Critically ill patients often are subjected to interventions that acutely increase oxygen demand and require increased output of the cardiac and respiratory systems. This study explored whether alfentanil could attenuate the response to chest physical therapy, a procedure that increases oxygen consumption by 40%-50%. Patients were examined during two consecutive therapy sessions. In random order, they received either a placebo or alfentanil (30 or 60 μg/kg) 2 min before treatment. In Group 1 (n = 11, 30 μg /kg alfentanil) only the arterial blood pressure increases induced by chest physical therapy were attenuated. In Group 2 (n = 12, 60 μg/kg) alfentanil attenuated the increases in heart rate, central venous pressure, and pulmonary artery systolic pressures as well as systemic blood pressure. Neither dose of alfentanil altered the increases in oxygen consumption, carbon dioxide elimination, oxygen delivery, or extraction ratio. Thus alfentanil attenuated the hemodynamic responses to chest physiotherapy in a dose-dependent fashion. This was likely due to its vagotonic actions. In contrast, alfentanil had no effect on the balance between oxygen demand and delivery during chest physiotherapy. There was thus a dissociation between the hemodynamic and metabolic responses.