The effects of deep barbiturate coma on multimodality evoked potentials

The authors report their investigation of the effects of high-dose barbiturates on the multimodality evoked response in 9 cats. After baseline evoked responses were obtained, boluses of pentobarbital were infused intravenously at regular intervals, amounting to cumulative total doses of 9, 18, 27, 45, 63, 123, and 183 mg/kg at respective infusions. This resulted in gradually increasing serum pentobarbital levels, reaching therapeutic coma levels (4 to 5 mg/dl) after the fifth infusion. At this point, the electroencephalogram was flat, and pressor agents were required to maintain cardiovascular stability. Evoked responses were obtained 15 minutes after each infusion. Brain-stem auditory evoked response (BAER) showed little change in wave latencies at therapeutic coma levels of pentobarbital. Further barbiturates resulted in delay of the late components of this response. In the somatosensory evoked responses (SER), early brain-stem components were relatively unaffected by therapeutic coma levels. Late brain-stem components and the initial cortical response showed progressive latency increase. Late cortical (association cortex) waves were abolished at relatively low doses. The central conduction time was relatively unaffected. The late waves of the visual evoked responses (VER) were abolished with low-dose barbiturates (9 mg/kg). A single positive-negative complex persisted despite massive infusions. It is concluded that evoked responses may prove useful in monitoring patients in deep barbiturate coma, but barbiturate effects must be kept in mind.