Physical chemistry applied to anaesthetic action.

The "dose" of inhalational anaesthetics is commonly expressed in terms of volume per cent (vol/vol) of the agent in an inspired or end-tidal gas sample. However, the "dose" should be expressed more correctly as a partial pressure which can be calculated as the fractional concentration multiplied by the total pressure of the system (e.g. the partial pressure of oxygen in air=0.21x760 = 160 mm Hg). Thus each gas in a gas mixture behaves as if it alone occupied the total volume of the sample and "the pressure of the mixture is the sum of the partial pressures of the component gases" (Dalton's law of partial pressures). At first sight rhis distinction between anaesthetic concentration and partial pressure appears to be somewhat semantic. For example, there has been some debate (Fink, 1971; Eger, 1971) about replacing the term MAC (minimum alveolar concentration of anaesthetic preventing response to surgical stimulation in 50% of patients) with MAP (minimum alveolar pressure of anaesthetic). However, the concept of partial pressures becomes particularly important when a system at equilibrium contains more than one phase. For example, when anaesthetic dissolves in blood, the concentration of the agent in the blood and gas phases is usually different but, provided equilibrium has been attained, the partial pressure of the agent is the same throughout the system. The situation is more complicated, of course, in that blood itself is not a homogeneous system but consists of many phases, and even a macromolecule such as haemoglobin may b ; considered to consist of a number of