Maintenance of anaesthesia

CLINICAL ANAESTHESIA

© 2004 The Medicine Publishing Company Ltd15ANAESTHESIA AND INTENSIVE CARE MEDICINE

Administration of a general anaesthetic can be considered in three stages: induction, maintenance and recovery. Maintenance is less commonly considered than the two other periods but usu-ally represents the longest stage of anaesthesia. The delivery of anaesthesia is achieved by the inhalational or intravenous route, though ketamine can be given by intramuscular administration in an emergency.

Inhalational anaesthesia

Inhalational anaesthesia is the most widely used technique and assuming the use of minimal fresh gas flow rates, is usually the most economical. A volatile agent is used in oxygen with air or nitrous oxide. The maintenance of anaesthesia relies on keeping the partial pressure of the volatile agent constant in the brain. The period of maintenance (as opposed to induction) begins when the required partial pressure of anaesthetic is reached at its site of action and ends when that partial pressure is allowed to fall to permit wakening.

OxygenWith modern machines it should be impossible to deliver a hypoxic mixture at the common gas outlet, but it should be noted that over time, minimal fresh gas flows in a circle system may result in a hypoxic mixture. The effects of general anaesthesia on the lung mean that it is usual to use a minimum of 30% oxygen rather than air, even in healthy subjects. In spontaneously breathing patients these effects include relative hypoventilation due to reduced tidal volumes and respiratory rate. They also include an increase in ventilation–perfusion mismatch due to the development of atel-ectasis in dependent areas. A reduction in the functional residual capacity (FRC) occurs so that it approaches the closing capacity

Maintenance of anaesthesiaRuth Spencer

Andrew K McIndoe

and airway collapse can occur during normal tidal breathing. The usually protective reflex of hypoxic pulmonary vasoconstriction, which reduces perfusion of poorly ventilated areas, is lost under anaesthesia. Oxygen toxicity is seldom a problem during the time-scale imposed by surgery, but is relevant when patients are ventilated in intensive care for prolonged periods. In general, the lowest inspired oxygen level that produces acceptable gases should be used. A few patients with long-standing chronic obstructive pulmonary disease may rely on their hypoxic drive to breathe and this can pose problems when weaning them from controlled ventilation.

Nitrous oxide (N2O)N2O is commonly used as a carrier gas and to supplement vol-atile anaesthesia. Although only weakly anaesthetic, it is a potent analgesic and reduces the requirement for other agents. It speeds the induction of anaesthesia because the rapid absorption of N2O results in an increase in the alveolar concentration of the volatile agent. It usually has little effect on the pulse and blood pressure but depresses myocardial contractility, especially in combination with volatiles and opioids. N2O is associated with increased postoperative nausea and vomiting (PONV), with up to 15% of patients being affected. It is best avoided in those with a history of PONV or those whose risk is additionally increased by their pathology or surgery. N2O also expands air-filled cavities because it is 35 times more soluble than nitrogen and able to pass from the blood into the space faster than nitrogen can diffuse out. For this reason, it may contribute to bowel dilatation and expands any air embolus or pneumothorax. It should be avoided in surgery to the inner ear and in some ophthalmic retinal procedures. If either intraocular air or sulphur hexafluoride are used, the expansion of that gas by N2O will initially raise the intraocular pressure but then cause an unintended reduction in the final volume. Concerns about the safety of N2O are controversial, with a number of conflicting studies. Occupational exposure has been linked to spontaneous abortions, reduced fertility and congenital abnormalities, though there is little recent work. The Control of Substances Hazardous to Health Regulations state that exposure to all anaesthetic agents should be kept to the lowest practical limit. In the UK, the occupational exposure limit for N2O is 100 parts per million (ppm) over an 8-hour average, but in the USA the limit is as low as 25 ppm. A recent survey conducted by the Association of Anaesthetists of Great Britain and Ireland has identified a change in practice with regard to the use of N2O. Over the last 5 years, 49% of consultant anaesthetists thought that they had reduced their routine use of N2O. In most cases this was because of the side-effects for the patient and not because of concerns over environmental pollution. The decrease was also attributed to the wider use of intravenous anaesthetic techniques and the increased availability of medical air on anaesthetic machines.

