (Friesen and Henry 1986). Dosesof 2 mg/kg intravenously provided brief analgesia and sedation with low incidenceof hypoventilation (Cotsen et al. 1997). When doses of 5 mg/kg were givenintravenously to 10 critically ill preterm infants aged 1 to 10 days prior toprocedure, no changes in cerebral blood flow and transcutaneous gas pressureswere noted. Moreover, while arterial pressure decreased significantly, no changesoccurred in heart rate or cardiac output (Betremieux et al. 1993). Doses of 1 mg/kgketamine were used to sedate 25 preterm infants for cryotherapy at 3 monthspostnatal age with no hemodynamic effects (Tashiro et al. 1991).

Adverse effects include emergence phenomena, increased intracranial pressure(Friesen and Henry 1986), and hypertension. Concomitant midazolam use canprevent emergence phenomena, described as a combination of bad dreams,hallucinations, and delirium. However, emergence reactions have not been seen inchildren younger than five years of age. Increased production of salivary and upperrespiratory secretions caused by ketamine can be relieved with atropine orglycopyrolate.

Pharmacokinetic and pharmacodynamic data on ketamine during the neonatalperiod is limited (Betremieux et al. 1993, Hartvig et al. 1993). In adults, analgesiceffect was observed with plasma concentrations of 150 ng/ml after intramuscularadministration and 40 ng/ml after oral administration, and awakening fromanesthesia occurred at plasma concentrations of 640 – 1120 ng/ml (Grant et al.1981). In ten infants aged 1 week to 30 months, 1 mg/kg/h or 2 mg/kg/h ketaminewas used for postoperative analgesia and sedation (Hartvig et al. 1993). Childrenwere arousable at ketamine concentrations of 1000 – 1500 ng/ml. The mean plasmaclearance was 15.7 ml/min/kg and the elimination half-life was 3.1 hours. The rateof conversion of two enantiomers is unknown in infants and the relationship ofisomers may differ from that of adults. The relationship between plasmaconcentration and analgesic effect is also unclear.

Ketamine is metabolized in the liver by CYP enzymes to norketamine, an activemetabolite with anesthetic potency one third that of ketamine. Norketamine ishydroxylated and excreted into the urine as conjugates.

AIMS OF THE STUDY

30

6 AIMS OF THE STUDY

The objectives of this study were :

To evaluate the efficacy, safety, and pharmacokinetics of opioid treatment forpersistent pain and distress in the early neonatal period (I, II, III).

To assess pain during brief standard procedures in the neonatal period and to findan appropriate premedication (IV, V).

The specific objectives were:

1. To compare the efficacy of fentanyl and morphine in treating persistent pain (I).2. To evaluate the efficacy of alfentanil in relieving acute pain (IV).3. To evaluate the efficacy of ketamine in relieving acute pain (V).4. To determine the pharmacokinetics of the drugs studied (II, III).5. To assess adverse events of drugs used (I, IV, V).

SUBJECTS AND METHODS

33

Ten newborn infants were enrolled in the alfentanil trial. The median gestationalage was 32 weeks (interquartile range (IQR) 29 to 36) and median birthweight 1440g (IQR 1040 to 3160). The median age at enrollment was three days (IQR 2 to 6).

Sixteen newborn infants were enrolled in the ketamine trial. The median gestationalage was 31 weeks (IQR 29 to 33) and median birthweight 1160 g (IQR 1070 to1900). The median age at enrollment was three days (IQR 2 to 5).

7.2 Study design

Fentanyl or morphine for ventilated newborn infants (I, II, III)

The randomization procedure of this double-blind study for allocation to either thefentanyl or morphine group was carried out in five blocks using sealed andnumbered envelopes. The infants were stratified for birthweight into two groups,less than 1500 g and greater than or equal to 1500 g.

The drug solution was prepared in 10% dextrose according to strict directions andusing the same drug concentration [morphine (MorphinR, Leiras, Turku, Finland)40 µg/ml, and fentanyl (FentanylR, Janssen Pharmaceutica, Beerse, Belgium) 3µg/ml] for all infants. The infusion was started as soon as possible after theenrollment, at a rate of 3.5 ml/kg/h for one hour, to obtain a loading dose of 140µg/kg morphine or 10.5 µg/kg fentanyl. The infusion provided 5.8 mg/kg/minglucose. After 60 minutes, the infusion was decreased to a rate of 0.5 ml/kg/h,corresponding to a dose of 20 µg/kg/h morphine, 1.5 µg/kg/h fentanyl, and 0.8mg/kg/min glucose, respectively, and continuing for at least 24 hours.

Intensive care procedures and monitoring were performed according to standardclinical practice. Mechanical ventilation was provided with Infant Star ventilators(Infrasonics Inc, San Diego, CA, USA), primarily on the synchronized intermittentmandatory ventilation mode. High-frequency oscillatory ventilation was used whenmean airway pressure exceeded 10 cmH20 or in case of air leak.

When the caretaking nurse evaluated the response to treatment procedures to bepainful on the basis of infant’s behavior, additional boluses of the investigationaldrug solution were administered. A bolus of 0.5 ml/kg (morphine 20 µg/kg or


34

fentanyl 1.5 µg/kg) could be given at most four times a day. A muscle relaxant wasused if the infant was struggling against the ventilator. Weaning from the opioidinfusion occurred gradually over 0.5 to 2 days depending on the duration of thetreatment.

Alfentanil for procedural pain relief in neonates (IV)

This double-blind, randomized, crossover trial contained three phases. Ten infantsreceived saline as placebo and two different doses of alfentanil (RapifenR, JanssenPharmaceutica, Beerse, Belgium), 10 µg/kg and 20 µg/kg, intravenously in randomorder before three painful procedures spaced at least six hours apart. Seven infantscompleted the entire protocol, one received two study doses, and two received onlyone study dose each, because of removal of the arterial line.

Endotracheal suction, part of the routine treatment of the infants, was used as astandardized painful procedure. The nurse responsible for drug preparationperformed the dilution of alfentanil from commercial vials according to randomizedinstructions in sealed numbered envelopes. The diluted volume was the same (0.8ml/kg) for all doses. Another nurse administered the drug slowly as a bolus overtwo minutes, and after another two minutes, the endotracheal suction was started.The infant was bag ventilated during the procedure, and if oxygen desaturationoccurred, the FiO2 was increased.

Ketamine for procedural pain relief in neonates (V)

This double-blind, randomized, crossover trial contained four phases. Each patientreceived saline as placebo and three different doses of racemic ketamine (KetalarR,Parke-Davis, Dublin, Ireland), 0.5 mg/kg, 1 mg/kg, and 2 mg/kg, intravenously inrandom order before four painful procedures (endotracheal suction) spaced at leasttwelve hours apart. One nurse performed the dilution of ketamine from commercialvials and an equal volume (0.5 ml/kg) was infused for all doses. Another nurseadministered the drug slowly over two minutes. Five minutes after beginning ofinjection, the endotracheal suction was started. The infant was bag ventilated duringthe procedure, and if oxygen desaturation occurred, the FiO2 was increased.


35

7.3 Blood sampling

Fentanyl-morphine trial (I, II, III) An arterial blood sample (1.5 ml blood inethylenediamine tetra-acetic acid vials containing 15 µl of 1% sodium-metabisulphite) was obtained for determination of plasma adrenaline,noradrenaline, and ß-endorphin concentrations upon entry to the study, and at 2 and24 hours after beginning infusion. The samples were centrifuged and stored at-70°C until analyzed. Timed arterial blood samples (0.5 ml) for determination offentanyl or morphine, M3G, and M6G concentrations were collected intoMicrotainer Brand Serum Separator Tubes (Becton Dickinson and Company,Franklin Lakes, NJ, USA) at 2, 12, 24, 48, and 60 hours after beginning infusion.Serum was separated within 60 minutes and stored at -70°C until analyzed.

Alfentanil trial (IV) Arterial blood was sampled (1.5 ml) before the solution wasadministered and at 30 minutes after endotracheal suction for determination ofplasma adrenaline, noradrenaline, and ß-endorphin concentrations. A simultaneousblood sample (0.5 ml) for determination of alfentanil concentration was obtained.The samples were centrifuged and stored at -70°C until analyzed.

Ketamine trial (V) Arterial blood was sampled (1.0 ml) before the solution wasadministered and at 10 minutes after endotracheal suction for determination ofplasma adrenaline and noradrenaline concentrations. A simultaneous blood sample(0.5 ml) for determination of ketamine concentration was obtained. The sampleswere centrifuged and stored at -70°C until analyzed.

7.4 Outcome measures

Behavioral pain assessment

Behavioral pain responses before, during, and after a routine tracheal suction wereassessed blindly by the caretaking nurse in the fentanyl-morphine trial and by theresearcher (ES) in alfentanil and ketamine trials. Scoring was not performed duringmuscle relaxation in the fentanyl-morphine trial. Definition of the pain scaleadapted from the Neonatal Infant Pain Scale (NIPS) of the Children´s Hospital ofEast Ontario Pain Scale (CHEOPS) is presented (IV, Table 1). Changes occurring


36

in the five parameters during the procedure, as compared with before the procedure,i.e. pain intensity difference (PID) was registered.

Physiological parameters

The heart rate, arterial blood pressure, and oxygen saturation were continuouslymonitored (Hewlett Packard Neonatal Component Monitoring System, Andover,MA, USA). Changes (from baseline) at certain time points (2, 12, 24, and 48 hoursafter start of infusion) and in association with the standardized painful procedure,endotracheal suction, were registered. Maximal changes within five minutes of theadministration of the investigational solution in the ketamine trial were alsoregistered.

Hormonal assays

Plasma noradrenaline and adrenaline concentrations were measured with high-performance liquid chromatography (HPLC) using electrochemical detector (EsaCoulochem Multi-Electrode, model 5100 A) (Eriksson and Persson 1982). Thedetection limit was 10 pg/injection. Radioimmunoassay was used for determinationof ß-endorphin concentrations (Vuolteenaho et al. 1981). The detection limit was 2pg/tube.

Determination of drug concentrations

Fentanyl-morphine trial (II) Serum fentanyl concentrations were determined byradioimmunoassay (Janssen Research Foundation, Belgium) (Woestenborghs et al.1987). The quantification limit was 1 ng/ml. The interassay coefficient of variation(CV) was 7.2% (at mean 3.75 ng/ml; n=6).

