Use of EEG spectral edge as index of equipotency in a comparison of propofol and isoflurane for maintenance of general anaesthesia

Propofol and isoflurane were compared for haemodynamic stability and recovery time as main anaesthetic agents for maintenance in a total of 43 ASA I and II patients in uncomplicated operations lasting at least 30 min. The premedication (oral diazepam), induction (thiopentone-fentanyl-suxamethonium) and maintenance protocol (N₂O-O₂ 2:1 litre min^-1, fentanyl and vecuronium in incremental doses) were identical for all patients. In one group of 20 patients, propofol was used in continuous intravenous (i.v.) infusion (starting dose 3 mg kg^-1 h^-1), while in the second group of 23 patients, isoflurane was administered in a starting concentration of 1%. To improve the equipotency of the two agents in all cases the dose of the main anaesthetic was titrated to keep the EEG 90% spectral edge frequency (SEF - recorded by processed EEG monitor) between 8 and 12 Hz. Mean blood pressure (BP) and heart rate (HR) deviations of ± 20% from the baseline were recorded. The incidence of mean blood pressure deviations/case in the propofol group was less than half of those occurring in the isoflurane group (0.45 vs. 0.96 P = 0.04). The mean duration of blood pressure deviation from baseline value was 5.5 min in the propofol group vs. 16.8 min in the isoflurane group (P = 0.01). The recovery intervals were significantly shorter in the propofol group (P < 0.05). We conclude that continuous i.v. administration of propofol for maintenance of general anaesthesia produces a significantly more stable blood pressure, as well as a quicker recovery, in comparison with those encountered with normally acceptable concentrations of isoflurane. EEG spectral edge frequency titration in the range of 8-12 Hz is a useful method for equipotent comparison of propofol and isoflurane and also appears to prevent awareness with these agents.