Induction of anaesthesia

CLINICAL ANAESTHESIA

© 2003 The Medicine Publishing Company Ltd364ANAESTHESIA AND INTENSIVE CARE MEDICINE

The induction of anaesthesia aims to produce an unconscious state in which the patient neither perceives nor recalls noxious stimuli. The choice of technique depends on patient factors, available time, type of surgery and the anaesthetist’s preference. Induction should produce a clinically appropriate level of CNS depression that permits surgical intervention with the best possible operat-ing conditions. The stages of anaesthesia were described in 1937 by Arthur Guedel using gas inductions with ether in air. His four discrete stages are indistinguishable with modern intravenous anaesthesia. The titration of current agents to features such as loss of eyelash reflex may result in overdosage. Usually now, a dose estimated as appropriate to the weight, age and physical health of the patient is delivered slowly, using motor relaxation and loss of verbal response to judge the end point. Anaesthetic rooms are a feature of UK practice; in most other countries, induction takes place on the operating table in theatre. A separate induction room saves the patient the psychological stress of being awake in the operating theatre and makes it easier for a parent to remain with their child until the child is uncon-scious. The theatre can also be cleaned and prepared for the next case while the patient is being anaesthetized. However, expensive monitoring equipment has to be duplicated in both rooms and the patient has a period of reduced or absent monitoring when they are disconnected for the transfer. Most anaesthetists induce in theatre when dealing with high risk, unstable patients or if extra movement should be kept to a minimum (e.g. in the morbidly obese).

Equipment

Emergency drugs, fully checked equipment, a trolley allowing Trendelenburg tilt and skilled assistance should always be available

before induction begins. A third member of staff, for example the ward nurse, should also be present to summon help in the event of an emergency. It is the responsibility of the anaesthetist to ensure that all the necessary equipment is present and in good working order. A complete machine check includes the gas supplies (both cylinders and pipelines), a correctly calibrated oxygen analyser, vaporizers, breathing systems, suction and ventilators. In addition, laryngo-scopes with a variety of blades and a selection of tracheal tubes and laryngeal masks should be ready. It is vital that equipment to deal with a failed or unexpectedly difficult intubation is immediately to hand.

Monitoring

Induction of anaesthesia produces many physiological changes. During induction, the patient is at risk of arrhythmias, apnoea, hypotension, laryngospasm and regurgitation of gastric contents. Susceptible individuals may show signs associated with allergies or unusual drug reactions. Comprehensive monitoring is an integral part of anaesthesia. Several groups have produced guidelines recommending the minimum standards of monitoring acceptable for good practice. It cannot be overstressed that the most important monitor is the continual presence of an experienced anaesthetist. Monitoring allows detection of early subtle changes in physiological variables, but this is useful only if the anaesthetist responds to the informa-tion appropriately. Technology will never be a substitute for clini-cal skill or anaesthetic vigilance. It is also important to remember that most monitors suffer from artefactual changes and technical limitations. The anaesthetist should adopt a healthy suspicion about numbers that are not corroborated by clinical signs. Alarm limits should be set to those appropriate for the patient rather than the default settings. A high incidence of false alarms downgrades their value and may lead to the hazardous practice of inactivating them. Whenever possible, monitoring in the form of ECG, blood pres-sure and pulse oximetry should be instituted before induction. All monitoring systems have uses and limitations, which must be recognized to allow correct interpretation of the values they deliver. Ultimately, the welfare of the patient depends on optimal tissue oxygenation and the maintenance of normal haemodynamic vari-ables. For these reasons, monitors of cardiovascular performance and respiratory adequacy are the most important.

Cardiovascular monitoringECG is a routine minimum standard of monitoring. It allows detec-tion of changes in rhythm and the development of ischaemia. It is not usually practical to measure the full 12-lead system in theatre, so most intraoperative ECG monitors use a 3-lead system of modified, bipolar limb leads. Two electrodes (positive and negative) detect the electrical activity of the heart and the third acts as a neutral or reference electrode. Many configurations of leads are described but all represent some form of compromise compared with a 12-lead recording and vary in their ability to detect arrhythmias or ischaemia. Lead II measures the potential difference between the right arm and left leg. It is close to the cardiac axis and is the most useful measure for detecting rhythm change because it gives good definition of the QRS complex and

Induction of anaesthesiaRuth Spencer

Andrew K McIndoe

Ruth Spencer is 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

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

Medical Simulation Centre where he has developed a specifi c interest in

human factors and crisis management.

spencer&mcind.indd 364 14/10/03, 13:38:55



its relationship to the P wave. The facility to print out a hard copy as a rhythm strip is valuable when reviewing the diagnosis or asking for advice. The ECG is not a sensitive monitor of ischaemia, which may have been present for some time before ST changes are apparent. The precordial lead V5 is the most sensitive single lead, with the CM5 configuration (right arm electrode on manubrium and left arm electrode at V5 ) best for demonstrating anterior ischaemia. Leads II, III and aVF detect inferior changes but posterior ischaemia is detected badly by any conventional lead arrangement. Ideally, the choice of lead configuration should be guided by the patient’shistory of arrhythmias or knowledge from stress testing of the areas most at risk of ischaemia.

