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.