Use of isoflurane in closed circuit. Measurement of anesthetic uptake.

A totally closed circuit is described into which a liquid volatile anesthetic is injected by a servomechanism to maintain end-tidal anesthetic concentration at a constant preset level during positive pressure ventilation. This equipment has been used to measure the uptake of isoflurane, at an end-tidal concentration of 1.55% (1.3 MAC) in the first half hour of anesthesia in 26 patients undergoing elective surgery. The rate of uptake was found to be rapid initially but became constant after 15 minutes. Uptake varied considerably, the standard deviation of the rate of uptake being approximately one third of the mean at any one time. The total uptake of isoflurane at 30 minutes was correlated with 5 anthropometric variables, body surface area, body weight, body weight 3/4, fat free body mass and fat body mass. Significant (P less than .01) correlations were found with the first four of these, the best being with surface area (correlation coefficient 0.629). No anthropometric variable provided a correlation on which anesthetic dose could be reliably predicted and we therefore conclude that administration of isoflurane in a closed circuit is best controlled in response to direct in-circuit analysis. Methods based on anthropometric variables will be inaccurate.