7 - General anaesthetics

Summary

The general anaesthetics comprise a large group of compounds of diverse chemical structure which cause a loss of sensation and perception leading in adequate doses to a complete, but reversible loss of consciousness. Although they may cause an attenuation of pain sensation in subanaesthetic doses, this is not invariably so, and the anaesthetics as a class are distinct from the analgesic drugs which selectively reduce the sensation of pain but do not lead to a loss of consciousness at therapeutic concentrations.

The general anaesthetics depress function at all levels of the central nervous system and so may depress respiration, circulation, temperature control, voluntary reflex movements and pain, but the degree of depression of each of these functions is not the same with all agents, even at a similar depth of anaesthesia. It therefore follows that the characteristics of anaesthesia with different agents may vary.

Types of general anaesthetic

The chemical structures of a variety of different general anaesthetics are given in Fig. 7.1. The diversity of structures is readily apparent and must be taken into consideration in evaluating the mechanisms of action. The physical form of anaesthetics also varies from gaseous substances such as the inert gases, nitrous oxide and cyclopropane, or volatile substances such as ether, chloroform and halothane, to soluble substances typified by the barbiturates, eugenols, steroid anaesthetics and cyclohexylamine. To a large degree, the physical form and properties of the anaesthetic determine the manner in which the anaesthetic is used, and some of the advantages and disadvantages of each class.