Isoflurane prevents EEG depression during trimetaphan-induced hypotension in man

We have studied the EEG analysed with the cerebral function analysing monitor (CFAM) during trimetaphan (TMP)-induced hypotension to a mean arterial pressure (MAP) of 40 mm Hg in 20 normocapnic patients anaesthetized with either 1% end-tidal isoflurane or 0.5% halothane. During the acute reduction in MAP, the average reduction in mean EEG amplitude with halothane was 14%, two patients showing short periods of EEG suppression; the decline in EEG amplitude correlated with declining MAP in four patients. In contrast, the average reduction in mean EEG amplitude with isoflurane was only 0.3% and there were neither periods of suppression nor any correlation between EEG amplitude and MAP. No significant changes in EEG frequency occurred in either group. Isoflurane prevented EEG amplitude depression during TMP-induced hypotension.