Monitoring of multimodality evoked potentials during open heart surgery under hypothermia

Multimodality evoked responses (ERs) were monitored in 16 adults who had cardiac surgery under cardiopulmonary bypass and moderate hypothermia (19-25 degrees C). Cooling affected all sensory ERs by progressively increasing the latencies of the major components. The effect was more profound on the later than on the earlier ER components. Visual evoked responses (VERs) were most inconsistent and always disappeared at temperatures below 25 degrees C. The later components of the long latency somatosensory evoked responses (SERs) also attenuated or disappeared rather early during hypothermia. On the other hand, short latency SERs were more resistant to the effects of hypothermia. They were always recordable at temperatures of 25 degrees C or above; and usually persisted even at temperatures between 20 and 25 degrees C. Brain-stem auditory evoked responses (BAERs) were consistently present at temperatures above 25 degrees C, wave V was recordable in majority between 20 and 25 degrees C. All sensory ERs disappeared with severe hypothermia (20 degrees C or less) except the components generated more peripherally such as N10 of the short latency SERs. We feel that BAERs and short latency SERs may serve as useful intraoperative monitors of brain function during hypothermia.