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 1.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.