Effect of hyperthermia on brain auditory evoked potentials in the conscious sheep

To assess the effect of hyperthermia on brain function of conscious sheep, auditory evoked potentials (AEPs) were studied. Auditory brain-stem potentials (BAEPs) and mid-latency potentials (MLPs) to monaural rarefaction click stimuli were recorded as the potential difference between midline skull screws and mastoid electrodes. Hyperthermia was induced by a combined passive heat and work stress in a climatic chamber. Brain temperature was monitored with a thermistor in the parietal lobe. Hyperthermia resulted in a progressive decrease in the absolute latencies of the BAEPs up to the time of heat-induced collapse. There was a similar decrease in the latencies of MLPs up to a brain temperature of 42.0 degrees C +/- 0.3 degrees C (mean +/- S.D.), while there was prolongation of latencies at higher brain temperatures. The wave form of the BAEPs (I-V) persisted up to the time of heat-induced collapse. In contrast those of MLPs showed reproducible changes in the form of flattening or splitting of the Pa wave at a brain temperature of 41.8 degrees C +/- 0.7 degrees C, reversible with cooling. That temperature was also associated with behavioural changes reversible with cooling. A complete loss of the MLP waves occurred at a brain temperature of 42.9 degrees C +/- 0.6 degrees C which was not reversed with whole body cooling and immediately preceded the heat-induced collapse. This study demonstrates that hyperthermia in the conscious sheep produces potentially damaging effects on the central nervous system once a critical brain temperature (41.8 degrees C +/- 7.0 degrees C) is exceeded and that the MLPs are more sensitive indicators of this damage than BAEPs.