Fentanyl-morphine trial (III) Serum morphine, M3G, and M6G concentrationswere determined using the reversed-phase ion-pair HPLC method (Svensson et al.1982). The quantification limit for morphine and M6G was approximately 1 ng/ml,and that for M3G approximately 3 ng/ml. The interassay CV was less than 5% formorphine and M6G, and less than 3% for M3G.


37

Alfentanil trial (IV) Serum alfentanil concentrations were determined withradioimmunoassay, the detection limit of which was 1 ng/ml (Janssen ResearchFoundation, Belgium).

Ketamine trial (V) Plasma ketamine (racemate) concentrations were determined byHPLC using etidocaine as an internal standard (Adams et al. 1992). The limit ofquantification was 20 ng/ml, and the interassay CV was 6.5% at 196 ng/ml (n=5).

Pharmacokinetic calculations

Fentanyl and morphine steady states were considered achieved when theconcentrations in two consecutive samples were within 15% of each other, withouta consistent increase or decrease in the slope of the concentration-time curve duringthe constant infusion. Total body clearance was calculated by dividing the infusionrate with the concentration at steady state (Rowland and Tozer 1995).

7.5 Adverse events

Fentanyl-morphine trial (I,II,III) Gastrointestinal motility was assessed daily basedon passing of meconium and gastric retention defined as fluid reflux from thenasogastric tube. Necrotizing enterocolitis was defined as stage II or III with Bell’sstaging system (Bell et al. 1978). Urinary retention was evaluated daily bothclinically and ultrasonographically. Retention was defined as loss of spontaneousurination with enlarged bladder or reversible hydronephrosis. Attention wasfocused on rigidity, which was repeatedly assessed during the infusion by thecaretaking nurse or neonatologist.

Alfentanil trial (IV) Special attention was paid to rigidity, which was graded interms of passive resistance to movement of the limbs relative to the pretreatmentlevel in four categories: muscle tone decreased or unchanged, slightly increasedmuscle resistance, severe rigidity, or convulsive activity. Urinary retention wasevaluated clinically and at least once in each infant with ultrasound measurement.


38

Ketamine trial (V) The caretaking nurse observed reactions to routine treatmentprocedures. Attention was paid to possible unexpected behavioral features of theinfant while undisturbed, in an attempt to register occurrence of emergencephenomena.

7.6 Statistical analysis

Data are presented as mean and standard deviation (± SD) or 95% confidenceinterval (CI), or as median and interquartile range (IQR), when distribution ofvalues was skewed. Statistical significance was defined as p<0.05.

Clinically significant side effects, such as urinary retention and decreasedgastrointestinal motility, were observed in two thirds of cases when using morphinein NICU prior to the trial, and this background prevalence was used whencalculating sample size in the fentanyl and morphine trials. To show a 40%decrease (from 60% to 35%) of these side effects with a power of at least 80% (α =0.05), a total sample size of 160 was needed. In all, 163 patients were enrolled.

Reduction of the behavioral pain score was chosen as a clinically significantresponse in the alfentanil and ketamine trials. To calculate sample size in thealfentanil trial, tracheal suction without pain relief was hypothesized to cause anincrease in behavioral pain score of 6 points. To show a 25% reduction in the painscores produced by either dose of alfentanil with a power of 90% (α = 0.05), asample size of 9 was needed; 10 patients were studied.

The sample size for the ketamine trial was calculated using the results of thealfentanil trial. To show a reduction in pain score from a mean value of 5 to 2 (SD3), corresponding to the observed scores after administering placebo and 10 µg/kgalfentanil, with any dose of ketamine with a power of 80% (α = 0.05), 16 infantsshould be enrolled.

In the fentanyl-morphine trial, comparison of the fentanyl and morphine baselinedata was performed with Student’s t-test or chi-squared test. Differences inhormone concentrations were analyzed with Student’s t-test, and in case of skeweddistribution, a logarithmic transformation or the Mann-Whitney U-test was used.The paired t-test was used for comparison of paired measurements from the same


39

individual. The relationships between fentanyl and morphine clearances andgestational age and birthweight were analyzed with Pearson’s correlationcoefficient. Comparison of the steady-state concentrations and fentanyl clearance ininfants delivered before or after 30 weeks of gestational age was performed withStudent’s t-test. The relation of morphine steady-state concentrations to responseswas analyzed with Student’s t-test (parametric data) or the Mann-Whitney U-test(nonparametric data). In the alfentanil trial, the Wilcoxon signed-rank test andSpearman's rank correlation coefficient were used, and in the ketamine trial thepaired Student’s t-test and the Wilcoxon signed-rank test were used. Analyses wereperformed using the SPSS (version 8.0 for Windows; SPSS, Chicago, IL, USA)software package.

7.7 Ethical considerations

The Ethics Committee of the hospital, and in the ketamine trial (V) also the FinnishNational Agency for Medicines, approved the study protocol in accordance withcurrent legislation. Written informed parental consent was obtained. Bloodsampling volumes were restricted because of the small size of the infants. If infantswere receiving blood transfusions on clinical indications, the sampling could becompleted according to protocol, since additional volume of packed red blood cellswas given to replace the blood volume sampled.

RESULTS

40

8 RESULTS

8.1 Comparison of analgesic effects of fentanyl and morphine (I)

In the fentanyl and morphine groups, the pain score values during opioid infusionprior to tracheal suction were similar [median 1 (IQR 0-3)], as were the scorechanges in response to tracheal suction (I, Figure). The heart rate and mean arterialblood pressure remained stable during drug infusions. Vasoactive agents were usedequally in both treatment groups.

Plasma catecholamine concentrations decreased significantly in both treatmentgroups at 24 hours, but ß-endorphin decreased significantly only in the fentanylgroup (I, Table III) (Figure 1).

Decreased gastrointestinal motility occurred significantly less frequently in thefentanyl group as compared with the morphine group (23% vs. 47%, p<0.01).Urinary retention occurred in 56% of fentanyl-treated and 55% of morphine-treatedinfants based on clinical evaluation, and in 36% and 37%, respectively, based onultrasound measurement. No adverse effects were observed in 28% of fentanyl-treated and 32% of morphine-treated infants.

RESULTS

47

Pain score did not correlate significantly with serum morphine steady-state valuesduring the second day of life; the steady-state morphine concentration was similarin infants with and without effective pain relief (158 ± 60 and 148 ± 61 ng/ml,respectively, III, Figure 3). Neither was there a correlation between metaboliteconcentrations, M3G+morphine, or M6G+morphine concentrations and the ratio ofthese concentrations to that of morphine. Infants with decreased gastrointestinalmotility had higher steady-state morphine concentrations (187 ± 82 ng/ml) thaninfants with normal motility (128 ± 51 ng/ml) (III, Figure 3). Serum morphinesteady-state concentration correlated significantly with the postnatal age whenenteral feeding began (r=0.44, p<0.05).

8.4 Alfentanil as procedural pain relief (IV)

Attenuation of the pain score change was significant (p<0.05) in infants receiving20 µg/kg alfentanil, but not in infants receiving 10 µg/kg alfentanil, when comparedwith placebo (IV, Figure 3).

Serum alfentanil concentration increased with dose [median 21 (IQR 17 to 22)ng/ml after 10 µg/kg and median 49 (IQR 40 to 50) ng/ml after 20 µg/kg]. Twoinfants receiving placebo as their second dose still had detectable alfentanilconcentrations (11 ng/ml and 13 ng/ml) six hours after the alfentanil dose.

Tracheal suction was associated with an increased heart rate of 15 (IQR 13 to 16)beats/min and an arterial blood pressure of 8 (IQR 6 to 14) mmHg in infantsreceiving placebo. With 20 µg/kg alfentanil, a significant attenuation of the heartrate change was noted; -7 (IQR -17 to -1) beats/min; p<0.05 compared withplacebo. Attenuation of the blood pressure change was not significant; 1 (IQR -6 to6) mmHg (IV, Figure 1).

Plasma noradrenaline concentrations tended to decrease with increasing alfentanildose, but the change was not significant (IV, Figure 2). After 20 µg/kg of alfentanil,the adrenaline concentration decreased (IV, Table 2). No significant changes werefound in ß-endorphin concentrations after any of the doses.

DISCUSSION

49

9 DISCUSSION

Morphine and fentanyl in the doses administered seemed to provide efficientanalgesia; however, fentanyl may be superior for short-term postnatal use becauseof the lesser risk for gastrointestinal dysmotility. The clearances of both morphineand fentanyl were correlated with gestational age and birthweight. The dose of 20µg/kg alfentanil relieved endotracheal suction-induced pain, but caused a highincidence of rigidity, and thus should be used only in conjunction with a musclerelaxant. Ketamine in the doses used had only a small or moderate analgesic effect,and therefore seems to be a poor pretreatment for painful procedures.

9.1 Methodological considerations

The study was performed in only one NICU to achieve strict adherence to theprotocol and to standardize evaluation of pain and adverse effects. A randomized,double-blind study design was used in fentanyl and morphine trials. Randomizationwas performed in five blocks to minimize the influence of changing treatmentsduring the fairly long study period. A randomized, balanced, double-blind,crossover study design was used in alfentanil and ketamine trials. The effect ofinterindividual variation on the results in alfentanil and ketamine trials wasminimized because the subjects served as their own controls. Possible carry-overeffect in these trials was only partly prevented by randomization, since for clinicalreasons, wash-out periods between study phases were only 6 (alfentanil) and 12(ketamine) hours. Due to the small sample size, a more detailed analysis of carry-over effect and period effect were not possible. Moreover, the previous treatmentswith morphine boluses in the alfentanil trial and fentanyl infusion in the ketaminetrial may have influenced results, although the time interval in both trials was atleast 12 hours.

The design of the randomized trial comparing morphine and fentanyl infusionsgave rise to speculation on the ethics of not including a placebo group (Kennedyand Tyson 1999), which we found unacceptable because of the frequent need foranalgesia in these patients. One resolution to this problem might have been toinclude a placebo group with an open-label use of analgesic bolus medication for

DISCUSSION

50

clinical discomfort. However, in our setting, most placebo infants would have beentreated with boluses, and almost the entire control group would have beencontaminated and thus not informative. In addition, previous controlled trials haveshown that placebo-treated infants had increased plasma adrenaline (Quinn et al.1993) and ß-endorphin (Ionides et al. 1994) concentrations and had suffered morepain as judged by a pain scale (Jacqz-Aigrain et al. 1994) than morphine-, fentanyl-,or midazolam-treated infants.

