A comparison of the effects on bispectral index of mild vs. moderate hypothermia during cardiopulmonary bypass

Abstract

Background and objective: Bispectral Index correlates with depth of hypnosis in adult patients. Bispectral index monitoring has been separately examined and validated during both mild and moderate hypothermic cardiopulmonary bypass. However, the effect on bispectral index of these two differing cardiopulmonary bypass temperature regimes has not previously been compared, particularly with the use of volatile anaesthetic agents.

Methods: Following Institutional Ethics Committee approval and informed consent, 30 patients undergoing coronary artery bypass grafting had additional bispectral index monitoring. Sixteen patients had mild hypothermic bypass (32-34 degrees C), and 14 had moderate hypothermic bypass (28-30 degrees C). Anaesthesia was standardized, and was maintained with enflurane throughout surgery. Scores were recorded at 14 specific event-related time points during surgery.

Results: This study has demonstrated that in this adult population, a relationship exists between bispectral index score and temperature throughout the duration of cardiac surgery, which included mild or moderate hypothermic cardiopulmonary bypass. It also demonstrated that patients which were categorized as having received moderately hypothermic cardiopulmonary bypass had lower bispectral index scores than those patients who received mild hypothermic cardiopulmonary bypass, at time points corresponding approximately with median steady-state bypass temperature, pre-re-warming and early re-warming periods.

Conclusions: Moderate hypothermic cardiopulmonary bypass has lower bispectral index scores when compared to mild hypothermic cardiopulmonary bypass, at a steady state of inspired enflurane. This may reflect an actual increase in the depth of hypnosis. However, as this study did not address the physico-chemical changes of the volatile agent or the accuracy of the bispectral index algorithm at lower temperatures, we would not advocate a change in practice based solely on this report.