Our understanding of the mechanisms involved in the action of general anaesthetics has increased considerably in recent times and is discussed below. This is followed by sections discussing intravenous and inhaled anaesthetic agents.

Mechanisms of general anaesthetic action

Any mechanism of general anaesthetic action must be able to explain: loss of conscious awareness, loss of response to noxious stimuli (anti-nociceptive effect) and perhaps most important of all, reversibility.

Anatomical sites of action

General anaesthetic agents affect both brain and spinal cord to account for physiological responses to nociception, loss of consciousness and inhibition of explicit memory. Auditory and sensory evoked potential data implicate the thalamus as the most likely primary target, but secondary sites such as the limbic system (associated with memory) and certain cortical areas are also important. Halogenated volatile anaesthetics appear to have a greater influence on spinal cord than do the intravenous agents.

Molecular theories

At the beginning of the 19th century, Overton and Meyer independently described the linear correlation between the lipid solubility of anaesthetic agents and their potency (Figure 8.1). This correlation was so impressive, given the great variation in structure of these agents, that it suggested a non-specific mechanism of action based on this physicochemical property. Later interpretation pointed out that any highly lipophilic area was a potential site of action, with cell membranes being the most likely contender given the high concentration of lipids.