Anaesthetics

Abstract

Anaesthesia is a state of reversible unconsciousness that comprises some or all of the ‘triad of anaesthesia’— hypnosis, analgesia, and muscle relaxation. Safe and ef­fective anaesthesia requires information of the drug’s potency at effector sites and knowledge of administration concentrations, as well as an understanding of the degree of noxious stimulus and how a patient’s physiology may modulate drug actions. Historically, the first compounds used as anaesthetics were diethyl ether, nitrous oxide, and chloroform. Diethyl ether was demonstrated to the wider medical commu­nity in 1846 by William Morton in the removal of a jaw lump from Gilbert Abbot, and the introduction of chloro­form followed within the year. It was noted by James Simpson, Professor of Obstetrics in Edinburgh in 1847, that chloroform was much more potent, but had a ten­dency to precipitate death in the anxious and could cause severe liver damage. This tendency demonstrates clearly that the depth of anaesthesia is critical. Too much cir­culating drug can lead to respiratory depression, cardiac arrhythmias, and death, while too little permits persistent consciousness, pain, and muscular spasm. This is of particular concern with regards to laryngospasm, which when combined with an unsecured airway can rapidly lead to hypoxia and death. Nowadays, death is incredibly rare, with signs of hypotension, tachy- , or bradycardia detected early and easily reversed by controlling drug dosage. The risk of drug- induced side effects when using anaesthetic drugs means that the depth of anaesthesia must be closely monitored. This is achieved subjectively with experience and training, in combination with ob­jective clinical assessment, such as pulse, BP, and mean alveolar concentration. See Table 8.1 for ideal properties of anaesthetic agents. There are many approaches to the application of gen­eral anaesthesia, and these depend on clinical situation, depth, and length of anaesthesia required, the type of sur­gical or interventional procedure to be undertaken and as­sociated patient risk factors. The stages of anaesthesia (outlined in Table 8.2) was a concept introduced at a time when induction was rou­tinely achieved through the use of inhalational anaes­thetics. More recently the use of IV induction agents has meant that transition between these stages is smoother, resulting in a rapid induction with minimal excitation responses, compared with inhalation agents. Inhalation agents also carry the risk of airway irritation.