The effect of cardiac output changes on end‐expired volatile anaesthetic concentrations – a theoretical study

Cardiac output is one of the major determinants of the rate of uptake, and therefore the end‐expired concentration ( F E′ ) of volatile anaesthetic agents. The purpose of this theoretical study was to explore the effect of cardiac output changes on F E′ for a range of volatile anaesthetics. A multicompartment model of anaesthetic uptake and distribution which produces constant values of F E′ was used. The minimum detectable change in cardiac output was determined for a variety of anaesthetic agents for four patterns of cardiac output change. The effect of a step change in cardiac output from 5 to 10 l.min −1 was also recorded. The smallest cardiac output changes (average 33%) were detected with isoflurane. As blood solubility increased or decreased, larger cardiac output changes were needed before they could be detected. With a large step change in cardiac output and with increasing solubility, the final change in F E′ increased but the initial rate of change of F E′ is decreased. A significant cardiac output change will produce a change in volatile anaesthetic uptake. An unexpected change in F E′ should be considered as a possible signal of a sudden cardiac output change. The difference between agents may represent a balance between the amount of agent taken up and the size of the tissue ‘sink’ for that agent.