Influence of high-dose alfentanil anaesthesia on the electroencephalogram: correlation with plasma concentrations

The influence of high-dose alfentanil-oxygen-air anaesthesia on the electroencephalogram was studied in 10 patients undergoing elective coronary artery surgery. Premedication was either lorazepam 5 mg orally (n = 5) or morphine 10 mg and hyoscine 0.4 mg i.m. (n = 5). Anaesthesia was induced with alfentanil 125 micrograms kg-1 h-1 and maintained by a continuous infusion at a rate of 0.25-0.5 mg kg-1. The EEG was recorded on the day before surgery, continuously from before induction of anaesthesia until the end of surgery and at hourly intervals after operation. Blood samples were withdrawn at intervals during and after surgery for measurement of plasma alfentanil concentrations. Anaesthesia was associated with large increases in delta activity and reduction in higher frequency components. There was less synchronization of the EEG than is observed with fentanyl or sufentanil. Spindle activity was prominent in 70% of patients. The changes in the EEG during recovery tended to occur abruptly and there was no obvious correlation between the EEG and plasma concentrations of alfentanil. There was a significant correlation between the total dose of alfentanil and the awake plasma concentration (r = 0.94; P less than 0.01). Plasma concentrations decreased exponentially after operation, with a mean half-life of 304 +/- 54 min.