Quantitative EEG and brainstem auditory evoked potentials: comparison of isoflurane with halothane using the cerebral function analysing monitor.

We studied EEG and brainstem auditory evoked potentials (BAEP) during routine surgery at various concentrations of isoflurane (12 patients) or halothane (11 patients) or during prolonged (mean 2.5 h, range 1.9-3.5 h) administration of 1% isoflurane (five patients). Recording and analysis was performed with the cerebral function analysing monitor (CFAM). At equivalent MAC, the two agents exhibited distinctive neurophysiological profiles. Increasing concentrations of isoflurane produced a clear sequence of EEG changes (decreasing fast and increasing slow components) then burst suppression activity suggesting cortical depression. With halothane, changes in EEG amplitude were less pronounced and those in frequency content less systematic, with no periods of suppression. Simultaneous BAEP showed greater latency increase with halothane than with isoflurane. Prolonged administration of 1% isoflurane was associated with a stable EEG (no periods of suppression) and BAEP.