ELECTRICAL ACTIVITY OF THE CEREBRAL CORTEX DURING INDUCED HYPOTENSION IN MAN

During the routine use of controlled hypotension the electroencephalogram (EEG) and mean arterial pressure (MAP) were monitored in 20 normotensive patients (younger than 70 years-of-age) receiving either trimetaphan (TMP) or sodium nitroprusside (SNP). The reduction in MAP was quicker and greater with SNP. Significant differences in EEG voltage between the two agents were seen in the range 55–40 mm Hg, electrical activity being better maintained with SNP. However, all patients showed some decline in EEG voltage with hypotension and half of these showed significant correlations with MAP. These pressure-dependent cerebral effects were not predictable in terms of age, preoperative arterial pressure or hypotensive agent. Our work supports previous experimental evidence that, during more profound hypotension, cerebral electrical activity is better maintained with SNP than with TMP. A simple measure of total EEG power, or filtered EEG voltage envelope (CFM) was shown to be a more useful monitor of cerebral electrical activity during controlled hypotension than measurements of power distribution in different frequency bands.