Methods of induction
The objective of modern anaesthesia is to rapidly obtain a state of unconsciousness, to maintain this state, and then to achieve a rapid recovery. For any anaesthetic agent to be effective, whether administered intravenously or by inhalation, it must achieve a sufficient concentration within the central nervous system. Inhalational anaesthetic agents are administered by concentration rather than dose, and as the concentration delivered rapidly equilibrates between alveoli, blood and brain, this allows a way of quantifying the anaesthetic effect for each agent. The minimum alveolar concentration (MAC) is defined as that concentration of anaesthetic agent that will prevent reflex response to a skin incision in 50% of a population. MAC is, therefore, an easily defined measure of depth of anaesthesia. Intravenous induction agents are, in contrast, administered by dose rather than concentration. To administer a ‘sleep dose’ of an induction agent requires an assessment of the likely response from an individual and knowledge of the pharmacokinetics and pharmacodynamics of the particular agent used. Figures IA1 and IA2 summarise the main advantages and disadvantages of each route of administration. Regardless of the technique used, minimal standards of monitoring must be commenced prior to induction and maintained throughout anaesthesia (see Section 1, Chapter 3 for more details).
The intravenous route is the most common method of induction, allowing delivery of a bolus of drug to the brain, which results in rapid loss of consciousness.