Sampled gas need not be returned during low-flow anesthesia

Abstract

Objective. The purpose of this investigation was to study the N2 flux between the patient and the breathing circuit, and the excess gas during N2O anesthesia with the low, fresh gas flow technique. Methods. Forty patients were studied. After a 6-minute high, fresh gas flow denitrogenation period, the O2 fresh gas flow was set at about 4 ml/kg/min and the N2O fresh gas flow was set to maintain an inspired O2 fraction of 0.30. The excess gas flow and N2 excretion were measured by a variant of the Douglas bag method. Results. The mean inspired N2 concentration reached a peak of 5.9% at 40 minutes. The estimated mean N2 excretion was 39 ml/min at 10 minutes, declining to 18 ml/min at 60 minutes. A calculation of N2 homeostasis during closed-circuit anesthesia based on the results of the patient study indicated that sampling for gas analysis actually reduces the gas costs if the sampled gas is scavenged instead of returned to the circle system, since intermittent flushing with high, fresh gas flow for denitrogenation is unnecessary in the former situation. Conclusions. Regardless of the fresh gas flow used, sampled gas need not be returned during N2O anesthesia. © 1993 Little, Brown and Company.