Monitoring oxygenator expiratory isoflurane concentrations and the bispectral index to guide isoflurane requirements during cardiopulmonary bypass

Abstract

Objective: The purpose of this study was to measure the changes in isoflurane requirements during the rewarming phase of cardiopulmonary bypass with moderate hypothermia. Design: An observational study. Setting: University hospital, single center. Participants: Forty patients undergoing elective coronary artery bypass surgery with cardiopulmonary bypass. Interventions: Isoflurane requirements were quantified by measuring the concentrations in the oxygenator expiratory gas. Anesthesia was guided by bispectral index monitoring. Measurements and Main Results: Isoflurane concentrations required to maintain the bispectral index between 40 and 50 during the rewarming phase of cardiopulmonary bypass were measured. There was a progressive increase in expiratory isoflurane requirements during rewarming from 30°C to 37°C, with a Pearson correlation coefficient of 0.78. There was a significant difference in the concentration required at 30°C (0.41% ± 0.14%) compared with 37°C (1.00% ± 0.12%). Conclusion: Isoflurane requirements are reduced during hypothermic cardiopulmonary bypass. Monitoring anesthetic concentrations in the oxygenator expiratory gas may be a useful adjunct to monitoring the depth of anesthesia. © 2005 Elsevier Inc. All rights reserved.