Effect-site estimation of volatile anaesthetic agents: Beyond MAC fractions as a target for anaesthesia delivery

Minimum alveolar concentration (MAC) fractions provide targets for inhalational anaesthesia delivery. Routine measurement of end-tidal concentration has provided a valuable tool for guiding inhalational delivery for more than 20 years. Experimentally derived MAC values represent equilibrium values and are of less value during periods of rapid change. Using derived effect-site levels provides an insight into anaesthetic needs during these periods. As with all monitors and measurements an understanding of the underlying principles and limitations is important. The aim of this paper is to discuss the concept of effect-site guided delivery in place of end-tidal, demonstrate how effect-site values may be calculated in real time and discuss the limitations of this approach.