As a standard painful stimulus, endotracheal suction was used, which as part ofroutine treatment is performed repeatedly. Endotracheal suction is associated withacute cardiovascular changes, hypoxemia (Skov et al. 1992, Pokela 1994), releaseof catecholamines (Greisen et al. 1985, Esuri et al. 1997), and atrial natriureticpeptide (Esuri et al. 1997) in preterm neonates. While it is considered distressingand unpleasant (Anand et al. 1999), how much of the associated response seen innewborn infants is indeed caused by pain is unclear, since it may also cause vagalreflex. Heel prick would have been a more reliable painful stimulus, but it could notbe used for ethical reasons, as sampling was performed from the arterial line.

The pain scale used in the present study was adopted from the NIPS, the mostsuitable, valid pain-assessment tool at the time of beginning the study. NIPS as suchwas not optimal for mechanically ventilated infants with oxygen saturationmonitoring and at least two peripheral lines restricting movement of arms and legs.The scale used proved to be feasible and easy to manage, which is important whenrepeated measurements are performed by several observers in a busy environment.However, the scale is limited by its inclusion of only one physiological indicator,and thus, should be used in combination with other physiological measures.Recently, a study comparing three pain assessment measures (NIPS, SUN, andComfort scale) has been published (Blauer and Gerstmann 1998); NIPS proved tobe the easiest, and thus, the fastest to use (Blauer and Gerstmann 1998).

Evaluation of urinary retention, gastrointestinal motility, and rigidity were based onpreviously defined criteria. Although subjective assessments, the blindness of theobserver and standardized evaluation made these measures reliable.

Because of blood volume restrictions, all parameters could not be measured in allinfants. In the subgroups with analyses performed, the patients did, however, notdiffer from the entire study group regarding clinical characteristics, morbidity, or

DISCUSSION

51

treatment. Sampling at time zero was not possible, sampling schedule was sparse,and sampling after discontinuing the infusion was also not possible. Therefore, theexact half-life could not be calculated. Urine collection was not performed becauseof difficulties in obtaining complete collection from premature infants.

The plasma catecholamines and ß-endorphin were determined by HPLC methodand by radioimmunoassay, respectively. The concentrations of plasma fentanyl andalfentanil were determined by radioimmunoassay, and ketamine, morphine, M3G,and M6G by HPLC. The sensitivities of the laboratory methods for measuring drugconcentrations and stress indicators were sufficient, and the coefficients of variationwere satisfactory.

9.2 Analgesia for persistent pain

Comparison of fentanyl and morphine pharmacodynamics (I)

Fentanyl and morphine treatments in the doses used (fentanyl 10.5 µg/kg over a 1-hour period followed by 1.5 µg/kg/h and morphine 140 µg/kg over a 1-hour periodfollowed by 20 µg/kg/h) provided equal analgesia, as judged by the pain score,hemodynamic parameters, and hormonal stress response, while examiningadrenaline and noradrenaline concentrations. However, when taking ß-endorphininto consideration, its reduction in plasma levels was significant only in the fentanylgroup, which ensued partly from a higher baseline ß-endorphin level in the fentanylgroup and small number of samples analyzed. Occurrence of the common opioid-related side effect, decreased gastrointestinal motility, differed, favoring fentanyl.

Morphine and fentanyl dosages were based on previous pharmacokinetic andpharmacodynamic results (Perlman and Volpe 1983, Roth et al. 1991, Chay et al.1992, Quinn et al. 1993) indicating an equipotent analgesic effect. In line with thesestudies, an equal analgesia was also achieved in the present study based on similarpain score values and use of boluses.

Significant changes in mean blood pressure were neither associated with thetreatments, nor with response to tracheal suctioning. The decrease in heart rate wasslight and similar in both treatment groups during the infusion, and no response totracheal suction occurred. Lack of these physiological responses to the stimulus

DISCUSSION

52

supports the view of sufficient analgesia. Vasopressors were given to most infantsprior to and during opioid infusion. Attention was also paid to correction ofpossible hypovolemia before the beginning of infusion. Weaning from the opioidinfusion occurred gradually over 0.5 to 2 days depending on the duration of theinfusion, and no withdrawal symptoms were observed. Overall, both treatmentsappeared clinically effective and safe when used over a short time period of 2 to 3days.

Endocrine response can be used for determining pain and stress in newborn infantswith certain limitations. Release of stress hormones usually increases when aninfant is experiencing pain, but these increases may also be caused by underlyingdisease, clinical condition, or handling. However, if no changes occur in stresshormone levels, it is unlikely that an infant is experiencing pain.

Limited studies are available on the range of plasma catecholamine concentrationsin preterm infants (Ekblad et al. 1992). Term babies born vaginally express agreater catecholamine surge than those born by cesarean section, withconcentrations falling rapidly by 2 hours and then declining more gradually until 24hours (Faxelius et al. 1984). Increased plasma catecholamine levels have beenobserved during surgery in newborn infants, adrenaline showing an immediate andnoradrenaline a delayed response (Anand et al. 1985, Anand et al. 1987, Anand etal. 1990). Increases of both adrenaline and noradrenaline have been reported inresponse to tracheal suctioning (Greisen et al. 1985). Evidence suggests thatadrenaline is a marker of acute pain and stress, while noradrenaline signifieschronic pain and stress (Lagercrantz and Slotkin 1986). However, no differencewas found in catecholamine response between the two treatment groups, with areduction occurring at 24 hours of infusion. On the other hand, baselinenoradrenaline concentrations were increased compared with adrenalineconcentrations, which probably reflects the immaturity of the infants studied. Therelative proportion of adrenaline to noradrenaline increases with increasing maturitydue to a maturation of the enzyme (phenylethanolamine N-methyl transferase)which converts noradrenaline to adrenaline (Quinn et al. 1992).

The usefulness of ß-endorphin as an indicator of pain has been debated. Previousstudies on newborn infants have shown a decrease in plasma ß-endorphinconcentration after opioid administration in mechanically ventilated neonates(Ionides et al. 1994), and in response to tracheal intubation (Pokela and Koivisto

DISCUSSION

53

1994). During cardiac surgery, the ß-endorphin increase occurred earlier than thecatecholamine response (Anand et al. 1990). In a study with preterm infantssuffering from respiratory difficulties, a significant increase in plasma ß-endorphinconcentration occurred from birth to the age of 2 hours, in comparison with infantsborn after spontaneous delivery (Ruth et al. 1986). During acute neonatalrespiratory difficulties requiring mechanical ventilation, the plasma ß-endorphinconcentration remained increased for the duration of the illness (Hindmarsh et al.1984). Overall, marked variability exists in plasma ß-endorphin values according toprevious findings (Ruth et al. 1986, Pokela and Koivisto 1994), with results of thepresent study supporting this. The plasma ß-endorphin concentration decreasedonly in the fentanyl group, and even though the baseline values in this groupseemed to be higher, the difference was significant. This may indicate lesserdistress in the infants treated with fentanyl.

The overall incidence of adverse effects was high. However, the exact relationshipbetween treatment and these adverse effects is unknown. Only one third of theinfants in both treatment groups had no side effects. Increased ventilatoryrequirements, which have been reported both with morphine (Quinn et al. 1992)and fentanyl (Orsini et al. 1996), were not noticed. Neither did we observewithdrawal symptoms, which have been reported with prolonged use of opioids andespecially with fentanyl doses exceeding 1.6 mg/kg or infusion lasting more than 5days (Arnold et al. 1990). The duration of opioid infusion in the present study wasabout 2-2.5 days.

Pharmacokinetic studies (II,III)

The total body clearances of both fentanyl and morphine correlated with gestationalage and birthweight in the immediate postnatal period. However, their contributionto variance (r2) was low or moderate (21 to 36%), indicating that other factors, suchas variations in volume of distribution with gestational age and disease state, play alarger role in determining fentanyl and morphine levels.

In the fentanyl subgroup, patients were somewhat heterogenous with regard todiagnoses, but postnatal age was similar and known inhibitors of the CYP3A4 werenot administered. Increased fentanyl levels were assumed to provide betteranalgesia since the pain score value correlated inversely with the fentanyl steady-

DISCUSSION

54

state concentration. Increased fentanyl levels also seemed to postpone beginning ofenteral feeding. Again, however, the contribution to variance (r2) was low ormoderate (16 to 32%). Higher steady-state concentrations were observed in verypremature infants than in those delivered after 30 gestational weeks. This seems toreflect maturation of drug metabolism after 30 gestational weeks, although largeinterindividual variations occur. The steady state was reached between 24 and 48hours, consistent with a fentanyl half-life of about 7-15 hours in this population. Ina previous study of 12 infants of postnatal age less than 4 days, a half-life of 6 hourswas reported (Koehntop et al. 1986).

Elimination of fentanyl depends both on liver blood flow and on activity ofCYP3A4 enzyme (Scholz et al. 1996). Conditions which cause increasedabdominal pressure, such as necrotizing enterocolitis, have been associated withdecreased fentanyl clearance (Koehntop et al. 1986). In the present study, four ofthe five infants with necrotizing enterocolitis had decreased fentanyl clearance.Immaturity of the hepatic enzyme system and the balance between fetalpredominant CYP3A7 and the major adult form CYP3A4 (Hakkola et al. 1998)may explain some of the age-related differences in fentanyl metabolism. A generalincrease occurs in the drug metabolism during the first weeks of life probablybecause of redistribution of hepatic blood flow and exposure of the neonatal liver toinducing agents (Morselli 1976).

Overall, the clearance of fentanyl was lowest in the most premature infants,indicating that elimination is slowed and half-life prolonged with immaturity. Theseresults are in line with the few previous studies performed in the immediatepostnatal period. Mean clearances have been reported at 7.4 ml/min/kg in 5 infantsduring the first postnatal week and 23.6 ml/min/kg in 5 infants during the secondpostnatal week, when fentanyl was given prior to surgery (Gauntlett et al. 1988).The most remarkable increase in fentanyl clearance occurred by two weeks of age(Gauntlett et al. 1988), with these authors concluding that approximately 30% ofthe variability could be explained by postnatal age (r2=0.31). Two infants who hadundergone trough abdominal surgery did not seem to eliminate the drug at all. Anexperimental study on newborn lambs showed a similar increase in clearanceduring the first two weeks of age, with more than half of the variability beingexplained by age (r2=0.56) (Gauntlett et al. 1988). A mean clearance of 17.9ml/min/kg and an elimination half-life of 5.3 hours were observed in 14 infantsundergoing major surgery during the early postnatal period (Koehntop et al. 1986).

