Remifentanil in combination with propofol for spontaneous ventilation anaesthesia

We have investigated the effect of four doses of remifentanil on the incidence of respiratory depression and somatic response at incision. Remifentanil was administered as a loading dose of 0.125, 0.25, 0.375 or 0.5 mu g kg(-1) and at a maintenance infusion rate of 0.025, 0.05, 0.075 or 0.1 mu g kg(-1) min(-1), respectively, with an infusion of propofol 6 mg kg(-1) h(-1). Responses occurred in 88% of patients with remifentanil 0.025 mu g kg(-1) min(-1) compared with 30-40% in the other groups. Respiratory depression after incision increased from 6% with remifentanil 0.025 mu g kg(-1) min(-1) to 73% with 0.1 mu g kg(-1) min(-1). Increases in propofol infusion rate to 7.2-8.4 mg kg(-1) h(-1) produced adequate maintenance of anaesthesia. Reductions in remifentanil doses to 0.025-0.05 mu g kg(-1) min(-1) resulted in adequate respiration at the end of surgery in 88% of patients. Maintenance infusions of the two drugs for spontaneous ventilation are likely to be in these ranges. However, the ideal loading doses and infusion rates for induction remain to be established.