Dose comparison of Remifentanil and Alfentanil for loss of consciousness

Background: This study evaluated the efficacy and safety of remifentanil, a potent mu agonist opioid with a rapid onset and offset of effect, as a sole induction agent for loss of consciousness (LOC) and compared it with alfentanil. Methods: Remifentanil and alfentanil were administered intravenously over 2 min in ascending doses (remifentanil 2, 3, 4, 5, 6, 8, 10, 15, 20 mu g/kg; alfentanil 40, 60, 80, 100, 120, 160, 200 mu g/kg) to unpremedicated healthy patients, Patients were observed for rigidity and LOC for 30 s after the end of infusion. if patients had not lost consciousness, 2 mg.kg(-1).min(-1) thiopental was administered until LOC was achieved. Arterial blood samples, obtained at specified time intervals, were analyzed for remifentanil and alfentanil whole-blood concentration. Blood pressure and heart rate were also recorded at preset time intervals. Results: Neither drug could reliably produce LOC. With both drugs, there was a dose-dependent decrease in thiopental requirements and a dose-dependent increase in the incidence and severity of rigidity (P < 0.05). The median effective dose (ED50) for LOC with remifentanil was 12 mu g/kg, and for alfentanil it was 176 mu g/kg. The median effective concentration (EC50; whole-blood concentration) of remifentanil was 53.8 ng/ml and for alfentanil it was 1,012 ng/ml. Minimal hemodynamic changes were observed after either drug was given. Conclusions: Remifentanil is 15 times more potent than alfentanil, based on the ED50 to achieve loss of response to a verbal command and 20 times more potent than alfentanil based on the EC50. Neither opioid is suitable as a sole induction agent.