DISCUSSION

55

A wide range of fentanyl clearance (2.0 to 85.0 ml/min/kg) was observed in infantsreceiving fentanyl for sedation and analgesia during intensive care because ofsevere respiratory distress syndrome, cardiac disorders, or cerebral disorders (Rothet al. 1991). This variability might be due to differences in postnatal age, underlyingdisease, or concomitant treatment with phenobarbital, an inducer of CYP3A4enzyme. However, in our study, fentanyl clearance in five infants treatedconcomitantly with phenobarbital, ranging from 8.1 to 13.9 ml/min/kg, was similarto that in the rest of the study group.

In the morphine subgroup, both metabolites M3G and M6G were detectable fairlyearly after the beginning of morphine infusion. This indicates that even prematureinfants are capable of metabolizing morphine. In fact, the ratio of both metabolitesto morphine increased with increasing gestational age, indicating enhancedglucuronidation with maturity. Serum morphine concentration seemed to reachsteady state between 24 and 48 hours, and even earlier in a few infants. Thus, theestimated half-life of morphine is about 7-15 hours. In a recent study performedduring a postnatal age period of less than 3 days, the elimination half-life ofmorphine was estimated at 5-21 hours in 26 preterm infants (Scott et al. 1999).M3G and M6G metabolites did not reach steady state during the 60-hour studyperiod, so their clearances could not be calculated. Neither concentration ofmorphine nor of either metabolite was correlated with analgesic effect, but highsteady-state morphine concentration was associated with decreased gastrointestinalmotility.

Data on morphine pharmacokinetics and pharmacodynamics in preterm infants inthe immediate postnatal period are limited (Kart et al. 1997). In the present study,the clearance of morphine ranged from 0.8 to 6.5 ml/min/kg, which is in line withprevious studies on subjects of the same age range. The mean clearance of 2.3ml/min/kg was reported for the most immature infants delivered before thegestational age of 28 weeks (Scott et al. 1999). For infants delivered at thegestational age of 28 to 31 weeks, the mean clearance varied between 3.2 and 3.5ml/min/kg (Barret et al. 1991, Scott et al. 1999), whereas for infants delivered atgestational age of 32 to 35 weeks clearance was 1.7 ml/min/kg (Chay et al. 1992).In term infants, clearance has varied between 2 and 6 ml/kg/min (Lynn and Slattery1987, Chay et al. 1992, McRorie et al. 1992), which is in agreement with results forthe few term infants included in our study. However, despite the similar clearances

DISCUSSION

56

in previous studies, only the recent study by Scott et al. (1999) has demonstratedthe same relationship between gestational age and clearance as the present study.

The concentration of morphine providing analgesia varies widely in infants.Consistent with findings of Scott et al. (1999) no correlation between analgesiceffect and morphine levels was found. However, with our treatment regimen, meanconcentrations of 105 ng/ml at 2 hours and 125 ng/ml from 12 hours onward werereached, levels which have been considered sufficient for sedation in newborninfants (Chay et al. 1992). In Chay’s study, morphine concentrations above 300ng/ml were associated with adverse effects including bradycardia, carbon dioxideretention, and urinary retention. In the present study an even lower concentration of187 ng/ml was associated with decreased gastrointestinal motility. No associationbetween morphine or metabolite concentrations and other adverse effects could beshown. However, overall incidence of adverse effects was high, at 68% in all(n=80) morphine-treated infants.

Morphine metabolism is diminished during the neonatal period (Pacifici et al. 1982,Choonara et al. 1989). M3G, an antagonist of morphine analgesia (Smith et al.1990), is the predominant metabolite in the neonatal period, whereas M6G, a potentanalgesic and respiratory depressant (Osborne et al. 1992), is detected at muchlower concentrations than in full-term neonates and children (Choonara et al. 1992).In this study, the M3G concentration exceeded that of morphine at 24 hours, whilethe M6G concentration reached about one third of morphine concentration at 48hours. Decreased formation of M6G may explain the higher levels of morphineneeded for sufficient analgesia, even though no correlation between analgesic effectand M6G or the sum of morphine and M6G was found. The increasedconcentrations of M3G may in part explain the decreased sensitivity to therespiratory depressant effect of morphine (Hartley et al. 1994).

9.3 Procedural pain management (IV,V)

When performing tracheal suction without analgesia, an increase in both bloodpressure and heart rate was seen, which concurs with previous studies (Perlman andVolpe 1983, Skov et al. 1992). A increase in blood pressure and heart rate alsooccurred in response to tracheal suction after 10 µg/kg alfentanil and all doses ofketamine. Only after 20 µg/kg alfentanil was a significant attenuation in heart rate

DISCUSSION

57

found. These hemodynamic changes are probably related to vagal reflex and paincaused by tracheal suction. However, an effect of ketamine on release ofcatecholamines from endogenous sources (Takki et al. 1972) cannot be excluded.

The changes in plasma catecholamine concentrations were somewhat confounding(IV, Table 2; V, Table 4), although increased catecholamine levels have previoslybeen reported during suctioning (Greisen et al. 1985). Several samples in thealfentanil trial had concentrations below the detection limit, possibly relating to theimmaturity of the infants studied. Overall, catecholamine concentrations showedgreat variability, and only noradrenaline tended to decrease with increasingalfentanil dose.

The behavioral pain score increased in response to tracheal suction after placebo. Asimilar increase was also seen with 10 µg/kg alfentanil and all doses of ketamine.The dose of 20 µg/kg alfentanil abolished the pain score change and 1 mg/kgketamine attenuated it.

Alfentanil and ketamine concentrations increased with the increasing dose. After 20µg/kg alfentanil, the concentration (40-50 ng/ml) was comparable with theanalgesic concentration recommended for sedation in older age groups (35-50ng/ml) (Marlow et al. 1990). With the ketamine doses of 1 and 2 mg/kg theconcentrations (189 ng/ml and 379 ng/ml, respectively) corresponded with thosereported for sufficient analgesia (Grant et al. 1981). Detectable, but minorconcentrations were measured in some infants six hours after the previous alfentanildose and twelve hours after the previous ketamine dose.

As an adverse effect, severe muscle rigidity occurred in more than half of theinfants treated with the 20 µg/kg dose of alfentanil. Since rigidity was previouslyreported with 9 µg/kg alfentanil (Pokela et al. 1992), attention was paid to slowadministration of the solution. Because psychic emergence phenomenon is a knownadverse effect of ketamine in children and adults (White et al. 1982), the poor dose-effect relationship was speculated to be related to discomfort caused by the highestketamine dose. Thus, in infants in whom repeated tracheal suctions are judged to beclearly painful, a continuous infusion might be more appropriate than apretreatment.

DISCUSSION

58

10 CONCLUSIONS

The main conclusions of this study aimed at designing pain relief applicable inclinical practice to persistent pain and procedural pain are:

1. When comparing continuous infusions of fentanyl (10.5 µg/kg over a 1-hourperiod followed by 1.5 µg/kg/h) and morphine (140 µg/kg over a 1-hour periodfollowed by 20 µg/kg/h), fentanyl-treated infants had a lower incidence ofdecreased gastrointestinal motility. Thus, fentanyl may offer a superiortreatment, especially for premature infants with a high risk for gastrointestinaldysmotility.

2. Substantial interindividual variability was present in fentanyl and morphineclearances in newborn infants. However, the clearances correlated with bothgestational age and birthweight. The analgesic effect correlated with fentanylbut not morphine steady-state concentrations. Enteral feeding was postponedwith high fentanyl concentrations, and decreased gastrointestinal motility wasassociated with high morphine concentrations.

3. Dosages of fentanyl and morphine should not only be based on carefulmonitoring of individual responses, but also on the degree of newbornimmaturity. A loading dose followed by continuous infusion appeared clinicallysafe and effective.

4. Mechanically ventilated newborn infants experience endotracheal suction as anunpleasant and painful procedure. Alfentanil at a dose of 20 µg/kg relieved pain,but caused rigidity as an adverse effect, thus being an inappropriate optionwithout concomitant muscle relaxant. With ketamine, hemodynamic stabilitywas maintained. However, the doses of 0.5, 1, and 2 mg/kg had only small ormoderate analgesic effect, and thus, ketamine alone appears to be a poorpretreatment for endotracheal suction-induced pain.

ACKNOWLEDGMENTS

59

11 ACKNOWLEDGMENTS

This study was carried out at the Hospital for Children and Adolescents, and theDepartment of Clinical Pharmacology, University of Helsinki, Finland, during theyears 1993 to 1997. I wish to express my sincere gratitude to former EmeritusProfessor Jaakko Perheentupa, Professor Martti Siimes, and Professor Mikael Knip,Heads of the Hospital for Children and Adolescents during these years, andProfessor Pertti Neuvonen, Head of the Department of Clinical Pharmacology, forproviding excellent facilities and a stimulating atmosphere for research.

I express my deepest gratitude to both of my supervisors:Docent Vineta Fellman, who was my supervisor throughout the study. Herenormous clinical experience, scientific enthusiasm, and continuous support andencouragement have been invaluable.Professor Pertti Neuvonen, who become involved at the middle of the study periodand introduced me to the world of clinical pharmacology. I highly appreciate hispositive and diplomatic guidance as well as his vast knowledge in the field ofclinical pharmacology.

I am particularly indebted to Professor Kari Raivio, the Rector of the University ofHelsinki, for giving me the initial idea of this fascinating study subject. I thank himfor guidance at the beginning of the study, valuable help in revising the manuscriptsof my first publications, and unfailing encouragement throughout.

Professor Mikko Hallman and Docent Kalle Hoppu are acknowledged for theirconstructive criticism and advice during the preparation of the final manuscript ofthis thesis. The linguistic content was revised by Carol Ann Pelli, to whom I amgrateful.

The collaboration of Docent Pirkko Huttunen, responsible for catecholaminemeasurements, Professor Juhani Leppäluoto, for ß-endorphin measurements, andProfessor Per Rosenberg, for morphine measurements, has been invaluable for thisstudy and is deeply appreciated. I am also grateful to Docent Olli Meretoja, foranesthesiological advice and fruitful collaboration, Docent Kimmo Sainio, for

ACKNOWLEDGMENTS

60

stimulating discussions and cooperation, and Martti Virtanen, for statisticalguidance.

I warmly thank my colleagues and all the personnel on ward 7 for their cooperationand for providing a friendly working environment. Special thanks are due to AnneliArasola, Anne Salonen, Kati Nikula, and Eija Reen for their kind assistance. HelenaVertanen and Kaarina Mantila are also acknowledged for overseeing technicalprocedures and for secretarial help, respectively.

