Bispectral index monitoring in patients undergoing cardiac surgery under cardiopulmonary bypass: the influence of temperature changes should be considered

Abstract

EDITOR: We read the article by Drs Puri and Murthy reporting their results with bispectral index (BIS) monitoring during cardiac surgery [1] with great interest. We agree with the authors that BIS monitoring can be useful in guiding anaesthesia during cardiac surgery. However, there are some points we wish to comment on. Firstly, the authors compare the numbers of episodes of tachycardia and hypertension in the group managed with BIS monitoring and in the control group (without BIS monitoring) and find despite an equal number of patients experiencing haemodynamic disturbances that in the BIS managed group there were significantly less episodes of tachycardia and hypertension. However, the authors do not report on the left ventricular function, the ASA physical status nor on the use of preoperative drugs of these patients but give only values for the overall groups. It would be very interesting to compare the patients with such episodes with regard to these variables, since there are many factors causing haemodynamic instability during cardiac surgery apart from inadequate anaesthesia. Unfortunately, there are no data provided as to the BIS values before and during these haemodynamic disturbances, which probably would have shed some light on this question. Secondly, the authors present the hypothesis that BIS monitoring may be a tool to predict impending haemodynamic changes by providing additional information of the hypnotic state. However, several investigators have shown that BIS reflects the actual cortical state of the patient and is not suitable to anticipate an impending arousal [2]. The BIS value only reflects the state of consciousness of the patient averaged over a preceding time span [3]. Thirdly, it is suggested that 'the observed jump in the BIS values at the time of termination of cardiopulmonary bypass (CPB) may be due to the temperature rise in the brain which raises the anaesthetic requirement'. Unfortunately, there are no data provided with regard to the temperature changes in individual patients in both groups. It is well known that brain temperature affects the electroencephalogram (EEG) and, consequently, EEG-derived values like BIS [4,5]. It is crucial to state the brain temperature if one compares the BIS values of two groups of patients undergoing CPB under moderate hypothermia. Therefore, the rise in BIS after initiation and after termination of CPB in the control group should be interpreted with caution, and it should be considered that a comparison between groups is only valid if brain temperature were approximately the same. We also wondered if really only one anaesthesiologist supervised all procedures. As the authors admit, there is clearly a learning curve and the BIS monitor was shown to be an effective teaching tool. The very large standard deviation of BIS values at the end of bypass (56.4 ± 25) and the alarmingly high BIS value (82.8 ± 2) 5 min after termination of CPB in the control group suggest problems with controlling depth of anaesthesia at large rather than the inference that BIS monitoring is superior to conventional management. Taking these values into account, it is surprising, that only one patient in the control group suffered from explicit awareness. Finally, it would be of interest if the total amount of drugs delivered (minimal alveolar concentration in h, morphine in mg) was significantly different between groups. There are several studies reporting a reduced consumption of hypnotics during BIS monitoring [6]. The higher BIS values in the control group in the majority of time points during the study suggest to expect the opposite. B. Bein P. H. Tonner Department of Anaesthesiology and Intensive Care Medicine; University Hospital Schleswig-Holstein; Kiel, Germany