Induction of anaesthesia
Intravenous injection is most common in contemporary practice, although the recent introduction of nonpungent sevoflurane has led to renewed use of inhalational induction. Inhalational induction is useful in young children, or ‘needle phobic’ adults, and may also be used in patients who are at riskof airway obstruction or pulmonary aspiration (of, for example, blood) when the patient is put into the lateral position with head down tilt to drain the fluid away from the trachea. For intravenous induction, propofol with its rapid recovery is replacing the long-standing barbiturate agent, thiopentone. Analgesic agents are frequently also injected at the time of anaesthetic induction.
Maintenance of anaesthesia
Following the induction of anaesthesia, inhalational volatile or intravenous anaesthetic agents are continuously administered to maintain an adequate depth of anaesthesia. Adding nitrous oxide contributes analgesic and weak anaesthetic effects, which reduce the concentration of volatile anaesthetic agent required for maintenance. To provide a safety margin, at least 30 per cent oxygen is added to the inspired mixture. Although still employed in some parts of the world, ether has generally been replaced by halothane, enflurane and isoflurane. Desflurane and sevoflurane are the most recently introduced agents, conferring the advantages of fewer side effects and more rapid recovery.
The use of nitrous oxide is slowly waning, as oxygen-enriched air with volatile or intravenous maintenance gains popularity.
If compressed sources of oxygen, nitrous oxide or air are scarce, then air may be drawn into the anaesthetic circuit, either by the (unparalysed) patient’s own respiratory effort or by a mechanical ventilator.
Total intravenous anaesthesia, a technique undergoing contemporary resurgence, avoids the use of inhalational anaesthetic agents and is claimed to provide enhanced quality and rapidity of recovery. It is also used when inhalational anaesthesia may be impractical, such as during airway laser surgery or endoscopy, and is popular for cardiopulmonary bypass. It is also indicated in spinal surgery during neurophysiological monitoring of cord integrity, as evoked potential signals are suppressed by inhalational anaesthesia. Intravenous anaesthesia avoids atmospheric pollution, and is usually conducted by infusing propofol and a short acting opioid analgesic agent, such as fentanyl or alfentanil, in combination with neuromuscular block and pulmonary ventilation with a mixture of air and oxygen.
Management of the airway during anaesthesia
General anaesthesia reduces the tone of the muscles required to preserve airway patency, and hence the need for manual methods (e.g. jaw thrust), or devices such as the Guedel or laryngeal mask airways, or endotracheal tubes.
Sir Ivan Magill developed the endotracheal tube during World War I to facilitate plastic surgery around the mouthwithout a face mask. The addition of a cuff to the tube allowed a seal of the trachea to protect the lungs from aspiration of blood or secretions, and later mechanical positive pressure pulmonary ventilation.
The following means of airway control in the anaesthetised or unconscious patient are used .
Positioning of the tongue and jaw
The anaesthetist thrusts the jaw forward, from behind the temporomandibular joints, thereby elevating the tongue off the posterior pharyngeal wall, which may also be achieved by inserting an artificial oropharyngeal airway such as the ‘Guedel’. The anaesthetic gases are given through a face mask.
The laryngeal mask airway (LMA) (developed by Archie Brain) is also inserted via the mouth, and is positioned with the mask over the larynx, sealed by an inflatable cuff. It frees the anaesthetist’s hands from holding the patient’s jaw or face mask. Its placement is less stimulating than endotracheal intubation. It has proved to be a reliable means of maintaining a patent airway, and is a technique readily taught to nonanaesthetists for emergency airway management. It is likely to replace the face mask for immediate care prior to endotracheal intubation.
The endotracheal tube may be passed into the trachea via either the mouth or the nose. It is usually placed by direct laryngoscopy, using a laryngoscope, but it is occasionally impossible to visualise the larynx. A fibreoptic technique may be used in which the tracheal tube is ‘rail-roaded’ over the flexible laryngoscope, once the tip has been steered into the trachea.
A cuffed endotracheal tube is used to facilitate artificial ventilation or surgery around the face or airway, and to protect the lungs if there is a risk of pulmonary aspiration. If fluid may collect in the mouth from above (as in nasal surgery), a throat pack is placed in the oropharynx.
