Pharmacokinetics and pharmacodynamics of a constant rate infusion of fentanyl (5 μg/kg/h) in awake cats

Objective-To evaluate the pharmacokinetics and thermal and mechanical antinociceptive effects of a fentanyl constant rate infusion (CRI) in conscious cats. Animals-8 healthy adult cats. Procedures-At a >= 14-day interval, 7 cats received a loading dose (LD) of fentanyl (5 mu g/kg, IV [administered at 0 hours]) followed by fentanyl infusion (5 mu g/kg/h, IV) for 2 hours or similar administrations of equivalent volumes of 0.9% saline (NaCl) solution. One cat received only the fentanyl treatment. For both treatments, sedation and adverse events were evaluated and mechanical threshold (MT) and thermal threshold (TT) testing was performed prior to (baseline) and at predetermined times up to 26 hours after LD administration; plasma fentanyl concentrations were determined at similar times when the cats received fentanyl. Results-Fentanyl induced mild sedation during the infusion. The only adverse effect associated with fentanyl LD administration was profuse salivation (1 cat). Saline solution administration did not significantly change MT or TT over time. For the duration of the CRI, MT and TT differed significantly between treatments, except for TT 1 hour after LD administration. For the fentanyl treatment, MT and TT were significantly higher than baseline at 0.25 to 0.75 hours and at 0.25 to 1 hour, respectively. During the fentanyl CRI, mean +/- SD plasma fentanyl concentration decreased from 4.41 +/- 1.86 ng/mL to 2.99 +/- 1.28 ng/mL and was correlated with antinociception; plasma concentrations < 1.33 +/- 0.30 ng/mL were not associated with antinociception. Conclusions and Clinical Relevance-Fentanyl CRI (5 mu g/kg/h) induced mechanical and thermal antinociception in cats.