Pitfalls of Noninvasive Hemodynamic Monitoring

Review Article

STETHOSCOPY

Stethoscopy was recommended by Harvey Cushing in1908 to be used as a routine cardiopulmonary monitoringdevice during surgery. Oesophageal stethoscope providesclear breath sounds & distinct heart sounds. Oesophagealstethoscopy was widely used in anesthesia practice in 20thcentury. Stethoscopy is not used routinely now a days andis replaced by other sensitive monitors. Australian incidentmonitoring 1993 showed that stethoscope was notsensitive to detect morbid incidents- cardiac arrest. On theother hand it is not free from complications, may causepharyngeal, oesophageal or trauma to posterior wall oftrachea. Rare complications include hypoxia fromunintended tracheo-bronchial placement, loss down theoesophagus, detachment of acoustic cuff. It also requiresanesthetist to be close to the patient which may not bepossible in most of the cases [1]. A frequent blood pressuredetermination is fundamental cardiovascular sign duringanesthesia & intensive care unit. It aids drug titration andfluid management and provide warning of conditions thatcould affect patient safety [2].

NONINVASIVE BLOOB PRESSURE MONITORING

Intermittent blood pressure monitors

The majority of automated noninvasive monitorsemploy oscillometry [3-4]. The cuff is inflated above thepoint where pressure oscillations are present. As the cuff isdeflated, pressure pulsations (oscillations) caused bymovement of the arterial wall are transmitted to the cuff.

PITFALLS OF NONINVASIVE HEMODYNAMIC MONITORING

Sanjeev Aneja* and Pragati Nanda***Senior Consultant, Anaesthesia, **Registrar, Department of Anaesthesia, Indraprastha Apollo Hospitals,

Sarita Vihar, New Delhi 110 076, India.Correspondence to: Dr Sanjeev Aneja, Senior Consultant, Department of Anaesthesia,Indraprastha Apollo Hospitals,

Sarita Vihar, New Delhi 110 076, India.E-mail: sanjeevaneja@hotmail.com.

Appropriate level of monitoring is important in anaesthesia. Monitors should be sensitive enough to be able todetect early changes in hemodynamics. There have been major advance in non invasive monitoring in therecent past to make them more user friendly & also provide data which was till recently possible by invasivemonitors only. Non invasive monitoring has limitations because of physical principle involved and otherprerequisites required for accuracy. Thus non invasive monitors may not be sensitive enough to pick earlychanges in hemodynamic in sick patients. In this review we discuss the limitations of non invasivehemodynamic monitoring and factors that may influence their accurate working.

Key word: Non invasive hemodynamic monitoring.

The magnitude of these oscillations increases to amaximum, then decreases. The monitor measures theseoscillations. After the determination is complete, theremaining air in the cuff is rapidly exhausted.

Noninvasive blood pressure (NIBP) monitors rely onmeasurement, extrapolations. and clinically testedalgorithms to arrive at values for mean, systolic, anddiastolic pressures [4-6]. The point of maximumamplitude corresponds to the mean pressure [7-9].Systolic and diastolic blood pressures are then calculatedfrom the increasing and decreasing magnitude of theoscillations according to an empirically derivedalgorithm.

Factors affecting blood pressuredeterminations

Cuff/Arm relationship

Accurate blood pressure measurement depends on therelationship between arm circumference and cuff width[8]. Typically, a cuff that is too small for the patient’s armcircumference will overestimate blood pressure, whereasa cuff that is too large will underestimate blood pressure[10-11]. Current recommendations are that a cuff’s widthshould be approximately 40% to 50% of thecircumference (125% to 150% of the diameter) of thelimb on which it is used at the limb’s midpoint [12-14].Cuff length has little effect as long as the bladder encirclesat least 50% of the arm’s circumference [12].

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Site

The site where the cuff is placed will affect themeasured pressure. As the site moves more peripherally,the systolic pressure tends to increase and diastolicpressure to decrease.

Increased vascular tone may result in an increase inpulse pressure. Vascular disease and peripheral vaso-constriction may cause reduced pressures at distallocations.

Arm position

If the cuff and the patient’s heart are not at the samelevel, a correction must be made. For each 10 cm ofvertical height, 7.5 mm Hg (for every inch 1.80 mm Hg)above or below the heart level should be added to orsubtracted from the pressures measured [15].

Arrhythmias

The oscillometric technique is vulnerable to error inpatients with arrhythmias [16-19]. Several circumstancescan prevent accurate determination of blood pressure withNIBP devices. Highly irregular or rapid cardiac rhythmsmake it difficult to accurately determine blood pressureusing NIBP devices because of the great beat to beatvariability. Most NIBP devices employ oscillometrictechnology that is dependent on fairly regular cardiacrhythms to determine blood pressure. Excessive patientmovement such as shivering, restlessness, or externalmove-ment such as that from a helicopter, ambulancetransport, or a rapid-cycling ventilator can interfere withdetection of cardiac oscillations by the NIBP monitor. Thiscan lead to erroneous blood pressure measurements [20-21].

