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In doses typically administered during conscious sedation, remifentanil may be associated with ventilatory depression. However, the time course of ventilatory depression after an initial dose of remifentanil has not been determined previously.In eight healthy volunteers, the authors determined the time course of the ventilatory response to carbon dioxide using the dual isohypercapnic technique. Subjects breathed via mask from a to-and-fro circuit with variable carbon dioxide absorption, allowing the authors to maintain end-tidal pressure of carbon dioxide (PETCO2) at approximately 46 or 56 mmHg (alternate subjects). After 6 min of equilibration, subjects received 0.5 μg/kg remifentanil over 5 s, and minute ventilation (E) was recorded during the next 20 min. Two hours later, the study was repeated using the other carbon dioxide tension (56 or 46 mmHg). The E data were used to construct two-point carbon dioxide response curves at 30-s intervals after remifentanil administration. Using published pharmacokinetic values for remifentanil and the method of collapsing hysteresis loops, the authors estimated the effect-site equilibration rate constant (keo), the effect-site concentration producing 50% respiratory depression (EC50), and the shape parameter of the concentration–response curve (γ).The slope of the carbon dioxide response decreased from 0.99 [95% confidence limits 0.72 to 1.26] to a nadir of 0.27 l · min−1 · mmHg−1 [−0.12 to 0.66] 2 min after remifentanil (P < 0.001); within 5 min, it recovered to approximately 0.6l · min−1 · mmHg−1, and within 15 min of injection, slope returned to baseline. The computed ventilation at PET = 50 mmHg (E50) decreased from 12.9 [9.8 to 15.9] to 6.1 l/min [4.8 to 7.4] 2.5 min after remifentanil injection (P < 0.001). This was caused primarily by a decrease in tidal volume rather than in respiratory rate. Estimated pharmacodynamic parameters based on computed mean values of E50 included keo = 0.24 min−1 (T1/2 = 2.9 min), EC50 = 1.12 ng/ml, and γ = 1.74.After administration of 0.5 μg/kg remifentanil, there was a decrease in slope and downward shift of the carbon dioxide ventilatory response curve. This reached its nadir approximately 2.5 min after injection, consistent with the computed onset half-time of 2.9 min. The onset of respiratory depression appears to be somewhat slower than previously reported for the onset of remifentanil-induced electroencephalographic slowing. Recovery of ventilatory drive after a small dose essentially was complete within 15 min.