Although endotracheal intubation is generally straight forward, complications do occur:
• accidental and unrecognised oesophageal intubation;
• accidental intubation of a main bronchus;
• trauma to larynx, trachea or teeth;
• aspiration of vomitus during neuromuscular blockade for intubation;
• failure to intubate and loss of airway control;
•disconnection or blockage of the tube;
• delayed tracheal stenosis, in children or after prolonged intubation.
Careful observation of physical signs and constant vigilance, aided by pulse oximetry, capnography of the expiratory gases, inspiratory oxygen concentration measurement and ventilator disconnection alarms are mandatory to minimise these risks.
Anaesthesia can safely be conducted through a tracheostomy tube but it should have an inflatable cuff for airway control. Silver or fenestrated tracheostomy tubes should therefore be replaced by plastic cuffed tubes at induction of anaesthesia.
In pulmonary and open oesophageal surgery, selective intubation of either bronchus is usual to facilitate deflation of the lung on the operated side. Its use is essential to protect the normal lung in the presence of a bronchopleural fistula.
Ventilation through a bronchoscope
The lungs can be ventilated during bronchoscopy by intermittent jets of oxygen down a cannula within the bronchoscope. The oxygen entrains air by the Venturi effect to generate enough pressure and flow to inflate the lungs.
The technique demands constant observation of the patient’s chest movement.
Neuromuscular blockade during surgery
Pharmacological blockade of neuromuscular transmission provides relaxation of muscles to facilitate surgery and mechanical positive pressure ventilation. Muscle tone may also be reduced by very deep anaesthesia, but may compromise the circulation. Neuromuscular blockade demands complete control of the airway and ventilation by the anaesthetist. The depolarising muscle relaxant, suxamethonium, rapidly provides excellent intubating conditions of brief duration, but commonly causes postoperative diffuse muscle pains, and rarely may cause a prolonged block if the patient is deficient in plasma pseudocholinesterase.
The competitive neuromuscular blocking agents such as curare and its modern successors produce a prolonged effect which may persist into the postoperative period. Atracurium, cisatracurium, vecuronium and rocuronium share more predictable activity profiles and are less dependent on hepato renal function. A peripheral nerve stimulator is also used to check for adequate depth of blockade during surgery, and to confirm satisfactory recovery of neuromuscular function prior to extubation of the trachea. The advent of neuromuscular blockade in the 1940s facilitated many advances in abdominal and thoracic surgery, but introduced the risk of accidental patient awareness during surgery.
Haemostasis and blood pressure control
Although the dangers of profound hypotension are nowadays well accepted, a 20—3 0 per cent reduction of mean arterial blood pressure from the awake preoperative level in fit patients is still deemed acceptable, and can greatly improve the quality of the operative field and reduce total blood loss. Reduction of venous pressure at the wound by correct patient positioning and avoidance of any causes of venous obstruction (Fig. 6.2), and maintenance of satisfactorily deep anaesthesia and slightly reduced arterial carbon dioxide tension, further contribute to providing a dry surgical field.
Hypotensive drugs may be used to produce deliberate controlled hypotension if there is a clear surgical benefit to be obtained, although preservation of cerebral perfusion and oxygenation remains paramount. The surgeon must be aware of the prevailing blood pressure, particularly at the time of ensuring satisfactory haemostasis prior to wound closure. Usually, the anaesthetist will attempt to bring the blood pressure back to the normal level at this stage of the procedure.
Management of temperature during anaesthesia
Hypothermia develops quickly during anaesthesia and surgery due to vasodilation, cold infusions of fluid, and loss of body heat by radiation and fluid evaporation from open body cavities. It is a particular hazard in children because of the high ratio of body surface area to body mass. .The elderly are also at particular risk as hypothermia and shivering increase oxygen consumption and vascular resistance, predisposing to myocardial infarction. In high-risk patients and for long operations, warm air blowers and warming blankets should be used, and fluids for intravenous infusion, or irrigation of body cavities or organs (such as the bladder and renal pelvis), should be warmed to body temperature. Careful intraoperative temperature control greatly reduces postoperative morbidity.
Monitoring during anaesthesia
Accurate monitoring of vital functions during anaesthesia is now regarded as obligatory in all parts of the world. Surgery should not be practised where proper facilities for monitoring and cardiopulmonary resuscitation are not available. The basic parameters monitored are inspiratory oxygen concentration, oxygen saturation by pulse oximetry, expiratory carbon dioxide tension measurement, blood pressure and the electrocardiogram. For major surgery, invasive, direct monitoring of the circulation is used, but the potential value of information gained must be weighed against the possible dangers of placing intra arterial or central venous or pulmonary artery catheters. Hourly observation of urine output via a urinary catheter is most helpful in assessing renal perfusion. Ventilators should all have airway pressure monitors and disconnection alarms.
