Several different types of drug produce anaesthesia. The aim is to provide a pleasant induction and lack of awareness for the patient, using a technique that is safe and that provides good operating conditions. Unfortunately, the
ideal anaesthetic drug with all the desired qualities does not exist. It is common practice, therefore, to combine several drugs, each of which provides a single component of anaesthesia. This can be represented diagrammatically as a triangle whose corners represent sleep (unconsciousness), muscular relaxation, and analgesia (lack of response to painful stimulation) (Figure 14.1).

Certain drugs, such as thiopental, produce unconsciousness without relaxation or analgesia and are suitable only for inducing anaesthesia. In contrast, ether produces a mixture of sleep, analgesia and relaxation but, because of its pungent smell and high solubility in blood, it is rather inconvenient and slow (though safe) for induction of anaesthesia.

The muscle relaxants produce muscular relaxation alone and may therefore be used to provide good surgical relaxation during light anaesthesia, allowing the patient to recover rapidly at the end of anaesthesia.

Opiate drugs, such as morphine and pethidine, produce analgesia with little change in muscle tone or level of consciousness. The choice of the most suitable combination for any given patient and operation calls for careful thought and planning.