The pharmacology of drugs used in general anaesthesia

Abstract

INTRODUCTION WHY ARE SO MANY DRUGS INVOLVED IN ANAESTHESIA? In the early days of the development of anaesthesia, a single anaesthetic agent such as ether or chloroform was used throughout the operation. The agent would be used to both induce and maintain anaesthesia and would be administered in most types of surgery. This situation was complicated by the side effects of the anaesthetic agents, which would be dangerous if large doses were used. As the development of new anaesthetic agents progressed, it became apparent that it was possible to use several agents to provide anaesthesia, thus limiting the dose of any single agent. This balanced anaesthesia is based on the use of several agents, each of which is given to achieve a particular component of the desired anaesthetic effect. WHAT IS THE TRIAD OF ANAESTHESIA? The concept of a ‘triad’ of anaesthesia was developed to describe the three basic requirements of an anaesthetic that must be achieved during anaesthesia to ensure a successful outcome. These three conditions are: Hypnosis. Analgesia. Relaxation and are provided by the use of a ‘balanced’ combination of anaesthetic and other agents. Hypnosis is the general term used in this context to describe alterations in the patient's consciousness. In the case of general anaesthesia, this obviously means the patient is rendered unconscious. During operations performed under local anaesthesia, however, a sedative may be given to produce a state of drowsiness and this effect is also included under the general ‘hypnosis’ part of the triad. The analgesia component of the triad has two related meanings. It refers to the use of drugs and other techniques that ensure the patient recovers with as little pain as possible and also to the suppression of physiological reflexes that occur following surgical stimulation. A penetrating wound, such as a surgical incision, would cause a complex series of physiological responses if made in a conscious subject, including a dramatic increase in heart rate and blood pressure, hyperventilation, sweating and vomiting. The ‘analgesia’ component of the triad comprises those drugs and manoeuvres designed to limit these physiological responses to surgical stimulation. In many situations, this reflex suppression is achieved by the use of powerful narcotic analgesics.