Determination of the potency of remifentanil compared with alfentanil using ventilatory depression as the measure of opioid effect

Background: Remifentanil is a new opioid with properties similar to other m u-specific agonists. To establish its pharmacologic profile relative to oth er known opioids, it Is important to determine its potency. This study inve stigated the relative potency of remifentanil compared with alfentanil, Methods: Thirty young healthy males were administered double-blind remifent anil or alfentanil intravenously for 180 min using a computer-assisted cont inuous infusion device. Depression of ventilation was assessed by the minut e ventilatory response to 7.5% CO2 administered via a "bag in the box" syst em, The target concentration of the study drug was adjusted to obtain 40-70 % depression of baseline minute ventilation. Multiple blood samples were ob tained during and following the infusion. The concentration-effect relation ship of each drug was modeled, and the concentration needed to provide a 50 % depression of ventilation (EC50) was determined, Results: Only 11 subjects in each drug group completed the study; however, there were sufficient data in 28 volunteers to model their EC50 values. The EC50 (mean and 95% confidence interval) for depression of minute ventilati on with remifentanil was 1.17 (0.85-1.49) ng/ml and the EC50 for alfentanil was 49.4 (32.4-66.5) ng/ml. Conclusion: Based on depression of the minute ventilatory response to 7.5% CO2, remifentanil is approximately 40 (26-65) times more potent than alfent anil when remifentanil and alfentanil whole-blood concentrations are compar ed. As alfentanil is usually measured as a plasma concentration, remifentan il is approximately 70 (41-104) times more potent than alfentanil when remi fentanil whole-blood concentration Is compared with alfentanil plasma conce ntration. This information should be used when performing comparative studi es between remifentanil and other opioids.