My sincere thanks to the newborn infants and their families who participated in thisstudy, and thus, made it possible.

Special thanks are due to my colleagues at the Lastenlinnantie Office and at theDepartment of Clinical Pharmacology. The pleasant atmosphere, stimulatingdiscussions, and generous advice have been of great value to me while strugglingwith this study. I warmly thank Tuija Itkonen for her help in many practicalmatters. I also want to thank other colleagues and friends of mine at the Hospital forChildren and Adolescents.

I wish to express my heartfelt thanks to my father Kauko Tiitinen, my sisterMarianna Tiitinen and my parents-in-law Aira and Arvo Saarenmaa. I also want toexpress my gratitude to my mother, who died in July 1996.

My friends are thanked for support, sympathy, and enriching me with other lifeperspectives. I especially want to thank Maarit Ritakallio for the thousands ofkilometers that we’ve run together; it has been a perfect way to gather energy to thedaily challenges.

My deepest and most loving thanks are due to my husband Ari, for his unselfishand tolerant support and love, and to our dear children Sara, Miro, and Ilari, for allthe joy they have brought us. They remind me of what is really important in life.

This study was supported financially by the Foundation for Pediatric Research, theClinical Drug Research Graduate School, the Päivikki and Sakari SohlbergFoundation, Helsinki University Central Hospital Research Fund, the OrionCorporation Research Foundation, the Maritza and Reino Salonen Foundation, andthe Finnish Medical Society Duodecim.

REFERENCES

61

12 REFERENCES

Abad F, Diaz NM, Domenech E, Robayna M, Rico J. Oral sweet solution reduces pain-relatedbehaviour in preterm infants. Acta Paediatr 85:854-8, 1996.

Adams HA, Weber B, Bachmann-M B, Guerin M, Hempelmann G. The simultaneousdetermination of ketamine and midazolam using high pressure liquid chromatography and UVdetection. Anaesthesist 41:619-24, 1992.

Als H, Lawhon G, Brown E, Gibes R, Duffy FH, McAnulty G, Blickman JG. Individualizedbehavioral and environmental care for the very low birth weight preterm infant at high risk forbronchopulmonary dysplasia: neonatal intensive care unit and developmental outcome.Pediatrics 78:1123-32, 1986.

Ambuel B, Hamlett KW, Marx CM, Blumer JL. Assessing distress in pediatric intensive careenvironments: the COMFORT scale. J Pediatr Psychol 17:95-109, 1992.

Anand KJ, Brown MJ, Bloom SR, Aynsley-Green A. Studies on the hormonal regulation of fuelmetabolism in the human newborn infant undergoing anaesthesia and surgery. Horm Res22:115-28, 1985.

Anand KJ, Hickey PR. Pain and its effects in the human neonate and fetus. N Engl J Med317:1321-9, 1987.

Anand KJ, Sippell WG, Aynsley-Green A. Randomised trial of fentanyl anaesthesia in pretermbabies undergoing surgery: effects on the stress response. Lancet 1:62-6, 1987.

Anand KJ, Sippell WG, Schofield NM, Aynsley-Green A. Does halothane anaesthesia decreasethe metabolic and endocrine stress responses of newborn infants undergoing operation? BMJ296:668-72, 1988.

Anand KJ, Carr DB. The neuroanatomy, neurophysiology, and neurochemistry of pain, stress,and analgesia in newborns and children. Pediatr Clin North Am 36:795-822, 1989.

Anand KJ, Hansen DD, Hickey PR. Hormonal-metabolic stress responses in neonatesundergoing cardiac surgery. Anesthesiology 73:661-70, 1990.

Anand KJ, Hickey PR. Halothane-morphine compared with high-dose sufentanil for anesthesiaand postoperative analgesia in neonatal cardiac surgery. N Engl J Med 326:1-9, 1992.

Anand KJ, Craig KD. New perspectives on the definition of pain. Pain 67:209-11, 1996.

Anand KJ, McIntosh N, Lagercrantz H, Pelausa E, Young TE, Vasa R. Analgesia and sedationin preterm neonates who require ventilatory support: results from the NOPAIN trial. ArchPediatr Adolesc Med 153:331-8, 1999.

Andrews K, Fitzgerald M. The cutaneous withdrawal reflex in human neonates: sensitization,receptive fields, and the effects of contralateral stimulation. Pain 56:95-101, 1994.

REFERENCES

62

Andrews K, Fitzgerald M. Cutaneous flexion reflex in human neonates: a quantitative study ofthreshold and stimulus-response characteristics after single and repeated stimuli. Dev Med ChildNeurol 41:696-703, 1999.

Aperia A, Broberger O, Elinder G, Herin P, Zetterstrom R. Postnatal development of renalfunction in pre-term and full-term infants. Acta Paediatr Scand 70:183-7, 1981.

Arant BS Jr. Developmental patterns of renal functional maturation compared in the humanneonate. J Pediatr 92:705-12, 1978.

Arant BS Jr. Estimating glomerular filtration rate in infants. J Pediatr 104:890-3, 1984.

Arnold JH, Truog RD, Orav EJ, Scavone JM, Hershenson MB. Tolerance and dependence inneonates sedated with fentanyl during extracorporeal membrane oxygenation. Anesthesiology73:1136-40, 1990.

Ballantyne M, Stevens B, McAllister M, Dionne K, Jack A. Validation of the premature infantpain profile in the clinical setting. Clin J Pain 15:297-303, 1999.

Barker DP, Rutter N. Exposure to invasive procedures in neonatal intensive care unitadmissions. Arch Dis Child 72:F47-8, 1995.

Barrett DA, Elias-Jones AC, Rutter N, Shaw PN, Davis SS. Morphine kinetics afterdiamorphine infusion in premature neonates. Br J Clin Pharmacol 32:31-7, 1991

Barrett DA, Barker DP, Rutter N, Pawula M, Shaw PN. Morphine, morphine-6-glucuronide andmorphine-3-glucuronide pharmacokinetics in newborn infants receiving diamorphine infusions.Br J Clin Pharmacol 41:531-7, 1996.

Bell MJ, Ternberg JL, Feigin RD, Keating JP, Marshall R, Barton L, Brotherton T. Neonatalnecrotizing enterocolitis. Ann Surg 187:1-7, 1978.

Besunder JB, Reed MD, Blumer JL. Principles of drug biodisposition in the neonate. A criticalevaluation of the pharmacokinetic-pharmacodynamic interface (Part I). Clin Pharmacokinet14:189-216, 1988a.

Besunder JB, Reed MD, Blumer JL. Principles of drug biodisposition in the neonate. A criticalevaluation of the pharmacokinetic-pharmacodynamic interface (Part II). Clin Pharmacokinet14:261-86, 1988b.

Betremieux P, Carre P, Pladys P, Roze O, Lefrancois C, Malledant Y. Doppler ultrasoundassessment of the effects of ketamine on neonatal cerebral circulation. Dev Pharmacol Ther20:9-13, 1993.

Bhat R, Chari G, Gulati A, Aldana O, Velamati R, Bhargava H. Pharmacokinetics of a singledose of morphine in preterm infants during the first week of life. J Pediatr 117:477-81, 1990.

Bhat R, Abu-Harb M, Chari G, Gulati A. Morphine metabolism in acutely ill preterm newborninfants. J Pediatr 120:795-9, 1992.

Blass EM, Hoffmeyer LB. Sucrose as an analgesic for newborn infants. Pediatrics 87:215-8,1991.

REFERENCES

63

Blass EM. Behavioral and physiological consequences of suckling in rat and human newborns.Acta Paediatr 397:71-6, 1994.

Blauer T, Gerstmann D. A simultaneous comparison of three neonatal pain scales duringcommon NICU procedures. Clin J Pain 14:39-47, 1998.

Brans YW. Body fluid compartments in neonates weighing 1000 grams or less. Clin Perinatol13:403-17, 1986.

Bucher HU, Moser T, von Siebenthal K, Keel M, Wolf M, Duc G. Sucrose reduces painreaction to heel lancing in preterm infants: a placebo-controlled, randomized and masked study.Pediatr Res 38:332-5, 1995.

Camu F, Gepts E, Rucquoi M, Heykants J. Pharmacokinetics of alfentanil in man. Anesth Analg61:657-61, 1982.

Cazeneuve C, Pons G, Rey E, Treluyer JM, Cresteil T, Thiroux G, D'Athis P, Olive G.Biotransformation of caffeine in human liver microsomes from foetuses, neonates, infants andadults. Br J Clin Pharmacol 37:405-12, 1994.

Chay PC, Duffy BJ, Walker JS. Pharmacokinetic-pharmacodynamic relationships of morphinein neonates. Clin Pharmacol Ther 51:334-42, 1992.

Chiswick ML. Assessment of pain in neonates. Lancet 355:6-8, 2000.

Choonara IA, McKay P, Hain R, Rane A. Morphine metabolism in children. Br J ClinPharmacol 28:599-604, 1989.

Choonara I, Lawrence A, Michalkiewicz A, BowhayA, Ratcliffe J. Morphine metabolism inneonates and infants. Br J Clin Pharmacol 34:434-7, 1992.

Collins C, Koren G, Crean P, Klein J, Roy WL, MacLeod SM. Fentanyl pharmacokinetics andhemodynamic effects in preterm infants during ligation of patent ductus arteriosus. AnesthAnalg 64:1078-80, 1985.

Cotsen MR, Donaldson JS, Uejima T, Morello FP. Efficacy of ketamine hydrochloride sedationin children for interventional radiologic procedures. Am J Roentgenol 169:1019-22, 1997.

Craig KD, McMahan RJ, Morison JD, Zaskow C. Developmental changes in infant painexpression during immunzation injections. Soc Sci Med 19:1331-7, 1984.

Dahlström B, Bolme P, Feychting H, Noack G, Paalzow L. Morphine kinetics in children. ClinPharmacol Ther 26:354-65, 1979.

Davis PJ, Killian A, Stiller RL, Cook DR, Guthrie RD, Scierka AM. Pharmacokinetics ofalfentanil in newborn premature infants and older children. Dev Pharmacol Ther 13:21-7,1989a.

Davis PJ, Stiller RL, Cook DR, Brandom BW, Davis JE, Scierka AM. Effects of cholestatichepatic disease and chronic renal failure on alfentanil pharmacokinetics in children. AnesthAnalg 68:579-83, 1989b.

