21
The Practical Conduct of Anaesthesia
The conduct of anaesthesia is planned after details concerning the surgical procedure and the medical condition of the patient have been obtained at the preoperative visit. Preoperative assessment and selection of appropriate premedication are discussed in Chapter 17.
PREPARATION FOR ANAESTHESIA
The anaesthetic machine must be tested before use for leaks, misconnections and proper function. A checklist, e.g. that published by the Association of Anaesthetists of Great Britain and Ireland (AAGBI 2004), is recommended. This is discussed in Chapter 20. The breathing system to be used should be new for each patient, or a new filter of appropriate size for each patient should be placed between the patient and the system, according to the AAGBI recommendations (2008).
The availability and function of all anaesthetic equipment should be checked before starting (see Table 21.1). The anaesthetist should be satisfied that the correct operation is being performed upon the correct patient and that consent has been given. Surgical Safety checklists are available for all the theatre team. The patient must be on a tilting bed or trolley and the anaesthetist should have a competent, trained assistant.
TABLE 21.1
Equipment Required for Tracheal Intubation
Correct size of laryngoscope and spare (in case of light failure)
Tracheal tube of correct size + an alternative smaller size
Tracheal tube connector
Wire stilette
Gum elastic bougies
Magill forceps
Cuff-inflating syringe
Artery forceps
Securing tape or bandage
Catheter mount(s)
Local anaesthetic spray – 4% lidocaine
Cocaine spray/gel for nasal intubation
Tracheal tube lubricant
Throat packs
Anaesthetic breathing system and face masks – tested with O2 to ensure no leaks present
INDUCTION OF ANAESTHESIA
Anaesthesia is induced using one of the following techniques:
Inhalational Induction
The most common indications for inhalational induction of anaesthesia are listed in Table 21.2.
TABLE 21.2
Indications for Inhalational Induction
Young children
Upper airway obstruction, e.g. epiglottitis
Lower airway obstruction with foreign body
Bronchopleural fistula or empyema
No accessible veins
Intravenous Induction
Doses of the common i.v. agents are shown in Table 21.3. The induction dose varies with the patient’s weight, age, state of nutrition, circulatory status, pre-medication and any concurrent medication. A small test dose is commonly administered and its effects are observed. Slow injection is recommended in the aged and in those with a slow circulation time (e.g. shock, hypovolaemia, cardiovascular disease) while the effects of the drug on the cardiovascular and respiratory systems are assessed.
TABLE 21.3
Agent | Induction Dose (mg kg–1) |
Thiopental | 3–5 |
Etomidate | 0.3 |
Propofol | 1.5–2.5 |
Ketamine | 2 |
A rapid-sequence induction technique is indicated for patients undergoing emergency surgery and for those with potential for vomiting or regurgitation. After i.v. induction, a rapid transition to stage 3 anaesthesia (see below) is achieved; this is maintained by the introduction of an inhalational agent or by repeated bolus injections or a continuous infusion of an i.v. anaesthetic agent. Emergency anaesthesia is discussed fully in Chapter 37.
Complications and Difficulties
Histamine release. Thiopental in particular may cause release of histamine with subsequent formation of typical wheals. Severe reactions may occur to individual agents, and appropriate drugs and fluids should be available in the anaesthetic room for treatment. Guidelines for emergency management of acute major anaphylaxis are available (AAGBI) and may be displayed in the anaesthetic room. This is discussed further in Chapter 43.
POSITION OF PATIENT FOR SURGERY
Some commonly used positions are shown in Figure 21.1. Each may have adverse effects in terms of skeletal, neurological, ventilatory and circulatory effects.
FIGURE 21.1 Positions on the operating table. (A) Lithotomy position. (B) Lateral position. (C) Prone position. (D) Trendelenburg position.
The supine position carries the risk of the supine hypotensive syndrome during pregnancy (see Ch 35) or in patients with a large abdominal mass.
Positioning during anaesthesia is discussed extensively by Martin & Warner (1997).
MAINTENANCE OF ANAESTHESIA
Inhalational Anaesthesia with Spontaneous Ventilation
Minimum Alveolar Concentration
Minimum alveolar concentration (MAC) is the minimum alveolar concentration of an inhaled anaesthetic agent which prevents reflex movement in response to surgical incision in 50% of subjects. MAC values of commonly used inhalational agents are shown in Appendix C. MAC varies little with metabolic factors but is reduced by opioid medication and in the presence of hypothermia. MAC is higher in neonates and is reduced in the elderly (see Ch 2). The effects of inhalational anaesthetics are additive: thus 1 MAC-equivalent could be achieved by producing an alveolar concentration of 70% nitrous oxide (0.67 MAC) and 0.4% isoflurane (0.33 MAC).
The rate at which MAC is attained may be increased by raising the inspired concentration and by avoidance of airway obstruction. Increasing ventilation at a constant inspired concentration produces more rapid equilibration between inspired and alveolar concentrations. The time taken for equilibration increases with the blood/gas solubility coefficient of the agent; those with a high blood/gas solubility coefficient (e.g. halothane) do not reach equilibrium for several hours (see Ch 2). It follows, therefore, that the inspired concentration must be considerably higher than MAC to produce an adequate alveolar concentration when such agents are used.
Signs of Anaesthesia
Guedel’s classic signs of anaesthesia are those seen in patients premedicated with morphine and atropine and breathing ether in air. The clinical signs associated with anaesthesia produced by other inhalational agents follow a similar course, but the divisions between the stages and planes are less precise (Fig. 21.2).
Stage 1: the stage of analgesia. This is the stage attained when using nitrous oxide 50% in oxygen, as used in the technique of relative analgesia (see Ch 29).
Complications and Difficulties
Malignant hyperthermia. Volatile agents, succinylcholine or amide-type local anaesthetic agents may trigger this syndrome in susceptible individuals (see Ch 43).
Atmospheric pollution. The use of the appropriate scavenging apparatus helps to reduce levels of theatre pollution by volatile and gaseous agents (see Ch 20).