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 mu-specific agonists. To establish its pharmacologic profile relative to other known opioids, it is important to determine its potency. This study investigated the relative potency of remifentanil compared with alfentanil.

Methods: Thirty young healthy males were administered double-blind remifentanil or alfentanil intravenously for 180 min using a computer-assisted continuous infusion device. Depression of ventilation was assessed by the minute ventilatory response to 7.5% CO2 administered via a "bag in the box" system. The target concentration of the study drug was adjusted to obtain 40-70% depression of baseline minute ventilation. Multiple blood samples were obtained during and following the infusion. The concentration-effect relationship 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 ventilation with remifentanil was 1.17 (0.85-1.49) ng/ml and the EC (50) for alfentanil was 49.4 (32.4-66.5) ng/ml.