title: Effect of hyperthermia on brain auditory evoked potentials in the conscious sheep.
abstract: 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.
mesh_terms: D000818:Animals; D001921:Brain / Q000503:physiopathology*; D004569:Electroencephalography; D005072:Evoked Potentials, Auditory; D005334:Fever / Q000503:physiopathology*; D012756:Sheep