Volatile agents

The main factors that affect the uptake of volatile agents are their solubility, the cardiac output and the concentration gradi-ent between alveoli and blood. Volatile agents are administered by inhalation via a calibrated vaporizer. The rate of induction

Ruth Spencer is a Clinical Fellow in Paediatric Intensive Care at the Bristol

Children’s Hospital. She graduated from Bristol University and initially

trained in general medicine as an SHO and registrar. She has completed

the Bristol Specialist Registrar rotation in anaesthetics.

Andrew K McIndoe is Consultant Anaesthetist and Senior Clinical Lecturer

in the Sir Humphry Davy Department of Anaesthesia, Bristol Royal

Infirmary. He is also the Director of Research and Education at the Bristol

Medical Simulation Centre where he has developed a specific interest in

human factors and crisis management.

mcindoe.indd 15 17/12/03, 10:12:04



depends on the speed with which the partial pressure of agent in the brain can be made to rise. The less soluble the agent, the more rapidly alveolar concentration approaches the inspired agent concentration. This speeds the rate of induction. A high cardiac output increases uptake of agent from the alveoli and therefore prolongs the time it takes for the alveolar concentration to rise.A high cardiac output increases the time taken for induction while a reduced cardiac output speeds the process. Anaesthetic agents are initially taken up by organs with high blood flow (vessel-rich organs), including the brain. They are later redistributed to vessel-poor areas (e.g. muscle, fat, bone). A faster rate of induction can be achieved by initially increasing the fraction of inspired agent to a higher level than necessary for maintenance, to improve the driving concentration gradient (overpressure). Solubility also regulates the elimination of inhaled anaesthetic agents. With the exception of halothane, only a small percentage of these drugs are metabolized with most excreted via the lungs unchanged.

Minimum alveolar concentrationThe potency of an inhaled anaesthetic agent may be described in terms of its minimum alveolar concentration (MAC). This is defined as a concentration that prevents movement in response to skin incision in 50% of unpremedicated subjects. The MAC is therefore a measure of anaesthetic potency that allows comparison between agents, but it cannot be regarded as a clinically useful threshold. It is influenced by many patient factors, by other drugs and by the surgical stimulus to which the patient is about to be subjected. Factors affecting the MAC are listed in Figure 1. The MAC is a guide to help estimate the amount of anaesthetic required, but in practice, levels are adjusted according to clinical observations and monitoring that indicate the adequacy of anaesthetic depth. The individual volatile agents have many common attributes (Figure 2). All reduce myocardial contractility and depress ventila-tion, usually by decreasing the tidal volume. All agents decrease the normal ventilatory response to hypoxia and hypercapnia and abolish the protective effect of hypoxic pulmonary vasoconstric-tion. They all increase cerebral blood flow, with a consequent rise in intracranial pressure, but reduce cerebral oxygen consump-

tion. All agents potentiate the muscle-relaxing effects of non-depolarizing neuromuscular blockers and all are capable of trig-gering the development of malignant hyperthermia in susceptible individuals.

Isoflurane produces a marked fall in systemic vascular resistance and therefore in mean arterial pressure. This is often accompanied by a reflex tachycardia, especially in younger patients. It does not sensitize the myocardium to circulating catecholamines, so does not promote arrhythmias. It has been implicated in the ‘coronary steal’ syndrome in which, since stenosed vessels are unable to dilate, blood is diverted away from poorly perfused areas to those that have an adequate blood supply. It is irritant to the respiratory tract, which limits its usefulness for gas induction. Coughing, breath holding and an increase in bronchial secretions may occur. It is not nephrotoxic because fluoride ion production is minimal. Only 0.2% is metabolized, with the rest excreted via the lungs unchanged.

Sevoflurane is less soluble in blood so the alveolar concentration quickly reaches that of the inspired concentration, producing rapid induction. In higher doses, it can produce a bradycardia, especially in children but it does not sensitize the myocardium to catecholamines or precipitate coronary steal. It has a pleasant odour and is non-irritant, making it the usual first choice for gas induction. 3% of it is metabolized.

Desflurane is even more insoluble in blood and therefore fast acting and rapidly eliminated. Its low boiling point (23.5°C) and high saturated vapour pressure mean that it must be administered by a specific pressurized and heated vaporizer. Sympathetic tone is quite well preserved, as is cardiac index and left ventricular ejection fraction. It is valuable in the elderly, those with cardiac instability, or if postoperative assessment of conscious level is vital, such as after neurosurgical procedures. It is irritant to the airway and may cause an increased respiratory rate, though tidal volume is reduced. Only 0.02% is metabolized.

