Middle-latency auditory evoked responses during open-heart surgery with hypothermia.

Middle-latency auditory evoked responses (MLRs) were recorded from infants and adults before and during open heart surgery. Hypothermia was induced through perfusion cooling by cardiopulmonary bypass. In infants deep hypothermia (to 15 degrees C nasopharyngeal temp.) was often followed by the induction of total circulatory arrest. In adults nasopharyngeal temperatures of 25 degrees C were reached. The MLRs were elicited by unfiltered clicks presented through an insert type earphone and recorded with a vertex to ipsilateral earlobe electrode configuration. The MLRs proved to be resistant to muscle relaxation induced by pancuronium and to anesthesia induced and maintained in most cases by fentanyl. In most cases MLR peak latencies were progressively delayed as temperature decreased. Hypotension resulted in decreased MLR (Pa) amplitude.