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 agen ts. The purpose of this theoretical study was to explore the effect of card iac output changes on F-E' for a range of volatile anaesthetics. A multicom partment model of anaesthetic uptake and distribution which produces consta nt 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 ca rdiac output change. The effect of a step change in cardiac output from 5 t o 10 1.min(-1) was also recorded. The smallest cardiac output changes (aver age 33%) were detected with isoflurane. As blood solubility increased or de creased, larger cardiac output changes were needed before they could be det ected. With a large step change in cardiac output and with increasing solub ility, 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 chan ge in volatile anaesthetic uptake. An unexpected change in F-E' should be c onsidered as a possible signal of a sudden cardiac output change. The diffe rence between agents may represent a balance between the amount of agent ta ken up and the size of the tissue 'sink' for that agent.