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

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 degrees C to 37 degrees C, with a Pearson correlation coefficient of 0.78. There was a significant difference in the concentration required at 30 degrees C (0.41% +/- 0.14%) compared with 37 degrees 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.