Clonidine prolongs fentanyl-induced ventilatory depression.

The aim of the present study was to investigate the effect of intravenously (i.v.) administered fentanyl and clonidine on ventilation in 12 healthy male volunteers (age 30.8 +/- 4.9 years) who either received fentanyl alone (1.5 > or = micrograms kg-1) or fentanyl (1.5 > or = micrograms kg-1) in combination with clonidine (3 > or = micrograms kg-1). The effect on ventilation was measured with a CO2 rebreathing system. The respiratory depression caused by fentanyl disappeared 120 min after injection. The corresponding slopes were 7430 +/- 2075 mL kPa-1 prior to (t0) and 6263 +/- 1864 mL kPa-1 120 min post-application (base-line vs. t120; P = 0.106). An impaired ventilatory response was observed during CO2 rebreathing at t120 after the injection of fentanyl and clonidine. Before drug administration, the slope of the response curves was 7700 +/- 2800 mL kPa-1, which was reduced to 5480 +/- 2135 mL kPa-1 (P < 0.035) at t120. These data suggest a prolongation of a fentanyl-induced ventilatory depression when used in combination with clonidine.