REFERENCES

64

de Wildt SN, Kearns GL, Leeder JS, van den Anker JN. Glucuronidation in humans.Pharmacogenetic and developmental aspects. Clin Pharmacokinet 36:439-52, 1999a.

de Wildt SN, Kearns GL, Leeder JS, van den Anker JN. Cytochrome P450 3A: ontogeny anddrug disposition. Clin Pharmacokinet 37:485-505, 1999b.

Edwards DJ, Lalka D, Cerra F, Slaughter RL. Alpha1-acid glycoprotein concentration andprotein binding in trauma. Clin Pharmacol Ther 31:62-7, 1982.

Ekblad H, Kero P, Korvenranta H, Scheinin M. Sympathoadrenal activity in preterm infantsduring the first five days of life. Biol Neonate 61:294-301, 1992.

Elphick MC, Wilkinson AW. The effects of starvation and surgical injury on the plasma levelsof glucose, free fatty acids, and neutral lipids in newborn babies suffering from variouscongenital anomalies. Pediatr Res 15:313-8, 1981.

Eriksson BM, Persson BA. Determination of catecholamines in rat heart tissue and plasmasamples by liquid chromatography with electrochemical detection. J Chromatogr 228:143-54,1982.

Esuri M, Kurlak LO, Stephenson TJ. The effect of endotracheal suction on plasma levels ofatrial natriuretic peptide (ANP) in preterm infants. Early Hum Dev 47:212, 1997.

Faura CC, Collins SL, Moore RA, McQuay HJ. Systematic review of factors affecting the ratiosof morphine and its major metabolites. Pain 74:43-53, 1998.

Faxelius G, Lagercrantz H, Yao A. Sympathoadrenal activity and peripheral blood flow afterbirth: comparison in infants delivered vaginally and by cesarean section. J Pediatr 105:144-8,1984.

Field T, Goldson E. Pacifying effects of nonnutritive sucking on term and preterm neonatesduring heelstick procedures. Pediatrics 74:1012-5, 1984.

Fiselier T, Monnens L, Moerman E, Van Munster P, Jansen M, Peer P. Influence of the stress ofvenepuncture on basal levels of plasma renin activity in infants and children. Int J PediatrNephrol 4:181-5, 1983.

Fitzgerald M, Anand KJ. Developmental neuroanatomy and neurophysiology of pain. In: Painin Infants, Children, and Adolescents. Schechter NL, Berde CB, Yaster M (eds.). Williams &Wilkins, Baltimore, 1993:11-31.

Fitzgerald M. Developmental biology of inflammatory pain. Br J Anaesth 75:177-85, 1995.

Franck LS, Lawhon G. Environmental and behavioral strategies to prevent and manage neonatalpain. In: Pain in Neonates 2nd Revised and Enlarged Edition. Anand KJS, Stevens BJ, McGrathPJ (eds.). Elsevier, Amsterdam, 2000:203-16.

Friesen RH, Henry DB. Cardiovascular changes in preterm neonates receiving isoflurane,halothane, fentanyl, and ketamine. Anesthesiology 64:238-42, 1986.

Friis-Hansen B. Body water compartments in children: changes during growth and relatedchanges in body composition. Pediatrics 28:135-59, 1961.

REFERENCES

65

Friis-Hansen B. Body composition during growth. In vivo measurements and biochemical datacorrelated to differential anatomical growth. Pediatrics 47:264, 1971.

Friis-Hansen B. Water distribution in the foetus and newborn infant. Acta Paediatr Scand 305:7-11, 1983.

Gauntlett IS, Fisher DM, Hertzka RE, Kuhls E, Spellman MJ, Rudolph C. Pharmacokinetics offentanyl in neonatal humans and lambs: effects of age. Anesthesiology 69:683-7, 1988.

Gilles FH, Shankle W, Dooling EC. Myelinated tracts: growth patterns. In: The developinghuman brain: growth and epidemiologic neuropathology. Gilles FH, Leviton A, Dooling EC(eds.). Wright & Co., Boston, 1983:177-83.

Gladman G, Chiswick ML. Skin conductance and arousal in the newborn. Arch Dis Child65:1063-6, 1990.

Goresky GV, Koren G, Sabourin MA, Sale JP, Strunin L. The pharmacokinetics of alfentanil inchildren. Anesthesiology 67:654-9, 1987.

Gow PJ, Ghabrial H, Smallwood RA, Morgan DJ, Ching MS. Neonatal hepatic drugelimination. Pharmacol Toxicol 88:3-15, 2001.

Grant IS, Nimmo WS, McNicol LR, Clements JA. Pharmacokinetics and analgesic effects if IMand oral ketamine. Br J Anaesth 53:805-10, 1981.

Greeley WJ, de Bruijn NP, Davis DP. Sufentanil pharmacokinetics in pediatric cardiovascularpatients. Anesth Analg 66:1067-72, 1987.

Greenough A, Lagercrantz H, Pool J, Dahlin I. Plasma catecholamine levels in preterm infants.Effect of birth asphyxia and Apgar score. Acta Paediatr Scand 76:54-9, 1987.

Greenough A, Nicolaides KH, Lagercrantz H. Human fetal sympathoadrenal responsiveness.Early Hum Dev 23:9-13, 1990.

Greisen G, Frederiksen PS, Hertel J, Christensen NJ. Catecholamine response to chestphysiotherapy and endotracheal suctioning in preterm infants. Acta Paediatr Scand 74:525-9,1985.

Grunau RV, Craig KD. Pain expression in neonates: facial action and cry. Pain 28:395-410,1987.

Grunau RE, Oberlander T, Holsti L, Whitfield MF. Bedside application of the Neonatal FacialCoding System in pain assessment of premature neonates. Pain 76:277-86, 1998.

Grunau RV, Johnston CC, Craig KD. Neonatal facial and cry responses to invasive and non-invasive procedures. Pain 42:295-305, 1990.

Guignard JP, Drukker A. Why do newborn infants have a high plasma creatinine? Pediatrics103:E49, 1999.

Guignard JP, John EG. Renal function in the tiny, premature infant. Clin Perinatol 13:377-401,1986.

REFERENCES

66

Haehner BD, Gorski JC, Vandenbranden M, Wrighton SA, Janardan SK, Watkins PB, Hall SD.Bimodal distribution of renal cytochrome P450 3A activity in humans. Mol Pharmacol 50:52-9,1996.

Hakkola J, Pasanen M, Purkunen R, Saarikoski S, Pelkonen O, Mäenpää J, Rane A, Raunio H.Expression of xenobiotic-metabolizing cytochrome P450 forms in human adult and fetal liver.Biochem Pharmacol 48:59-64, 1994.

Hakkola J, Tanaka E, Pelkonen O. Developmental expression of cytochrome P450 enzymes inhuman liver. Pharmacol Toxicol 82:209-17, 1998.

Haouari N, Wood C, Griffiths G, Levene M. The analgesic effect of sucrose in full term infants:a randomised controlled trial. BMJ 310:1498-500, 1995.

Harpin VA, Rutter N. Making heel pricks less painful. Arch Dis Child 58:226-8, 1983.

Hartley R, Green M, Quinn M, Levene MI. Pharmacokinetics of morphine infusion inpremature neonates. Arch Dis Child 69:55-8, 1993a.

Hartley R, Quinn M, Green M, Levene MI. Morphine glucuronidation in premature neonates. BrJ Clin Pharmacol 35:314-7, 1993b.

Hartley R, Green M, Quinn MW, Rushforth JA, Levene MI. Development of morphineglucuronidation in premature neonates. Biol Neonate 66:1-9, 1994.

Hartvig P, Larsson E, Joachimsson P-O. Postoperative analgesia and sedation followingpediatric cardiac surgery using a constant infusion of ketamine. J Cardiothorac Vasc Anesth7:148-53, 1993.

Hindmarsh KW, Sankaran K, Watson VG. Plasma beta-endorphin concentrations in neonatesassociated with acute stress. Dev Pharmacol Ther 7:198-204, 1984.

Hodgkinson K, Bear M, Thorn J, Van Blaricum S. Measuring pain in neonates: evaluating aninstrument and developing a common language. Aust J Adv Nurs 12:17-22, 1994.

Holve RL, Bromberger PJ, Groveman HD, Klauber MR, Dixon SD, Snyder JM. Regionalanesthesia during newborn circumcision. Effect on infant pain response. Clin Pediatr 22:813-8,1983.

Horgan M, Choonara I. Measuring pain in neonates: an objective score. Paediatr Nurs 8:24-7,1996

Inoue K, Inazawa J, Nakagawa H, Shimada T, Yamazaki H, Guengerich FP, Abe T. Assignmentof the human cytochrome P-450 nifedipine oxidase gene (CYP3A4) to chromosome 7 at bandq22.1 by fluorescence in situ hybridization. Jpn J Hum Genet 37:133-8, 1992.

Ionides SP, Weiss MG, Angelopoulos M, Myers TF, Handa RJ. Plasma beta-endorphinconcentrations and analgesia-muscle relaxation in the newborn infant supported by mechanicalventilation. J Pediatr 125:113-6, 1994.

Jacobson S, Koren G. Drug administration in the newborn infant. In: Pediatric PharmacologyTherapeutic Principles in Practice. Yaffe SJ, Aranda JV (eds.). W.B. Saunders Company,Philadelphia, 1992:178-81.

REFERENCES

67

Jacqz-Aigrain E, Daoud P, Burtin P, Desplanques L, Beaufils F. Placebo-controlled trial ofmidazolam sedation in mechanically ventilated newborn babies. Lancet 344:646-50, 1994.

Jacqz-Aigrain E, Burtin P. Clinical pharmacokinetics of sedatives in neonates. ClinPharmacokinet 31:423-43, 1996.

Johnston CC, Strada ME. Acute pain response in infants: a multidimensional description. Pain24:373-82, 1986.

Johnston CC, Stevens B, Craig KD, Grunau RV. Developmental changes in pain expression inpremature, full-term, two- and four-month-old infants. Pain 52:201-8, 1993.

Johnston CC, Stevens BJ. Experience in a neonatal intensive care unit affects pain response.Pediatrics 98:925-30, 1996.

Johnston CC, Stremler RL, Stevens BJ, Horton LJ. Effectiveness of oral sucrose and simulatedrocking on pain response in preterm neonates. Pain 72:193-9, 1997.

Kart T, Christrup LL, Rasmussen M. Recommended use of morphine in neonates, infants andchildren based on a literature review: Part 1-Pharmacokinetics. Pediatr Anaesth 7:5-11, 1997.