Blood pressure: non-invasive measurement of blood pressure is undertaken automatically at set intervals and displayed as a numerical value. Most monitors use oscillometric methods of measurement that correlate well with invasive readings, provid-ing the value is within the normal range. Outside the normal range, non-invasive cuffs overestimate low blood pressure and underestimate high blood pressure. The cuff size is also vital to accuracy, because too small a cuff overestimates and too large a cuff underestimates the true value. Directly invasive monitoring of blood pressure provides beat-to-beat measurement, displayed as a continuous waveform. In addition to the blood pressure, the waveform can provide infor-mation about myocardial contractility and cardiac output. It can also suggest hypovolaemia, when the trace shows large falls in the peak blood pressure with positive pressure breaths. Indica-tions for invasive pressure monitoring are listed in Figure 1. The patient’s medical condition dictates whether invasive monitoring is established before or after induction of anaesthesia. The radial artery is usually selected because it is easily palpable, superficial and has a good collateral supply via the ulnar artery. Errors may be introduced by damping or resonance of the system and by in correct calibration, zeroing or placement of the transducer.

Central venous pressure (CVP): a CVP line allows rapid infu-sion of volume and central administration of vasoactive drugs. It gives an indication of intravascular volume and of right ventricu-lar function. The trend in the measurements and the response to interventions, such as filling, are often of more relevance than the absolute values. Observations of the waveform can be useful in the diagnosis of some cardiac conditions. In September 2002, the National Institute for Clinical Excellence published its recom-mendations on the use of ultrasound locating devices for insertion of central venous lines. Despite the huge implications for both equipment and training, it suggests that all doctors responsible for the placement of such lines should use ultrasound guidance rather than rely on anatomical landmarks alone.

Cardiac output measurement allows any treatment to be tailored to the aetiology of the cardiac failure. There are various invasive methods for the measurement of cardiac output and the subsequent calculation of derived values, such as systemic vascular resistance. The benefits of their use must be weighed against their risks. Non-invasive measurement may also be achieved by oesophageal Doppler probe insertion.

Respiratory monitoringPulse oximetry: clinical detection of cyanosis is poor. Pulse oximetry reduces the frequency of perioperative hypoxic events and should, whenever possible, be monitored before induction. It has technical and clinical limitations. The trace may be diffi-cult to obtain in the presence of hypothermia, vasoconstriction, movement or diathermy. Rapid atrial fibrillation can result in unstable measurements and the venous pulsation sometimes seen in right heart failure or tricuspid incompetence may produce errors because the pulsatile component does not originate only from the arterial inflow. Different types of haemoglobin affect the accuracy of pulse oximetry. The presence of carboxyhaemoglobin results in over-reading of the true level so that a hypoxic patient appears well saturated. Pulse oximetery is relatively insensitive to changes in arterial oxygen at the upper, flat end of the oxygen dissociation curve, where large falls in partial pressure produce only small changes in saturation. It is important to appreciate that oximetry measures only arterial oxygen saturation of haemoglobin and does not provide any information about delivery of oxygen to the tissues.

Capnography is considered the most valuable monitor by many anaesthetists. It provides a breath-by-breath analysis of inspired and expired carbon dioxide concentration and allows for the early detection of many adverse events. Its main value at induction is to aid the detection of accidental oesophageal intubation and therefore it should be available in anaesthetic rooms as well as in theatre. Its multiple uses are listed in Figure 2.

Regional blocks

The use of a regional analgesic block to supplement general anaesthesia is common, but whether it should be placed before or after induction remains controversial. Generally, patients prefer interventions to be carried out after they are unconscious and with poorly cooperative patients or children, this remains necessary. If the patient has painful fractures, correct positioning may be

Indications for invasive blood pressure monitoring

• Severe cardiac disease or instability

• Drug manipulation of the cardiovascular system by inotropic

therapy

• Cardiac or major vascular surgery

• Severe respiratory disease

• Thoracic surgery

• Major blood loss or fluid shifts

• Multiple trauma

• Intracranial procedures to ensure maintenance of cerebral

perfusion pressure

• Hypotensive anaesthesia

• Difficulty recording non-invasive pressure (e.g. extensive burns,

morbid obesity)

• To permit frequent blood gas analysis

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impossible before induction, but in most cases, an awake patient able to cooperate is an advantage. For an anaesthetist working alone, a block before induction avoids the added responsibility of monitoring an unconscious patient while trying to perform the procedure. Most importantly, the potential for undetected nerve damage is greater when the patient is unconscious and in the case of epidurals and spinals most anaesthetists prefer to perform these blocks in conscious patients.