Improper calibration and maintenance

Studies performed with instruments in common usehave shown problems with calibration, maintenance, andaccuracy [8]. Errors in the algorithm may occur [22-23].

Correlation with directly measured bloodpressure

Indirect pressure readings will never exactly matchinvasive pressures. One reason is that direct and indirectmonitors measure different physical properties. The directmethod measures both the systolic and diastolic pressuresand calculates the mean. Oscillometry measures the meanand calculates the systolic and diastolic pressures.

Many studies have compared pressures obtained withNIBP monitors with those obtained directly [10,24-30]. In

general, correlation of directly measured blood pressureswith indirectly measured blood pressures is best in normalhealthy patients and least accurate at the extremes of bloodpressure.

Clinical research studies have demonstrated that whenblood pressures (systolic, diastolic, and mean arterial)determined by NIBP monitors from various manufacturersare compared to direct arterial pressures, the two valuesare, on average, within 5 mm Hg of each other [31-34].

Oscillometric devices tend to overestimate lowpressures and underestimate high pressures [35]. In astudy by Thomson and colleagues, they found theMicrolife 3AC1-1PC was shown to accurately measureblood pressure in patients with end-stage renal disease. Asarterial stiffness increased and diastolic blood pressurefell, diastolic blood pressure was increasinglyoverestimated [36].

Most studies have concluded that an NIBP monitor isnot sufficiently accurate if vasoactive drugs are beingadministered or in critically ill patients although it may beuseful for trending purposes.

Another problem is that with moderate to severehypotension, the automated machine often cyclesrepeatedly before indicating a failure to measure.

There is an increasing number of wrist blood pressuremeasurement devices that successfully passed the valida-tion procedures of the British Hypertension Society(BHS) and the European Society of Hypertension (ESH).Even well-validated wrist blood pressure devices canshow a clinically relevant bias in patients with elevatedpulse pressure [37].

PROPER USE

An article discusses the practical skills andunderpinning knowledge needed when manually taking apatients blood pressure. The author also highlights thedifferent positions used for taking blood pressure and thearguments surrounding their respective merits [38].

If possible, the cuff should not be applied to the limb inwhich an intravenous infusion is placed because infusionof fluids and drugs will be slowed or blocked and theincreased pressure may cause blood to flow retrogradeinto the infusion set tubing or extravasation [5]. Checkvalves in the intravenous tubing between the fluid bag andthe patient usually prevent backflow into the tubing. Asimple method to avoid, or at least reduce, this problem isto route the infusion tubing under or through the bloodpressure cuff [39].

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Placement of noninvasive blood pressure cuffon upper arm

Damage to underlying tissues

Padding should be placed around the skin under thecuff to prevent skin creasing, petechiae, blistering, nervedamage, and problems from residual cleaning materials[40].Predisposing factors to the development of petechialhemorrhages include patients taking anti-inflammatorydrugs, steroids, or anticoagulants and patients with thin orredundant skin [41]. Prolonged use and frequent bloodpressure determinations can lead to venous pooling andcongestion. Excessive venous pressures can lead to tissueischemia and nerve damage [42-43].

Neuropathies

Neuropathies of the median, ulnar, and radial nerveshave been reported following use of an automatic NIBPmonitor [44,45].

Compartment syndrome

Cases of compartment syndrome associated withprolonged use have been reported [46].

Mechanical problems

Failure of an NIBP monitor is not uncommon and maybe associated with serious morbidity or mortality [2,47]. Acommon problem is a leaking cuff, hose, or connector [47-48])

Artifacts

Oscillometric devices are sensitive to both intrinsicand extrinsic motions [49]. Extrinsic motion artifacts arecaused by actions that compress the cuff, such as bumpingby personnel or equipment.

Intrinsic motion artifacts are caused by movementssuch as deliberate patient motion, shivering, tremors,convul-sions, restlessness, or vigorous skin preparation[49-50].

Air embolism

If the line used to inflate the cuff is connected to anintravenous line, a serious air embolus can result [51]

Patient discomfort

Patient discomfort is often associated with a prolongedcycle time. Cycle time will be prolonged with a large cuff,hypertensive patients, poor peripheral circulation, a leakin the monitor, low blood pressure, dysrhythmias, ormotion artifacts.

Noncontinuous measurements

After a blood pressure measurement has been made,the value is usually displayed until the next measurement.

CONTINUOUS NONINVASIVE BLOODPRESSURE INSTRUMENTS

Continuous or near-continuous blood pressure moni-toring is desirable in patients with cardiac orcerebrovascular disease, in critically ill patients, and insurgical procedures in which periods of hemodynamicinstability are anticipated.

