A Clinical Evaluation of Closed Circuit Anesthesia

Shin, Yang Sik; Kim, Jong Rae; Park, Kwang Won; Cho, Chung Hyun; Sohn, Yung Jai

Abstract

With the increased concern regarding air pollution in the operation theatre and the high cost of vapor anesthetics, closed circuit anesthesia has much to commend it. Since the demise of flammable anesthetics such as diethyl ether and cyclopropane, closed circuit anesthesia has been applied infrequently. Ernst and Lowe recently established a theoretic basis and reported quantitative data and a formula for closed circuit anesthesia. This involves frequent calculations to adjust the nitrous oxide flow and the vaporizer disls in accordance with the elapsed time. We attempted to evaluate their formula against our simplified method of adjusting only the N2O flow according to the N2O uptake rate calculated by Severinghaus. This method allows the output of the vapor from the plenum type vaporizer to diminish according to the decrease of N2O flow. Eighty patients undergoing closed circuit anesthesia were randomly divided into 4 groups of 20 patients each follows: Group l: Adjustment of the N2O flow and the halothane vaporizer dial by elapsed time, according to Ernst and Lowe. Group ll: Adjustment of the N2O flow and the enflurane vaporizer dial by elapsed time, according to Ernst and Lowe. Group lll: Adjustment of the N2O flow only and fixed dial setting on the halothane vaporizer, calculated from the measured end-tidal concentration at 16 min. Group lV: Adjustment of the N2O flow only and a fixed setting on the enflurane vaporizer, calculated from the measured end-tidal concentration at 16 min. The results were as follows: 1) In group l & ll, the inhalation anesthetic concentration decreased gradually and in 10 of 40 patients failed to maintain a satisfactory anesthetic condition after 50 minutes, requiring adjustment of the vaporizer dial setting. No failure occurred in groups lll & lV. 2) There were no abnormal values for the inspired oxygen fraction or the end-tidal CO2 tension at any time. 3) There were no significant differences between groups in the amount of a neuromuscular blocking agent, pancuronium, required. In conclusion, the method of closed circuit anesthesia when administer by adjusting only the flow of N20 and setting the vaporizer dial to a fixed point determined at 15 min may be convenient and efficacious. Attention to the patients safety can be maintained by the use of monitoring devices to measure the inspired oxygen fraction, the end-tidal CO2 tension and vital signs even without monitoring the anesthetic vapor concentration.