Induction methods

Induction is usually achieved by the intravenous or inhalational route. Rarely, intramuscular induction with ketamine can be used for uncooperative patients and rectal induction has been carried out in children.

Inhalational inductionInhalational induction is useful in children or adults with needle phobia, because it does not require intravenous cannulation while the patient is awake. However, it can be slow and often includes an agitated phase when there is a higher chance of laryngospasm. The main risks are those of having an unconscious patient, without

intravenous access, who does not yet have a secured airway. Many anaesthetists ask for a colleague’s assistance at a gas induction so that a second pair of hands is readily available to site an intra-venous cannula rapidly. Gas inductions are also valuable (with prior intravenous access) in patients in whom airway difficulties are anticipated. Spon taneous breathing is maintained until the anaesthetist is con-fident that they can control the airway and oxygenate the patient. It is especially useful if there is partial upper airway obstruction, because other techniques, particularly fibre-optic intubation, risk precipitating complete obstruction. Sevoflurane is the usual agent of choice, being well tolerated at high concentrations and less arrhythmogenic than halothane. A tight-fitting mask speeds induction, but for small children the initial use of a cupped hand may be less threatening. Some anaes-thetists use a mixture of nitrous oxide and oxygen first, because it often renders children sleepy enough not to resist when the smell of the volatile agent becomes apparent. Use of nitrous oxide also speeds induction, because its rapid absorption has the effect of increasing the alveolar concentration of the volatile agent (second gas effect.) Other anaesthetists prefer to induce in 100% oxygen, which although slower, has a wider margin of safety if airway difficulties are encountered. Intubation is possible under deep inhalational anaesthesia alone but in most cases it is usually best to gain intravenous access before any airway manipulation.

Intravenous inductionIntravenous induction is much faster, with rapid passage through the excitement phase. It is usually achieved with the combination of an opioid to suppress laryngeal reflexes, plus careful titration of an induction agent. For adults, it is normally more pleasant than inhalational induction. Dose reduction is necessary if patients are elderly, frail or premedicated and slow administration is vital in those with delayed circulation times. Induction agents vary in their pharmacological properties, with the difference in their dura-tion determined by the extent to which they are metabolized or redistributed. The main contraindication to intravenous induction is the presence of airway obstruction, because all agents carry a high chance of producing apnoea in a patient whose airway might then be difficult to maintain. Propofol produces a smooth and rapid induction although it commonly causes apnoea and a fall in blood pressure. It has no active metabolites and is therefore associated with a rapid recov-ery and minimal residual effects. Its depression of the laryngeal reflexes is particularly useful in any subsequent airway manipula-tion, especially the insertion of a laryngeal mask. It causes pain on injection, especially into small veins and remains unlicensed for use in children under 3 years of age. Thiopental induces unconsciousness in one arm–brain circula-tion. Its duration of action is governed by redistribution away from vessel-rich tissues, rather than metabolism, and up to 30% of the drug may still be present 24 hours after administration. It produces a dose-dependent depression of blood pressure and respiration but laryngospasm is more frequent than with propofol. It reduces intracranial pressure and is a highly effective anticonvulsant. It has been suggested that thiopental is a pragmatic choice for epi-leptic patients who have regained their driving licence, because it provides the best possible protection against a convulsion.

Uses of capnography

Respiratory• Breath-by-breath estimation of CO2 concentration

• Measurement of respiratory rate

• Aids recognition of oesophageal intubation

• Demonstrates airway obstruction or bronchospasm

• Identifies apnoea or ventilator failure

• Detects rebreathing

• Indicates onset of spontaneous breathing or decline of

neuromuscular blockade

• Allows ventilation to normocapnia during the procedure

Cardiovascular• Indicates changes in blood flow to the lungs (e.g. reduced

cardiac output, hypovolaemia, falling blood pressure)

• Detects changes in blood flow through the lungs (e.g. thrombus,

air or fat emboli)

Metabolic changes• Increased CO2 production (malignant hyperthermia, seizures)

• Decreased CO2 production (hypothermia, muscle paralysis)

• Effect of tourniquet release and reperfusion

• Absorption of CO2 from the peritoneum at laparoscopy

Anaesthetic machine• Detects breathing system disconnection

• Identifies inadequate fresh gas flow

• Shows when soda lime for CO2 absorption is exhausted

• Shows effects of increased dead space

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Etomidate demonstrates marked haemodynamic stability and remains popular for use in shocked or unstable patients. It is associ-ated with involuntary muscle movements, pain on injection and a high risk of postoperative nausea and vomiting. It interferes with adrenal steroid production and the normal rise in plasma cortisol levels seen after surgery is delayed by up to 6 hours after a single induction dose. Ketamine is traditionally used for induction in shocked patients or for field anaesthesia at sites of major trauma. The patient’s heart rate and blood pressure rise so that cardiac output is maintained. In addition, ketamine produces bronchodilation and intense analgesia. It preserves laryngeal reflexes but airway obstruction and laryngospasm can still occur. Limitations on its use are a result of the associated salivation, hallucinations and emergencephenomena. Benzodiazepines – midazolam is sometimes used as an induc-tion agent in the elderly or in combination with a high-dose opioid technique.