Enflurane is a structural isomer of isoflurane but is seldom used. It sensitizes the myocardium to catecholamines and is a powerful respiratory depressant. It may induce tonic–clonic muscle activity and produces epileptiform EEG traces. It should be avoided in those with epilepsy. 2.4% of an inhaled dose is metabolized.

Characteristics of volatile agents

Agent MAC1 in O2 MAC in Blood/gas solubility 70% N2O coefficientIsoflurane 1.15 0.5 1.4

Sevoflurane 2.0 0.8 0.6

Enflurane 1.68 0.57 1.91

Desflurane 5.7–10 2.5–3.5 0.42

(age dependent)

Halothane 0.75 0.29 2.5

1MAC, minimum alveolar concentration

2

Factors affecting minimum alveolar concentration (MAC)

Decreased MAC Increased MAC No effect• Increasing age • Decreasing age • Gender

• Premedication/ • Chronic alcohol use • Duration of

sedatives • Severe anxiety anaesthesia

• Use of N2O • Hyperthermia • Time of day

• Analgesia • Hyperthyroidism • Hypocarbia

• Hypotension/ • Moderate

hypovolaemia hypercarbia

• Hypoxia

• Acute alcohol ingestion

• Hypothermia

• Hypothyroidism

1

mcindoe.indd 16 17/12/03, 10:12:05



Halothane is less irritant to the airway than isoflurane or des-flurane, and results in bronchodilatation and reduced secretions. However, its use as an agent for gas induction has largely been superseded by sevoflurane, which has a much faster onset. The longer action of halothane was useful in operations on the airway performed during emergence but this has largely been replaced by intravenous techniques. Halothane causes marked sensitiza-tion of the myo cardium to catecholamines, both endogenous and administered. Its arrhythmogenic potential is further increased by the presence of hypoxia or hypercapnia. Bradycardias may also be seen secondary to increased vagal tone. Shivering is more common in the post operative period when halothane has been used. It is the agent that undergoes the most metabolism, with a rate of 20%. Halothane hepatitis is an extremely rare but serious side-effect. Its incidence and characteristics are controversial. The risk is increased by hypoxia, obesity and when there is a short interval between consecutive exposures. There is a recommendation that a minimum of 6 months should elapse between halothane anaes-thetics. However, similar hepatic toxicity has also been seen fol-lowing both enflurane and isoflurane administration.

Intravenous anaesthesia

Total intravenous anaesthesia (TIVA) is a technique in which intravenous agents alone are used to maintain unconsciousness while the patient breathes a combination of oxygen and air. Several agents have been used, but the pharmacokinetic profile of propofol, with its high clearance, short half-life and inactive metabolites, makes it the usual drug of choice. For short proced-ures it is sometimes used by intermittent bolus but the advent of reliable electronic syringe pumps, especially with target-controlled software, has contributed to its ease of administration for longer operations. Target-controlled infusion pumps require input of the patient’s age, weight and the desired plasma concentration before using a three-compartment model to describe the distribution, redistribution and elimination of the drug. The context-sensitive half-life is used to predict the time to wakening after discontinu-ation of the infusion. Calculations are based on population data and the minimum infusion rate (MIR) is analogous to the MAC of inhaled agents.

Advantages: TIVA reduces PONV and allows for a better quality of recovery. It is readily titratable, so that anaesthesia can be rapidly deepened with an intravenous bolus in response to increasing sur-gical stimulation. The delivery of anaesthesia is independent of the airway with TIVA. This makes it invaluable for situations such as bronchoscopic examination or airway surgery when jet ventilation may be required. It is also used during cardiopulmonary bypass when the patient is not ventilated. It is the method of choice for transferring anaesthetized patients to other hospital departments such as intensive care or for CT scanning. Propofol provides safe anaesthesia for those susceptible to malignant hyperthermia and avoids the problems of environmental pollution inherent to anaesthetic gases.

