Interaction of propofol and sevoflurane on loss of consciousness and movement to skin incision during general anesthesia

Background: Loss of consciousness (LOC) and immobility to surgical incision seem to be mediated at different levels of the central nervous system. Pharmacologic studies of hypnotic agents have previously focused on combinations of either volatile or intravenous anesthetics. This study examined the combination of inhaled sevoflurane and intravenous propofol at these two clinically relevant anesthetic end points. Methods. Thirty-six elective surgical patients were initially enrolled. Conditions approximating steady state were obtained for sevoflurane and target-controlled propofol infusions. Patients were sequentially evaluated for LOC (loud voice plus mild prodding) and immobility to surgical incision. The study was designed using the Dixon up-down method. Results: The observed propofol effect target with 50% response plus sevoflurane (0.46% end-tidal concentration) was 1.2 mu g/ml (95% confidence interval, 1.1-1.3 mu g/ml). it was not significantly different from that predicted (1-5 mu g/ml; 95% confidence interval, 1.2-1.7 mu g/ml) by simple additivity. The effective plasma concentration of propofol that suppressed movement to skin incision in 50% of patients was 5.4 mu g/ml (95% confidence interval, 4.8-6.0 mu g/ml) plus sevoflurane (0.86%) and was not significantly different from that predicted by additivity (5.4 mu g/ml; 95% confidence interval, 4.8-5.9 mu g/ml). Both analyses had adequate power (90%) to detect a significant change (+/- 19 to 25%) from predicted value. Repeated-measures analysis of variance identified a Bispectral Index value of 70 as the break point between those who responded at LOC or did not. Conclusions: Propofol and sevoflurane interact in a simple additive manner to produce LOC and immobility to surgical incision, suggesting a common mechanism or a single site of action. These clinical observations are consistent with a single site of interaction at the gamma-aminobutyric acid type A receptor.