Low and moderate Remifentanil infusion rates do not alter target-controlled infusion Propofol concentrations necessary to maintain Anesthesia as assessed by Bispectral index monitoring

BACKGROUND: We investigated whether the EC50 (the effective concentration that is required to achieve a response in 50% of patients) of propofol necessary to lower the Bispectral Index (BIS) value to 50 or less was reduced by coadministration of different remifentanil infusion rates.METHOD: Seventy-two adult ASA I or II patients undergoing endotracheal intubation and target-controlled infusion (TCI) propofol anesthesia were allocated to six groups by stratified randomization. Group B received remifentanil 0.1 mu g (.) kg(-1) (.) min(-1), Group C 0.15 mu g (.) kg(-1 .) min(-1), Group D 0.2 mu g (.) kg(-1 .) min(-1), Group E 0.3 mu g (.) kg(-1 .) min(-1) and Group F 0.4 mu g (.) kg(-1) (.) min(-1). Group A served as control and received no remifentanil. The response of the first patient to propofol TCI at 4 mu g/mL determined the effect-site concentration of propofol for the next patient in the same remifentanil group (Dixon's "up-and-down" method). If BIS was > 50, the next patient received more propofol, and if BIS was <= 50, the next patient received less propofol. The hemodynamic effects of the combinations were also studied. RESULTS: The EC50 varied from 2.4 to 2.9 mu g/mL. No additive effect of remifentanil on the EC50 of propofol was observed. However, there was a wider variation in the response to propofol when the patients received no remifentanil. There was a decrease in heart rate in the remifentanil groups.CONCLUSION: Infusion of remifentanil did not reduce propofol requirements in the unstimulated anesthetized patient. Propofol TCI levels should not be reduced because remifentanil is coinfused.