Disadvantages: the delivery of anaesthetic relies on dependable intravenous access. There is no monitored means of assessing this in the way that end-tidal agent analysis provides during

inhaled anaesthesia. Co-administration of other drugs via the same venous access can affect delivery and propofol accumu-lates after prolonged infusions. Once given, the drug is removed only by metabolism or excretion, rather than exhaled. Predicting plasma concentration is more difficult than with inhaled agents because the pharmacokinetics are more complicated and affected by intercurrent disease. It should be remembered that the target quoted is that of the blood concentration, whereas the actual site of action of propofol is in the brain. TIVA is a relatively expensive technique and this needs to be weighed against the fact that highly insoluble volatiles (e.g. sevoflurane, desflurane) now allow rapid induction and recovery.

Awareness

Awareness is the patient’s ability to recall events occurring during general anaesthesia and is a potential source of extreme distress to the patient and litigation against the hospital. It is most likely to occur during the induction or recovery phase when the levels of anaesthetic agent may be inadequate. The chance of awareness at induction is increased when there are repeated attempts at difficult intubation and during the maintenance stage if an inter-mittent bolus, rather than continuous infusion TIVA technique is used. The use of high oxygen, low volatile anaesthesia employed for the moribund patient or for caesarean section under general anaesthetic also carries a higher risk. The administration of an intravenous benzodiazepine following a possible episode of aware-ness does not guarantee retrograde amnesia. Assessing anaesthetic adequacy can be difficult. The discrete stages described by Guedel are not clearly seen with current intra-venous agents and important clinical signs may be altered by anaesthetic drugs. Light anaesthesia is suggested by tachycardia, hypertension, sweating, lacrimation, dilated pupils, laryngospasm or movement. Estimates of correct dose may be guided by MAC or MIR but both are subject to inter-individual variability. Many methods of assessing anaesthetic depth have been used including cerebral function monitors, bispectral analysis and evoked poten-tials, but there is no single measurement that reliably indicates the state of unconsciousness of an individual. The patient’s attitude to recall is influenced by whether or not they were also in pain. The psychological impact is similar to that of post-traumatic stress disorder. It is worsened if recounting their experience is met with professional denial rather than acceptance that such an incident of awareness could have occurred.

Ventilation

Ventilation during the procedure may be by the patient’s sponta-neous respiration or by intermittent positive-pressure ventilation. The airway is maintained with a face mask, laryngeal mask (LMA) or tracheal tube. Intubation and ventilation allow the control of carbon dioxide (even when using potent opioid analgesia), protect the lungs from aspiration and reduce intra-operative atelectasis. Neuromuscular blocking drugs and their reversal agents are usually required. The need to intubate is determined by the condition of the patient and by the nature and duration of the surgery. In a spontaneously breathing patient the respiratory rate can be used as a marker of anaesthetic depth and reduces the chances of inadvertent awareness. The use of LMAs provides for greater

mcindoe.indd 17 17/12/03, 10:12:06



haemodynamic stability at induction and may reduce minor anaes-thetic morbidity (e.g. sore throat). For short procedures, the airway can be maintained by holding a face mask alone. Many anaesthetists also use the LMA to deliver positive-pressure ventilation, either with or without muscle relaxation. While this avoids the hazards of intubation, it provides only limited protection against acid reflux or aspiration of stomach contents. When ventilatory pressures exceed the cuff seal pressure, gas can escape around the cuff and cause gastric insufflation. The use of larger sizes (size four for women and size five for men) ensures a better seal. The risks of aspiration are increased by light anaes-thesia, a pneumoperitoneum or placing the patient in either the Trendelenburg or lithotomy position. Currently, the very small number of critical incidents reported suggests that positive-pressure ventilation via an LMA is a safe practice in selected patients. Regardless of the mode of ventilation, the fresh gas flow must be set according to the size of the patient and the breathing system used. Initially, a high flow (equivalent at least to the patient's alveolar minute ventilation) is selected to allow equilibration of the inhaled anaesthetic agent during the early period of rapid uptake. After a few minutes the flow can be reduced to more economical levels providing the system contains a carbon dioxide absorber and there is appropriate monitoring of gas concentrations in the circuit. There are formulae for each breathing system, to calculate the fresh gas flows required to prevent rebreathing in both control-led and spontaneously ventilating modes. Alternatively, the flows can be gradually reduced until rebreathing becomes evident on the capnograph. There are some circumstances in which ventilation is likely to be particularly challenging. Thoracic surgical patients may require a period of one lung ventilation via a double lumen tracheal tube. In the lateral position, with positive-pressure ventilation, increased ventilation–perfusion mismatch occurs because the upper lung is better ventilated, while the lower lung is better perfused. When the upper lung is collapsed all the ventilation is then of the lower lung. However, since the upper lung may still have considerable blood flow, hypoxia can also result from a large shunt (lung that is perfused but not ventilated). The degree of hypoxia depends on the pre-existing state of the lungs and a complex interaction between cardiac output and the distribution of pulmonary blood flow. ICU patients are best managed using ventilators that offer the facility for volume-controlled and pressure-controlled ventilation, the application of positive end-expiratory pressure and the ability to adjust the inspiratory to expiratory ratio.