Vasotrac

The Vasotrac consists of a wrist module and a monitor/display connected by a cable [52]. The sensor is placed overthe radial artery at the wrist. A pressure-sensing mechanismapplies variable pressure directly above the artery. Thecounterpressure in the artery produces a pressure wave-form. Analysis of the waveform is used to calculate thesystolic, diastolic, and mean arterial pressures. Bloodpressure measurements, pulse rate, and an arterial wave-form display are updated several times a minute anddisplayed on a monitor. It is available in a handheld version.

When the Vasotrac was compared with direct arterialpressure, good correlation was found [52-55]. It wasfound to be accurate when deliberate hypotension wasused [56].

However, in liver transplantation patients, theVasotrac was not sufficiently accurate to substitute fordirect arterial blood pressure monitoring [57].

The Vasotrac system is susceptible to patient armmovement, but the waveform can alert the caregiver toartifacts [58].

Finapres (Ohmeda, Madison, USA) is a non-invasivedevice which continuously measures the arterial bloodpressure in a finger and produces a real-time display of thearterial pressure wave. It consists of a finger cuff with aninfra-red transmission plethysmograph, a servo controlbox and a monitor unit. While the Finapres has potential asa non-invasive continuous blood pressure monitor, thecurrent model Finapres, as supplied, displays too great avariability for it to be used as an alternative to intra-arterialpressure monitoring [59].

The Working Group on Blood Pressure Monitoring ofthe European Society of Hypertension has recentlypublished an International Protocol to facilitate thevalidation of more automated devices than was possiblewith the earlier more complicated protocols [60].

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NONINVASIVE MONITORS FOR VOLUMESTATUS.

Earlier, volume status of patients was assessed withclinical signs of dehydration like dry skin, tongue, urineoutput etc, which lacked precision and exact volumeassessment. Later, fullness of neck veins was tried toassess the fluid status, but had many limitations as positionand colour of the patient and personal perception bias.

Stroke volume variation as measured by the analysis ofthe arterial pressure waveform enables prediction of volumeresponsiveness in ventilated patients with normal cardiacfunction. stroke volume variation (SVV), which is thepercentage change between the maximal and minimal strokevolumes (SV) divided by the average of the minimum andmaximum over a floating period of 30s, continuouslydisplayed by the PiCCO continuous cardiac output monitor,was evaluated as a predictor of fluid responsiveness.Assessment of stroke volume variation for prediction offluid responsiveness using the modified FloTrac™ systemwas tried. In patients, SVVFloTrac was determined viaradial FloTrac sensor, and SVVPiCCO and pulse pressurevariation were assessed via a femoral PiCCO catheter. Lessinvasive haemodynamic monitoring systems based onarterial pulse contour analysis allow stroke volume variation(SVV) to be tracked continuously. SVV assessed using thePiCCOplus™ system (Pulsion Medical Systems, Munich,Germany; SVVPiCCO) has repeatedly been shown topredict fluid responsiveness well in various clinicalsettings[61-65], whereas only sparse data are available forSVV determined using the recently introduced FloTrac™/Vigileo™ system. In a study conducted by de Waal andcoworkers [66] SVVFloTrac failed to predict fluidresponsiveness. Neither PPV nor SVV have been validatedduring conditions of extreme hypovolaemia, where theseparameters may become less accurate.

As to the SVV, concerns have been raised over theability of the pulse contour technique to track accuratelySV changes that occur over a short period of time [67-68]and a recent study failed to demonstrate a correlationbetween the SVV and the response of the CO to volumeloading [69]. In addition, there is only limited informationabout the relationship between the SVV, PPV and SPVduring changes in volume status [70]. Since specificthreshold values of the PPV and SVV have already beensuggested for the clinical assessment of fluidresponsiveness, it is important to examine their reliabilityin varying volume states.

NONINVASIVE CARDIAC OUTPUT MONITORING

The value of noninvasive cardiac output (Qt) moni-toring lies first and foremost in its convenient, low-risk

application to a variety of clinical settings. Thenoninvasive thoracic electrical bioimpedance monitor iswell suited to hospital bedside, physician’s office,emergency department, ICU, and operating room settings.

Accurate clinical assessment of the circulatory statusis particular desirable in critically ill patients in the ICUand patients undergoing cardiac, thoracic, or vascularinterventions. As the patient’s haemodynamic status maychange rapidly, continuous monitoring of cardiac outputwill provide information allowing rapid adjustment oftherapy.

For more than three decades the pulmonary arterycatheter (PAC) thermodilution method has been generallyaccepted and is still the clinical standard to which all othermethods are compared.

