Effects of enflurane on brainstem auditory evoked responses in humans

The effects of enflurane anesthesia on brainstem auditory evoked responses (BAERs) was determined in 10 patients with normal hearing undergoing various surgical procedures. Arterial blood pressure, body temperature, and arterial Pco2 were controlled during the 2- to 5-hour recording sessions. End-tidal enflurane concentrations were continuously recorded on a chemetron Medspect II mass spectrometer in three subjects. BAERs were obtained by, and recorded on a Nicolet CA 1000, from C2 with reference to A1 or !2, with a 2000 click-averaging for each measurement. Enflurane administered at clinical concentrations (0.5% to 3%) produced a consistent changes in BAER latencies. The waves significantly affected (p less than 0.01) were waves III, IV, and V and interpeak latency I-V. The magnitude of theses changes was related to the concentration of enflurane and was magnified by temporarily decreasing the Paco2. These findings confirm similar data obtained in animals which have shown the same effects at doses that can produce generalized seizure activity. BAER analysis shows that changes predominate at the pons and midbrain levels and affect the brain stem conduction time, which likely reflects the action of enflurane on the activity of the reticular activating system.