Kennedy KA, Tyson JE. Narcotic analgesia for ventilated newborns: are placebo-controlledtrials ethical and necessary? J Pediatr 134:127-9, 1999.

Killian A, Davis PJ, Stiller RL, Cicco R, Cook DR, Guthrie RD. Influence of gestational age onpharmacokinetics of alfentanil in neonates. Dev Pharmacol Ther 15:82-5, 1990.

Kitada M, Kamataki T, Itahashi K, Rikihisa T, Kato R, Kanakubo Y. Purification and propertiesof cytochrome P-450 from homogenates of human fetal livers. Arch Biochem Biophys 241:275-80, 1985.

Koehntop DE, Rodman JH, Brundage DM, Hegland MG, Buckley JJ. Pharmacokinetics offentanyl in neonates. Anesth Analg 65:227-32, 1986.

Kolars JC, Lown KS, Schmiedlin-Ren P, Ghosh M, Fang C, Wrighton SA, Merion RM,Watkins PB. CYP3A gene expression in human gut epithelium. Pharmacogenetics 4:247-59,1994.

Koren G, Goresky G, Crean P, Klein J, MacLeod SM. Pediatric fentanyl dosing based onpharmacokinetics during cardiac surgery. Anesth Analg 63:577-82, 1984.

Koren G, Butt W, Chinyanga H, Soldin S, Tan YK, Pape K. Postoperative morphine infusion innewborn infants: assessment of disposition characteristics and safety. J Pediatr 107:963-7, 1985.

Krechel SW, Bildner J. CRIES: a new neonatal postoperative pain measurement score. Initialtesting of validity and reliability. Paediatr Anaesth 5:53-61, 1995.

Lacroix D, Sonnier M, Moncion A, Cheron G, Cresteil T. Expression of CYP3A in the humanliver -evidence that the shift between CYP3A7 and CYP3A4 occurs immediately after birth. EurJ Biochem 247:625-34, 1997.

Lagercrantz H, Nilsson E, Redham I, Hjemdahl P. Plasma catecholamines following nursingprocedures in a neonatal ward. Early Hum Dev 14:61-5, 1986.

REFERENCES

68

Lagercrantz H, Slotkin TA. The "stress" of being born. Sci Am 254:100-7, 1986.

Lane JC, Tennison MB, Lawless ST, Greenwood RS, Zaritsky AL. Movement disorder afterwithdrawal of fentanyl infusion. J Pediatr 119:649-51, 1991.

Lawrence J, Alcock D, McGrath P, Kay J, MacMurray SB, Dulberg C. The development of atool to assess neonatal pain. Neonatal Network 12:59-66, 1993.

Leeder JS, Kearns GL. Pharmacogenetics in pediatrics. Implications for practice. Pediatr ClinNorth Am 44:55-77, 1997.

Leff RD, Fischer LJ, Roberts RJ. Phenytoin metabolism in infants following intravenous andoral administration. Dev Pharmacol Ther 9:217-23, 1986.

Levine JD, Gordon NC. Pain in prelingual children and its evaluation by pain-inducedvocalization. Pain 14:85-93, 1982.

Lindahl S. Calming minds or killing pain in newborn infants? Acta Paediatr 86:787-8, 1997.

Lindemann R. Respiratory muscle rigidity in a preterm infant after use of fentanyl duringCaesarean section. Eur J Pediatr 157:1012-3, 1998.

Lindgren L, Rautiainen P, Klemola UM, Saarnivaara L. Haemodynamic responses andprolongation of QT interval of ECG after suxamethonium-facilitated intubation duringanaesthetic induction in children: a dose-related attenuation by alfentanil. Acta AnaesthesiolScand 35:355-8, 1991.

Lloyd-Thomas AR, Fitzgerald M. Do fetuses feel pain? Reflex responses do not necessarilysignify pain. BMJ 313:797-8, 1996.

Lowrie L, Weiss AH, Lacombe C. The pediatric sedation unit: a mechanism for pediatricsedation. Pediatrics 102:E30, 1998.

Lynn AM, Opheim KE, Tyler DC. Morphine infusion after pediatric cardiac surgery. Crit CareMed 12:863-6, 1984.

Lynn AM, Slattery JT. Morphine pharmacokinetics in early infancy. Anesthesiology 66:136-9,1987.

Lynn AM, Nespeca MK, Opheim KE, Slattery JT. Respiratory effects of intravenous morphineinfusions in neonates, infants, and children after cardiac surgery. Anesth Analg 77:695-701,1993.

Lynn A, Nespeca MK, Bratton SL, Strauss SG, Shen DD. Clearance of morphine inpostoperative infants during intravenous infusion: the influence of age and surgery. AnesthAnalg 86:958-63, 1998.

Marlow N, Weindling AM, van Peer A, Heykants J. Alfentanil pharmacokinetics in preterminfants. Arch Dis Child 65:349-51, 1990.

Masciello AL. Anesthesia for neonatal circumcision: local anesthesia is better than dorsal penilenerve block. Obstet Gynecol 75:834-8, 1990.

REFERENCES

69

McGrath PJ, Johnson G, Goodman JT, Schillinger J, Dunn J, Chapman J. CHEOPS: Abehavioral scale for rating postoperative pain in children. In: Fields HL, Dubner R, Cervero F(eds.). Advances in pain therapy. Vol. 9. New York: Raven Press, 1985:395-402.

McGrath PA. An assessment of children's pain: a review of behavioral, physiological and directscaling techniques. Pain 31:147-76, 1987.

McIntosh N, Van Veen L, Brameyer H. The pain of heel prick and its measurement in preterminfants. Pain 52:71-4, 1993.

McRorie TI, Lynn AM, Nespeca MK, Opheim KE, Slattery JT. The maturation of morphineclearance and metabolism. Am J Dis Child 146:972-6, 1992.

Meistelman C, Saint-Maurice C, Lepaul M, Levron J-C, Loose J-P, MacGee K. A comparisonof alfentanil pharmacokinetics in children and adults. Anesthesiology 66:13-6, 1987.

Merskey H, Albe-Fessard DG, Bonica JJ, Carmon A, Dubner R, Kerr FWL, Lindblom U,Mumford JM, Nathan PW, Noordenbos W, Pagni CA, Ranaer MJ, Sternbach RA, Sunderland S.Pain terms: a list with definitions and notes on usage. Recommended by the IASPSubcommittee on Taxonomy. Pain 6:249, 1979.

Merskey H. Classification of chronic pain. Pain 3:126, 1986.

Michelsson K, Sirviö P, Wasz-Höckert O. Pain cry in full-term asphyxiated newborn infantscorrelated with late findings. Acta Paediatr Scand 66:611-6, 1977.

Michelsson K, Järvenpää AL, Rinne A. Sound spectrographic analysis of pain cry in preterminfants. Early Hum Dev 8:141-9, 1983.

Mikkelsen S, Feilberg VL, Christensen CB, Lundström KE. Morphine pharmacokinetics inpremature and mature newborn infants. Acta Paediatr 83:1025-8, 1994.

Miller HD, Anderson GC. Nonnutritive sucking: effects on crying and heart rate in intubatedinfants requiring assisted mechanical ventilation. Nurs Res 42:305-7, 1993.

Morselli PL. Clinical pharmacokinetics in neonates. Clin Pharmacokin 2:81-98, 1976.

Morselli PL. Clinical pharmacology of the perinatal period and early infancy. ClinPharmacokinet 17:13-28, 1989.

Nelson DR, Koymans L, Kamataki T, Stegeman JJ, Feyereisen R, Waxman DJ, Waterman MR,Gotoh O, Coon MJ, Estabrook RW, Gunsalus IC, Nebert DW. P450 superfamily: update on newsequences, gene mapping, accession numbers and nomenclature. Pharmacogenetics 6:1-42,1996.

Nichols DG, Yaster M, Lynn AM, Helfaer MA, Deshpande JK, Manson PN, Carson BS,Bezman M, Maxwell LG, Tobias JD. Disposition and respiratory effects of intrathecal morphinein children. Anesthesiology 79:733-8, 1993.

Norton SJ. Aftereffects of morphine and fentanyl analgesia: a retrospective study. NeonatalNetw 7:25-8, 1988.

REFERENCES

70

Obara H, Sugiyama D, Maekawa N, Hamatani S, Tanaka O, Chuma R, Kitamura S, Iwai S.Plasma cortisol levels in paediatric anaesthesia. Can Anaesth Soc J 31:24-7, 1984.

Olkkola KT, Maunuksela E-L, Korpela R, Rosenberg PH. Kinetics and dynamics ofpostoperative intravenous morphine in children. Clin Pharmacol Ther 44:128-36, 1988.

Orsini AJ, Leef KH, Costarino A, Dettorre MD, Stefano JL. Routine use of fentanyl infusionsfor pain and stress reduction in infants with respiratory distress syndrome. J Pediatr 129:140-5,1996.

Osborne R, Thompson P, Joel S, Trew D, Patel N, Slevin M. The analgesic activity ofmorphine-6-glucuronide. Br J Clin Pharmacol 34:130-8, 1992.

Owen JA, Sitar DS, Berger L, Brownell L, Duke PC, Mitenko PA. Age-related morphinekinetics. Clin Pharmacol Ther 34:364-8, 1983.

Owens ME, Todt EH. Pain in infancy: neonatal reaction to a heel lance. Pain 20:77-86, 1984.

Pacifici GM, Säwe J, Kager L, Rane A. Morphine glucuronidation in human fetal and adultliver. Eur J Clin Pharmacol 22:553-8, 1982.

Pasternak GW. Multiple morphine and enkephalin receptors and the relief of pain. JAMA259:1362-7, 1988.

Perlman JM, Volpe JJ. Suctioning in the preterm infant: effects on cerebral blood flow velocity,intracranial pressure, and arterial blood pressure. Pediatrics 72:329-34, 1983.

Pokela ML, Ryhänen PT, Koivisto ME, Olkkola KT, Saukkonen AL. Alfentanil-inducedrigidity in newborn infants. Anesth Analg 75:252-7, 1992.

Pokela ML. Effect of opioid-induced analgesia on beta-endorphin, cortisol and glucoseresponses in neonates with cardiorespiratory problems. Biol Neonate 64:360-7, 1993.

Pokela ML, Olkkola KT, Seppälä T, Koivisto M. Age-related morphine kinetics in infants. DevPharmacol Ther 20:26-34, 1993.

Pokela ML. Pain relief can reduce hypoxemia in distressed neonates during routine treatmentprocedures. Pediatrics 93:379-83, 1994.

