Burst suppression or isoelectric encephalogram for cerebral protection: evidence from metabolic suppression studies

Metabolic suppression may have a role in cerebral protection. It is often assumed that the cerebral metabolic and protective effects of qualitative burst suppression are similar to those of the isoelectric encephalogram (EEG). We have examined the effect of different degrees of EEG suppression on blood flow and oxygen difference during general anaesthesia. We studied 11 patients undergoing general anaesthesia for resection of acoustic neuromas. The study was performed after surgery with propofol and remifentanil anaesthesia. Transcranial Doppler ultrasonography and jugular bulb venous saturations were measured at values of EEG suppression: 0%, 50% and 100% (isoelectric EEG). Data from nine patients were suitable for analysis. There were no significant differences in mean arterial pressure, heart rate or PaCO2 during EEG activity, 50% burst suppression ratio or isoelectric EEG. There was a significant decrease in middle cerebral artery flow velocity (vmca) with increasing EEG suppression (0% suppression, mean 38 (SEM 4) cm s-1; 50% suppression, 29 (3) cm s-1; and 100% suppression, 24 (2) cm s-1; P < 0.05). Jugular bulb venous saturations did not change consistently with the change in EEG activity, indicating intact flow-metabolism coupling. We conclude that the degree of EEG suppression had a significant effect on blood flow. If flow-metabolism coupling is maintained, the assumption that cerebral metabolism during 50% EEG burst suppression is equivalent to isoelectric EEG may not be justified. If cerebral protection is related to brain metabolism, then an isoelectric EEG may give more cerebral protection than 50% burst suppression.