Abstract
Objective—To evaluate the pharmacokinetics and pharmacodynamics of morphine after IV administration as an infusion or multiple doses in dogs by use of a von Frey (vF) device.
Animals—6 dogs.
Procedure—In the first 2 crossover experiments of a 3-way crossover study, morphine or saline (0.9%) solution was administered via IV infusion. Loading doses and infusion rates were administered to attain targeted plasma concentrations of 10, 20, 30, and 40 ng/mL. In the third experiment, morphine (0.5 mg/kg) was administered IV every 2 hours for 3 doses. The vF thresholds were measured hourly for 8 hours. Plasma concentrations of morphine were measured by highpressure liquid chromatography.
Results—No significant changes in vF thresholds were observed during infusion of saline solution. The vF thresholds were significantly increased from 5 to 8 hours during the infusion phase, corresponding to targeted morphine plasma concentrations > 30 ng/mL and infusion rates ≥ 0.15 ± 0.02 mg/kg/h. The maximal effect (EMAX) was 78 ± 11% (percentage change from baseline), and the effective concentration to attain a 50% maximal response (EC50) was 29.5 ± 5.4 ng/mL. The vF thresholds were significantly increased from 1 to 7 hours during the multiple-dose phase; the EC50 and EMAX were 23.9 ± 4.7 ng/mL and 173 ± 58%, respectively. No significant differences in half-life, volume of distribution, or clearance between the first and last dose of morphine were detected.
Conclusions and Clinical Relevance—Morphine administered via IV infusion (0.15 ± 0.02 mg/kg/h) and multiple doses (0.5 mg/kg, IV, every 2 hours for 3 doses) maintained significant antinociception in dogs. (Am J Vet Res 2005;66:1968–1974)