Pokela ML, Koivisto M. Physiological changes, plasma beta-endorphin and cortisol responsesto tracheal intubation in neonates. Acta Paediatr 83:151-6, 1994.

Quinn MW, Otoo F, Rushforth JA, Dean HG, Puntis JWL, Wild J, Levene MI. Effect ofmorphine and pancuronium on the stress response in ventilated preterm infants. Early Hum Dev30:241-8, 1992.

Quinn MW, Wild J, Dean HG, Hartley R, Rushforth JA, Puntis JWL, Levene MI. Randomiseddouble-blind controlled trial of effect of morphine on catecholamine concentrations in ventilatedpre-term babies. Lancet 342:324-7, 1993.

Ramenghi LA, Griffith GC, Wood CM, Levene MI. Effect of non-sucrose sweet tasting solutionon neonatal heel prick responses. Arch Dis Child 74:F129-31, 1996.

REFERENCES

71

Rautiainen P. Alfentanil infusion for sedation in infants and small children during cardiaccatheterization. Can J Anaesth 38:980-4, 1991.

Reich DL, Silvay G. Ketamine: an update on the first twenty-five years of clinical experience.Can J Anaesth 36:186-97, 1989.

Robillard JE, Segar JL, Smith FG, Jose PA. Regulation of sodium metabolism and extracellularfluid volume during development. Clin Perinatol 19:15-31, 1992.

Rosow CE, Moss J, Philbin DM, Savarese JJ. Histamine release during morphine and fentanylanesthesia. Anesthesiology 56:93-6, 1982.

Roth B, Schlunder C, Houben F, Gunther M, Theisohn M. Analgesia and sedation in neonatalintensive care using fentanyl by continuous infusion. Dev Pharmacol Ther 17:121-7, 1991.

Roure P, Jean N, Leclerc AC, Cabanel N, Levron JC, Duvaldestin P. Pharmacokinetics ofalfentanil in children undergoing surgery. Br J Anaesth 59:1437-40, 1987.

Rowland M, Tozer TN. Clinical Pharmacokinetics. Concepts and applications. 3. edit. Williams& Wilkins, USA, 1995:18-33.

Rushforth JA, Levene MI. Behavioural response to pain in healthy neonates. Arch Dis Child70:F174-6, 1994.

Ruth V, Pohjavuori M, Rovamo L, Salminen K, Laatikainen T. Plasma beta-endorphin inperinatal asphyxia and respiratory difficulties in newborn infants. Pediatr Res 20:577-80, 1986.

Scholz J, Steinfath M, Schulz M. Clinical pharmacokinetics of alfentanil, fentanyl andsufentanil. An update. Clin Pharmacokinet 31:275-92, 1996.

Schuttler J, Stoeckel H. Clinical pharmacokinetics of alfentanyl. Anaesthesist 31:10-4, 1982.

Schwartz GJ, Feld LG, Langford DJ. A simple estimate of glomerular filtration rate in full-terminfants during the first year of life. J Pediatr 104:849-54, 1984.

Scott CS, Riggs KW, Ling EW, Fitzgerald CE, Hill ML, Grunau RVE, Solimano A, Craig KD.Morphine pharmacokinetics and pain assessment in premature newborns. J Pediatr 135:423-9,1999.

Shelly MP, Cory EP, Park GR. Pharmacokinetics of morphine in two children before and afterliver transplantation. Br J Anaesth 58:1218-23, 1986.

Singleton MA, Rosen JI, Fisher DM. Plasma concentrations of fentanyl in infants, children andadults. Can J Anaesth 34:152-5, 1987.

Skogsdal Y, Eriksson M, Schollin J. Analgesia in newborns given oral glucose. Acta Paediatr86:217-20, 1997.

Skov L, Ryding J, Pryds O, Greisen G. Changes in cerebral oxygenation and cerebral bloodvolume during endotracheal suctioning in ventilated neonates. Acta Paediatr 81:389-93, 1992.

Smith MT, Watt JA, Cramond T. Morphine-3-glucuronide – a potent antagonist of morphineanalgesia. Life Sciences 47:579-85, 1990.

REFERENCES

72

Sparshott M. The development of a clinical distress scale for ventilated newborn infants:identification of pain and distress based on validated behavioural scores. J Neonatal Nursing2:5-11, 1996.

Stanski DR, Greenblatt DJ, Lowenstein E. Kinetics of intravenous and intramuscular morphine.Clin Pharmacol Ther 24:52-9, 1978.

Stevens B, Johnston C, Petryshen P, Taddio A. Premature Infant Pain Profile: development andinitial validation. Clin J Pain 12:13-22, 1996.

Stevens B, Taddio A, Ohlsson A, Einarson T. The efficacy of sucrose for relieving proceduralpain in neonates - a systematic review and meta-analysis. Acta Paediatr 86:837-42, 1997.

Stevens B, Johnston C, Franck L, Petryshen P, Jack A, Foster G. The efficacy ofdevelopmentally sensitive interventions and sucrose for relieving procedural pain in very lowbirth weight neonates. Nurs Res 48:35-43, 1999.

Strauss J, Daniel SS, James LS. Postnatal adjustment in renal function. Pediatrics 68:802-8,1981.

Suresh S, Anand KJ. Opioid tolerance in neonates: mechanisms, diagnosis, assessment, andmanagement. Semin Perinatol 22:425-33, 1998.

Svensson JO, Rane A, Säwe J, Sjöqvist F. Determination of morphine, morphine-3-glucuronideand (tentatively) morphine-6-glucuronide in plasma and urine using ion-pair high-performanceliquid chromatography. J Chromatogr 230:427-32, 1982.

Taddio A, Katz J, Ilersich AL, Koren G. Effect of neonatal circumcision on pain responseduring subsequent routine vaccination. Lancet 349:599-603, 1997.

Takki S, Nikki P, Jäättelä A, Tammisto T. Ketamine and plasma catecholamines. Br J Anaesth44;1318-21, 1972.

Talbert LM, Kraybill EN, Potter HD. Adrenal cortical response to circumcision in the neonate.Obstet Gynecol 48:208-10, 1976.

Tashiro C, Matsui Y, Nakano S, Ueyama H, Nishimura M, Oka N, Respiratory outcome inextremely premature infants following ketamine anaesthesia. Can J Anaesth 38:287-91, 1991.

Tateishi T, Nakura H, Asoh M, Watanabe M, Tanaka M, Kumai T, Takashima S, Imaoka S,Funae Y, Yabusaki Y, Kamataki T, Kobayashi S. A comparison of hepatic cytochrome P450protein expression between infancy and postinfancy. Life Sci 61:2567-74, 1997.

Treluyer JM, Jacqz-Aigrain E, Alvarez F, Cresteil T. Expression of CYP2D6 in developinghuman liver. Eur J Biochem 202:583-8, 1991.

Truog R, Anand KJ. Management of pain in the postoperative neonate. Clin Perinatol 16:61-78,1989.

Valman HB, Pearson JF. What the fetus feels. BMJ 280:233-4, 1980.

REFERENCES

73

Vandenberghe H, Mac Leod S, Chinyanga H, Endrenyi L, Soldin S. Pharmacokinetics ofintravenous morphine in balanced anesthesia: studies in children. Drug Metab Rev 14:887-903,1983.

Vanpee M, Ergander U, Herin P, Aperia A. Renal function in sick, very low-birth-weightinfants. Acta Paediatr 82:714-8, 1993.

Ward-Platt MP, Lovat PE, Watson JG, Aynsley-Green A. The effects of anesthesia and surgeryon lymphocyte populations and function in infants and children. J Pediatr Surg 24:884-7, 1989.

Wasz-Höckert O, Koivisto M, Vuorenkoski V, Partanen TJ, Lind J. Spectrographic analysis ofpain cry in hyperbilirubinemia. Biol Neonate 17:260-71, 1971.

Wasz-Höckert O, Michelson K, Lind J. Twnty-five years of Scandinavian cry research. In:Infant Crying: Theoretical and Research Perspectives. Lester BM, Boukydis CFZ (eds.). PlenumPress, New York, 1985:83-104.

Watkins PB, Wrighton SA, Schuetz EG, Molowa DT, Guzelian PS. Identification ofglucocorticoid-inducible cytochromes P-450 in the intestinal mucosa of rats and man. J ClinInvest 80:1029-36, 1987.

White PF, Ham J, Way WL, Trevor AJ. Pharmacology of ketamine isomers in surgical patients.Anesthesiology 52:231-9, 1980.

White PF, Way WL, Trevor AJ. Ketamine - its pharmacology and therapeutic uses.Anesthesiology 56:119-36, 1982.

White PF, Schuttler J, Shafer A, Stanski DR, Horai Y, Trevor AJ. Comparative pharmacologyof the ketamine isomers. Studies in volunteers. Br J Anaesth 57:197-203, 1985.

Williamson PS, Williamson ML. Physiologic stress reduction by a local anesthetic duringnewborn circumcision. Pediatrics 71:36-40, 1983.

Wilson AS, Stiller RL, Davis PJ, Fedel G, Chakravorti S, Israel BA, McGowan FX Jr. Fentanyland alfentanil plasma protein binding in preterm and term neonates. Anesth Analg 84:315-8,1997.

Woestenborghs RJ, Stanski DR, Scott JC, Haykants JJ. Assay methods for fentanyl in serum:gas-liquid chromatography versus radioimmunoassay. Anesthesiology 67:85-90, 1987.

Vuolteenaho O, Leppäluoto J, Vakkuri O, Karppinen J, Hoyhtya M, Ling N. Development andvalidation of a radioimmunoassay for beta-endorphin-related peptides. Acta Physiol Scand112:313-21, 1981.

Yang HY, Lee QP, Rettie AE, Juchau MR. Functional cytochrome P450 3A isoforms in humanembryonic tissues: expression during organogenesis. Mol Pharmacol 46:922-8, 1994.

Zeskind PS, Sale J, Maio ML, Huntington L, Weiseman JR. Adult perceptions of pain andhunger cries: a synchrony of arousal. Child Dev 56:549-54, 1985.

Zeskind PS, Barr RG. Acoustic characteristics of naturally occurring cries of infants with"colic". Child Dev 68:394-403, 1997.

Documents

ANALGESIA FOR NEWBORN INFANTS DURING MECHANICAL VENTILATIONethesis.helsinki.fi/julkaisut/laa/kliin/vk/saarenmaa/analgesi.pdf · ANALGESIA FOR NEWBORN INFANTS DURING MECHANICAL VENTILATION