Circulation

Appropriate fluid management is an important component of the maintenance period. The amount and type of fluid is determined by any existing preoperative deficit, the usual maintenance requirements and the need to replace continuing losses during the surgery. Elective patients seldom have a major fluid deficit unless they have received bowel preparation. However, preoperative starvation reduces their margin of reserve to withstand any further losses, such as surgical bleeding or postoperative vomiting. Many anaes-thetists routinely administer 500–1000 ml of crystalloid during even short procedures and this may improve the quality of recovery. There are specific formulae to calculate the fluid requirement of

children according to their weight. Children have less reserve than adults to tolerate fluid losses and in the case of infants are more prone to hypoglycaemia. Assessment of the fluid deficit in emergency patients is difficult. They may be seriously dehydrated secondary to trauma, vomit-ing, interstitial losses, nasogastric aspirates or pyrexia. Whenever possible, such deficits should be replaced before induction of anaesthesia. The exception is major continuing blood loss (e.g. ruptured aneurysm) where the priority is to stop the bleeding rather than prolong attempts to normalize the circulation preoperatively. During surgery, evaporative losses, which are hard to estimate, may be high, especially from laparotomy or thoracotomy incisions. The management of major burns relies on specific formulae when, in addition to maintenance requirements, fluid is administered according to the weight of the patient and the percentage of burnt area. Measurements such as pulse rate, blood pressure, central venous pressure, urine output and core–peripheral temperature gap help to guide volume replacement.

Blood loss: estimating intraoperative blood loss is difficult. Weighing swabs, along with observation of the wound, gloves, instruments and suction bottles can indicate the degree of loss, but all are inaccurate. Measurement of the haemoglobin from a finger-prick test or from an arterial line sample also helps, but is influenced by volume status. Whenever major blood loss is anticipated it is essential to involve senior staff and to communicate efficiently with the blood transfusion service. Massive blood loss and transfusion results in a coagulopathy that requires the use of fresh frozen plasma,platelets or cryoprecipitate. There are also additional complications of deranged biochemistry, hypothermia and acute lung injury. The patient must have reliable wide-bore intravenous access and there should be the facility to infuse warmed fluids rapidly. Reducing the need for blood transfusion is becoming increas-ingly important (Figure 3) because donor blood is expensive, in short supply and carries the risk of disease transmission. Most patients tolerate a reduced level of haemoglobin well and accepting a lower transfusion trigger is increasingly common. Many anaes-

Reducing the need for blood transfusion

• Acute normovolaemic haemodilution

• Use of tourniquets

• Local infiltration with vasoconstrictors

• Positioning

• Spinal or epidural anaesthesia

• Avoiding hypertension and raised venous pressure

• Deliberate hypotensive anaesthesia

• Use of antifibrinolytic agents (e.g. tranexamic acid)

• Good haemostasis/surgical technique

• Intraoperative cell salvage

• Tolerating a lower haematocrit

• Postoperative cell salvage from drains

• Maintenance of normothermia

3

mcindoe.indd 18 17/12/03, 10:12:06



thetists accept levels of 8 g/dl before giving blood, though this has to be weighed against the patient’s underlying medical problems. Pre operative measures that reduce the need for transfusion include the administration of iron supplements, control of hypertension and stopping drugs that promote bleeding. In circumstances when avoiding transfusion is critical (e.g. Jehovah’s Witnesses) or those for whom it is difficult to find compatible blood, the use of pre-donation or erythropoietin may be considered.

