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 a ssumed that the cerebral metabolic and protective effects of qualitative bu rst suppression are similar to those of the isoelectric encephalogram (EEG) . We have examined the effect of different degrees of EEG suppression on bl ood flow and oxygen difference during general anaesthesia. We studied I I p atients undergoing general anaesthesia for resection of acoustic neuromas. The study was performed after surgery with propofol and remifentanil anaest hesia. Transcranial Doppler ultrasonography and jugular bulb venous saturat ions were measured at values of EEG suppression: 0%, 50% and 100% (isoelect ric EEG). Data from nine patients were suitable for analysis. There were no significant differences in mean arterial pressure, heart rate or Pace, dur ing 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 c hange in EEG activity, indicating intact flow-metabolism coupling. We concl ude that the degree of EEG suppression had a significant effect on blood fl ow. If flow-metabolism coupling is maintained, the assumption that cerebral metabolism during 50% EEG burst suppression is equivalent to isoelectric E EG may not be justified. If cerebral protection is related to brain metabol ism, then an isoelectric EEG may give more cerebral protection than 50% bur st suppression.