Effects of hypothermia on the cat brain-stem auditory evoked response

Effects of systemic hypothermia on the brain-stem auditory evoked responses (BAERs) in 4 pentobarbital anesthetized adult cats placed on total cardiopulmonary bypass were investigated. Hypothermia was achieved by slowly cooling the bypass blood through a heat exchanger. Serial BAERs were recorded at 2 min intervals as brain temperature was lowered from 37 to 22 degrees C and then rewarmed over a 1-2 h period. Temperatures were recorded from the brain, esophagus and rectum. Three effects were produced by controlled systemic hypothermia. First, latencies of each component wave (I-V) of the BAER increased exponentially as brain temperature was lowered to 19 degrees C. Latencies of earlier waves (I-III) increased less than those of the later waves (IV-V). Arrhenius plots of inverse latency (rate) versus reciprocal of the absolute temperature generated a family of straight lines of similar slope for each of the 5 component waves of the BAER. The activation energy for each of the 5 BAER waves was derived from the slope of the Arrhenius plot. The mean and standard deviation of the activation energy of all 5 waves was 9.7 +/- 0.5 kcal/mole degree C. The fact that the activation energy was similar for each BAER component wave (I-V) indicated that the increase in latencies for all 5 waves was governed by the same rate-limiting, temperature-dependent process(es). Second, the rise time and duration of each of the component waves of the BAER increased with decreasing temperature. Third, wave amplitudes increased from 37 to 32 degrees C in a quasiparabolic relation and then, decreased at approximately a linear rate. The BAER wave form completely disappeared below 20.3 degrees C. Slow rewarming of the brain to its initial temperature restored the BAER component waves to their original latencies and amplitudes.