In the UK, the Association of Anaesthetists recommends the following standards of routine monitoring in the anaesthetised patient (Fig. 6.3):
• the continuous presence of an adequately trained anaesthetist;
• regular blood pressure and heart rate measurements (recorded);
• continuous monitoring of the electrocardiography (ECG) throughout anaesthesia;
• continuous analysis of the oxygen content in the inspiratory gas mixture;
• oxygen supply failure alarm;
• ventilator disconnection alarm;
• pulse oximeter;
• capnography (measurement of end-tidal carbon dioxide content);
• temperature measurement availability;
• neuromuscular monitoring availability.
In the USA, spirometry during anaesthesia is regarded as necessary.
Recovery from general anaesthesia
Recovery from general anaesthesia should be closely supervised by trained nursing staff in an area equipped with the means of resuscitation and with adequate monitoring devices. An anaesthetist should be readily available. For the seriously ill patient, a high dependency unit or intensive care unit may be necessary until the patient’s condition is satisfactory and stable. The transition from tracheal intubation with ventilatory support to spontaneous breathing with an unprotected airway is a time of increased risk, when respiratory arrest or obstruction may occur.
The common causes of failure to breathe after general anaesthesia are:
• obstruction of the airway;
• central sedation from opioid drugs or anaesthetic agents;
• hypoxia or hypercarbia of any cause;
• hypocarbia from mechanical overventilation;
• persistent neuromuscular blockade;
• pneumothorax from pleural damage during anaesthesia or surgery;
• circulatory failure leading to respiratory arrest.
Management of blood pressure in the recovery room
This may be due to hypovolaemia, prolonged vasodilation or myocardial depressant effects of anaesthetic, drugs, cardiac dysrhythmia or hypoxaemia. Management is by treatment of the cause.
Hypertension is common postoperatively, usually of brief duration and associated with peripheral vasoconstriction due to pain, fear, cold or shivering, or pre-existing hypertensive disease. Hypertension predisposes to cerebral and myocardial damage, and needs active management. If it persists in the presence of adequate pain relief, sublingual nifedipine and intravenous labetalol or hydralazine are useful. Rarely, control with more powerful intravenous drugs such as sodium nitroprusside or glyceryl trinitrate is necessary, in conjunction with direct intra-arterial blood pressure monitoring.
General anaesthesia for day-case surgery
Day-case management already accounts for about 40 per cent of procedures in UK hospitals and is intended to reach over 60 per cent. While the principles remain the same, it is even more necessary for the day-case patient to recover rapidly from general anaesthesia and mobilise with the minimum of side-effects. Longer and more complex operations are now conducted as day cases, demanding high-quality anaesthesia and effective analgesic strategies.
Careful selection of patients is essential with regard to coexisting diseases, the nature of the proposed surgery, the availability of a suitable escort and transport home, and domiciliary care. Well-controlled nondebilitating chronic diseases do not preclude day care, but may require a longer period of postoperative supervision before permitting discharge home.
General anaesthesia combined with regional anaesthesia, as for inguinal hernia repair, is often suitable. Anaesthetics which promote rapid recovery such as propofol, sevoflurane and desflurane are used. Drugs with prolonged depressant central action, including premedicant drugs, are avoided and, where possible, analgesics should act peripherally or be of
short duration if centrally acting. Where possible, patients are managed with a laryngeal mask or face mask, although endotracheal intubation is the necessary airway control for many day cases such as oral or ear, nose and throat (ENT) procedures, and is generally uncomplicated.
With the patient relatively isolated from immediate medical supervision and advice, postoperative analgesia must be carefully tailored to the procedure, especially in the case of the more painful procedures (such as hernia repair, haemorrhoidectomy, tubal surgery, meniscectomy) for which stronger analgesics and combinations should be provided. Regular postoperative dosing is recommended to avoid breakthrough pain, as may particularly occur following the initial benefits of local anaesthetic.
General anaesthesia and cardiopulmonary bypass
Anaesthetic agents and oxygen cannot be delivered to the circulation through the lungs when the lungs are bypassed, so all drugs are given directly into a vein or into the blood while it passes through the oxygenator.
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