Differential effects of isoflurane on SEP recorded over parietal and frontal scalp

We evaluated the effects of different concentrations of isoflurane on short-latency somatosensory evoked potentials recorded over the parietal and frontal scalp in 14 patients during general anesthesia at stable end-tidal concentrations between 0% and 2.5%. At concentrations between 0% and 1%, there was a marked discrepancy between the amplitude of the parietal N20, which remained stable, and the striking increase of the frontal P22. At concentrations above 1.5%, the parietal N20 disappeared in 6 patients whereas the P22 was clearly identified in 12. The central conduction time increased significantly whereas the spinal conduction was not delayed by increasing levels of isoflurane. These results show that isoflurane anesthesia is a useful pharmacologic model to demonstrate the independence of the N20 recorded over the parietal scalp and the P22 recorded over the frontal scalp.