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  • Print publication year: 2009
  • Online publication date: May 2010

2 - Induction of anaesthesia

from Section 1 - Clinical anaesthesia

Summary

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).

Intravenous induction

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.

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References and further reading
,Association of Anaesthetists of Great Britain and Ireland. Suspected Anaphylactic Reactions Associated with Anaesthesia, revised edn. London: AAGBI, 2003. www.aagbi.org.
Cormack, RS, Lehane, J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984; 39: 1105–11.
Gataure, PS, Hughes, JA. The laryngeal mask airway in obstetrical anaesthesia. Can J Anaesth 1995; 42: 130–3.
McCoy, EP, Mirakhur, RK. The levering laryngoscope. Anaesthesia 1993; 48: 516–19.
Mendelson, CL. Aspiration of stomach contents into lungs during obstetric anaesthesia. Am J Obstet Gynecol 1946: 52: 191–205.
,Confidential Enquiry into Maternal and Child Health. Saving Mothers' Lives: Reviewing Maternal Deaths to Make Motherhood Safer, 2003–2005. London: CEMACH, 2007. www.cemach.org.uk.
Sellick, BA. Cricoid pressure to control regurgitation of gastric contents during induction of anaesthesia. Lancet 1961; 2: 404–5.