Temperature control

Prevention of hypothermia during the maintenance period is important. Hypothermia causes a reduced cardiac output, impaired tissue oxygen delivery, prolonged drug action, a coagulopathy and postoperative shivering, which increases the oxygen requirement. Anaesthesia can result in hypothermia for several reasons. Periph-eral vasodilatation causes redistribution of heat and the absence of muscle activity decreases metabolic heat production. Patients are often exposed and lose heat by radiation from the skin, convec-tion (especially with the frequent air changes in modern theatres) and evaporation from the respiratory tract, skin and viscera. It is important to identify those most at risk. These include those at the extremes of age, the malnourished and those undergoing lengthy procedures with major blood loss or open body cavities. Heat loss can be minimized by increasing the ambient temperature, using overhead heaters for infants, and using a forced-air warmingblanket, warmed fluids and humidified, warmed gases.

Positioning

Positioning of the patient is the joint responsibility of the anaesthet-ist and surgeon. Unconscious patients cannot move to relieve an uncomfortable position and therefore may be exposed to potential injury of their limbs, joints, pressure areas or nerves. At particular risk in the upper limb are the brachial plexus, and the radial and ulnar nerves, which can be inadvertently overstretched. In the lower limb the common peroneal and saphenous nerves are at risk if the lithotomy position is used, when they may be compressed against the support poles. Lithotomy may also damage the lower back, hips and knees, and must be used with caution if patients have had previous hip or knee replacement surgery. The position of the patient must facilitate the surgery and may be supine, prone, lateral or sitting. After primary positioning, patients may also be tipped or tilted to allow other procedures, such as venous drainage of a limb prior to surgery, venous distension to help cannulate a vein, or to manipulate the spread of spinal anaesthesia. Any movement of an unconscious patient risks displacing intravenous lines or the tracheal tube. It also carries the risk of injury to staff, especially if the patient is overweight or there are an inadequate number of people present to lift safely.

Supine positioning impairs lung function and is accompanied by a drop in FRC. If it approaches the closing capacity, airway collapse occurs during tidal breathing, increasing ventilation–perfusion mismatch and atelectasis. If the table is tipped into the Trende-lenburg position, diaphragmatic movement can be further limited by the weight of the abdominal viscera. The risk of regurgitation of gastric contents is also increased, as is venous engorgement of the neck, which raises intraocular and intracranial pressures.

Aortocaval compression occurs during pregnancy when the gravid uterus compresses the great vessels against the vertebral bodies. Venous return and cardiac output are reduced. A similar problem may be seen with large abdominal tumours. Those at risk require insertion of a wedge or a left lateral table tilt.

The prone position causes similar respiratory problems, with chest and abdominal movement hindered. It is important that any supports used leave the abdomen free, because venous return is impeded by a compressed abdomen. Turning an unconscious patient prone requires careful attention to all potential pressure areas, especially the face and eyes, which must be protected and padded. The tracheal tube must be firmly secured, because rapid emergency access to the airway is not subsequently possible.

Lateral positioning may result in increased ventilation–perfusion mismatch and hypoxia. The lower arm may also become compressed. Often, patients are turned into a lateral position for recovery so that the airway is protected.

Head up: this may range from a simple reverse Trendelenburg tilt to a full sitting position for some neurosurgical procedures. There is a risk of hypotension, reduced cerebral perfusion and air embolism in the presence of low venous pressure with open veins.

Preparation for the postoperative period

The maintenance period is often a convenient time to complete the anaesthetic record and to consider postoperative prescriptionsand instructions. Adequate analgesia and anti-emetics, by the appropriate route, should be prescribed for the immediate recov-ery period and a clear plan determined for later on the ward. The anaesthetist should ensure that the patient has sufficient intra-venous access to meet their postoperative needs and, when neces-sary, prescribe an intravenous fluid regimen. Antibiotics and the need for thromboprophylaxis should also be considered. Oxygen should be prescribed, specifying the flow rate, need for humidifi-cation and duration of treatment. The extent to which the patient requires postoperative monitoring should be charted along with the actions to be taken if the observations deviate from acceptable targets. The anaesthetic record should always be completed in a way that is comprehensive and legible so that it provides all the necessary information for subsequent anaesthetists.

mcindoe.indd 19 17/12/03, 10:12:07

Documents

Maintenance of anaesthesia