Elimination of Nitrous Oxide Accelerates Elimination of Halothane

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Abstract
The effect of N2O on the elimination of halothane was studied in 10 patients ranging in age from 20-50 yr. After establishing a stable baseline (inspired halothane concentration: 0.85%, end-tidal halothane concentration: 0.75%), halothane administration was stopped and the rate of decrease in alveolar concentration of halothane (FE/FE0, FE: measured end-tidal concentration of halothane; FE0: the endtidal concentration immediately preceding the cessation of halothane administration) was measured continuously. The rate of decrease in FE/FE0 was more rapid when N2O (70%) is discontinued abruptly and replaced by the same concentration of N (part 2) than when the N2O is continued (part 1). At 1.5 min after the cessation of halothane administration, FE/FE0 was 0.38 .+-. 0.05 (mean .+-. SD) in part 2 and 0.45 .+-. 0.04 in part 1 (P < 0.01). In part 2, the fall in the alveolar CO2 from 4.27 .+-. 0.01% to 4.16 .+-. 0.01% at 1.5 min and an increase in the mean expired tidal volume from 522 .+-. 39 ml to 557 .+-. 29 ml. Thus, the elimination of N2O accelerates the elimination of halothane both by dilution and by an increased expired ventilation.

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