Rapid-sequence induction is used to reduce the risks of aspiration in patients who have a potentially full stomach. Preoxygenation is mandatory, using 100% oxygen for 3 minutes of tidal volume breathing or, in an emergency, three vital capacity breaths. The aim is to replace air in the resting volume of the lungs (functional residual capacity) with oxygen. The oxygen then acts as a reservoir so that manual inflation of the lungs is avoided before intubation. Until the airway is protected by the cuffed tracheal tube cricoid pressure is used to compress the oesophagus between the trachea and vertebral column, to prevent aspiration of gastric contents. It must be relaxed or released if it interferes with the view at laryn-goscopy or the passage of the tube. Induction is traditionally with thiopental but both propofol and etomidate are used. Suxamethonium is the muscle relax-ant of choice owing to its rapid onset and short duration of action. It can cause severe postoperative muscular pains and iscontraindicated if there is a history of allergy, suxamethonium apnoea or a susceptibility to malignant hyperthermia. It is contra-indicated in muscular dystrophies and must be used with extreme caution in muscle trauma, spinal cord injury or burns of more than a few days’ duration, when it may result in a dangerous rise in serum potassium. Rocuronium is often used as the alternative, because, with high doses, rapid intubation is possible, but its duration of action prevents the onset of spontaneous breathing in time to be of any help at a failed intubation.

Cervical spine injury: until a patient has had their spine cleared clinically and radiologically, intubation must be performed using manual-in-line stabilization of the spine and bimanual cricoid pressure.

Airway management

The most hazardous period of induction occurs between rendering a patient unconscious and successfully establishing an adequate, patent airway. The use of laryngeal masks has revolutionized intraoperative airway management, but has reduced experience in tracheal intubation. When intubation is planned, it is sensible to confirm that the lungs can be adequately ventilated with a bag and mask before administration of a paralysing agent.

No preoperative assessment of the airway reliably predicts difficulty and it is therefore essential that anaesthetists have an immediate strategy for dealing with an unexpected airway problem. Difficulty in obtaining a good view of the laryngeal inlet is relatively common, but failure to intubate is much more unusual and failure to ventilate with a bag and mask is rare. Initial management should ensure optimal positioning, followed by the use of a gum elastic bougie, a different laryngoscope blade and external manipulation of the larynx in an attempt to improve the view. However, the overwhelming clinical imperative is to oxygenate the lungs, not to intubate the patient. The quality of the airway may deteriorate with multiple attempts and a decision should be made to abandon intubation before this is the case. The failed intubation drill must then be followed. If oxygenation cannot be achieved by other means, transtracheal ventilation via a cricothyroidotomy should be used early and not deferred until the patient is moribund.

Anaesthesia outside the operating theatre

Many of the most difficult, high-risk inductions are carried out in the accident and emergency department. Patients who require emergency airway management are not usually able to give any history and afford little time for assessment or preparation. Patients may be agitated, have suspected cervical spine injuries and, almost inevitably, a full stomach. Changing patterns of work are making it more difficult to guarantee the immediate provision of a suitably experienced anaesthetist. Increasingly, emergency physicians are managing these patients, as they do in the USA and Australia. Anaesthetists may be asked to intervene only in particularly challenging situations or when the non-anaesthetist has already failed to intubate. It is also common for anaesthetists to induce patients in sites such as radiology departments, obstetric units or isolated theatres, where immediate back-up is not available. The most important considerations are the presence of an appropri-ately qualified anaesthetic assistant at these sites, the availability of familiar, well-maintained equipment and the doctor’s ability to recognize, before induction, when a situation is beyond their experience.

FURTHER READINGAssociation of Anaesthetists of Great Britain and Ireland (AAGBI).

Recommendations for standards of monitoring during anaesthesia

and recovery. 3rd ed. London: AAGBI, 2000. www.aagbi.org/pdf/

absolute.pdf

Association of Anaesthetists of Great Britain and Ireland (AAGBI).

Checklist for anaesthetic apparatus. London: AAGBI, 1997.

www.aagbi.org/pdf/checkbkd.pdf


Induction of anaesthesia

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