Novel techniques to monitor cardiac output

Indicator dilution techniques. Recent advances inindicator dilution techniques involve: the re-introductionof transpulmonary thermodilution (PiCCO, Pulsion), thetranspulmonary lithium dilution method (LiDCO), thepulmonary artery catheter based continuousthermodilution methods. CO is calculated with use of theSteward-Hamilton equation. Of the mentioned methodsthe transpulmonary indicator dilution methods as well asthe so-called ‘continuous cardiac output’ thermodilutionmethods [71] have been reasonably accepted in clinicalpractice, but is invasive and requires pulmonary arterycatheter insertion.

Fick principle. The NICO (Novametrix) system is anon-invasive device that applies Fick’s principle on CO2and relies solely on airway gas measurement. The methodactually calculates effective lung perfusion, i.e. that part ofthe pulmonary capillary blood flow that has passedthrough the ventilated parts of the lung. The effects ofunknown ventilation/perfusion inequality in patients mayexplain why the performance of this method shows a lackof agreement between thermodilution and CO2-rebreathing cardiac output [72].

Bio-Impedance and conduction techniques. The bio-impedance analysis (changes in voltage of electricalcurrents traversing the patient’s chest) method was intro-duced, five decades ago, as a simple, low-cost method thatgives information about the cardiovascular system and/or(de)-hydration status of the body in a non-invasive way. Toimprove the related thoracic impedance method, over theyears, a diversity of thoracic impedance measurementsystems appeared. These systems determine CO on a beat-to-beat time base. More than 150 validation studies havebeen reported, mostly with poor and exceptionally with

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good correlations compared to a reference method. Inorder to explain these controversial results, many of thesestudies refer to the poor physical principles of the thoracicimpedance method [73]. The accuracy of this technique isdramatically increased (along with its invasiveness) whenthe electrodes are placed directly in the left ventricle,rather than on the chest.

Arterial pulse contour analysis. The estimation ofcardiac output base on pulse contour analysis is an indirectmethod, since cardiac output is not measured directly, aswith an electromagnetic flow probe, but is computed froma pressure pulsation on basis of a criterion or model. Theorigin of the pulse contour method for estimation of beat-to-beat stroke volume goes back to the classic Windkesselmodel described by Otto Frank in 1899. Most pulsecontour methods are, explicitly or implicitly based on thismodel [74-76]. They relate an arterial pressure or pressuredifference to a flow or volume change. Nowadays, threepulse contour methods are available; PiCCO (Pulsion),PulseCO (LiDCO) and Modelflow (TNO/BMI). All thesethree pulse contour methods use an invasively measuredarterial blood pressure and they need to be calibrated.PiCCO is calibrated by transpulmonary thermodilution,LiDCO by transpulmonary lithium dilution andModelflow by the mean of 3 or 4 conventionalthermodilution measurements equally spread over theventilatory cycle. Output of these pulse contour systems iscalculated on a beat-to-beat base, but presentation of thedata is typically with a 30 seconds window. A no invasivepulse contour development is the combination of non-invasively measured arterial finger blood pressure withModelflow [77].

None of the CO techniques combines all criteriamentioned above. With respect to accuracy and precision,a number of methods may replace the thermodilutionmethod with a precision of 15%. But none of the newtechniques can reliably replace the conventionalthermodilution based on the averaged result of 3 or 4measurements done equally spread over the ventilatorycycle [78]. Under research conditions the use of thisconventional thermodilution method remains the methodof choice.

PROS AND CONS OF TRANSESOPHAGEALECHOCARDIOGRAPHY

Transesophageal echocardiography (TEE) is a well-established technology in anesthesiology, cardiology andthe management of trauma patients. A specialized probecontaining an ultrasound transducer at its tip is passed intothe patient’s esophagus. This allows image and Dopplerevaluation which can be recorded. Volume status, great

vessel injury, cardiac lesions, and evidence of ischemiacan be visualized quickly, thus assisting in directingtreatment. While TEE has been shown to be safe, it is notwithout risk and carriers an overall morbidity of 0.2%[79].

DISADVANTAGES

• TEE requires a fasting patient, (the patient mustfollow the ASA NPO guidelines(i.e. usually not eator drink anything for eight hours prior to theprocedure)

• Requires a team of medical personnel

• May be uncomfortable for the patient

• May require sedation or general anesthesia

• has some risks associated with the procedure(esophageal perforation[80]-1 in 10,000, andadverse reactions to the medication)

• Direct trauma to esophagus- tearing, bleeding,dysphagia severe odynophagia, UGI hemorrhage,and airway- laryengeal dysfunction,bacteraemia andairway compression in children have been reported[81].

• Direct cardiac irritation from probe may evokedysrrhythmias [82].

In a study [83], it was found, that TEE is of limitedvalue in pridicting left atrial clot, before cardioversion inpatiens with atrial flutter.

Failure to successfully insert or advance the TEEprobe occurs in 0.7%-1.9% of sedated adult patients and in0.8% of anesthetized pediatric patients [84].

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