Electroencephalographic burst suppression in elderly and young patients anesthetized with isoflurane

The electroencephalograms of seven elderly (70-85 years) and seven younger patients (23-31 years) anesthetized with a concentration of isoflurane sufficient to produce burst suppression were studied. Anesthesia in these unpremedicated subjects was induced by inhalation of nitrous oxide, isoflurane, and oxygen. Tracheal intubation was facilitated with succinylcholine and the lungs were next ventilated with oxygen and isoflurane to produce an end-tidal concentration of 1.7%. Isoflurane concentration was determined by infrared analysis of expired gas collected from a Teflon catheter inserted through the endotracheal tube. After 25 minutes at steady state, the EEG was recorded for 5 minutes prior to surgical stimulation. Arterial blood pressure, temperature, and ventilation were maintained at normal values. In elderly patients the EEG had both a greater proportion of total time in electrical silence (76.0 +/- 10.8% vs 37.6 +/- 15.4%; P less than 0.01) and a greater number of isoelectric periods (19.7 +/- 8.1 vs 10.7 +/- 5.4; P less than 0.05). This demonstrates a discrete alteration with age in the central nervous system sensitivity to isoflurane.