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.
Kennedy
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RR
Department of Anaesthesia, The Christchurch School of Medicine, Christchurch, New Zealand. ross.kennedy@chmeds.ac.nz
Baker
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AB
D018685:Anesthetics, Inhalation / Q000493:pharmacokinetics*; D002302:Cardiac Output; D004334:Drug Administration Schedule; D006801:Humans; D007530:Isoflurane / Q000493:pharmacokinetics; D008955:Models, Cardiovascular; D014018:Tissue Distribution
D018685:Anesthetics, Inhalation; D007530:Isoflurane