Lateralisation of the BIS Sensor during Carotid Endarterectomy Does it Really Matter?

Bispectral index (BIS) monitoring is commonly used to provide a measure of depth during general anaesthesia in attempt to reduce the rare occurrence of awareness [1,2]. The Bispectral Index is a generated number between 0 and 100 that is derived through utilization of a number of advanced EEG signal processing techniques including bispectral analysis, power spectral analysis, and time domain analysis [3]. The recommended range to reduce awareness is 40‐60 arbitrary BIS units. During carotid endarterectomy, clamping of the carotid artery may alter cerebral perfusion resulting in ischaemia with altered EEG activity [4]. When monitored using a continuous EEG changes are commonly observed bilaterally, however asymmetrical changes are often seen [3]. Electrocephalogram (EEG) changes during endarterectomy have shown global but not hemispheric changes which was correlated with systolic blood pressure variability during clamping [5]. Bonhomme et al. [6] recently investigated the use of BIS monitoring during carotid cross clamping with the sensor placed on the operative side of the forehead. They demonstrated that clamping of the carotid artery was associated with variable changes in BIS (increase, decrease, or no change at all) and suggested caution when interpreting BIS values during carotid cross‐clamping.