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Cardiorespiratory, gastrointestinal, and analgesic effects of morphine sulfate in conscious healthy horses

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  • 1 Department of Small Animal Medicine and Surgery, School of Veterinary Medicine, St George's University, Grenada, West Indies.
  • | 2 Animal Medical Center, 510 E 62nd St, New York, NY 10065.
  • | 3 HFL Sport Science Inc, 1745 Alysheba Way, Lexington, KY 40509.

Abstract

Objective—To compare the cardiorespiratory, gastrointestinal, analgesic, and behavioral effects between IV and IM administration of morphine in conscious horses with no signs of pain.

Animals—6 healthy adult horses.

Procedures—Horses received saline (0.9% NaCl) solution (IM or IV) or morphine sulfate (0.05 and 0.1 mg/kg, IM or IV) in a randomized, masked crossover study design. The following variables were measured before and for 360 minutes after drug administration: heart and respiratory rates; systolic, diastolic, and mean arterial blood pressures; rectal temperature; arterial pH and blood gas variables; intestinal motility; and response to thermal and electrical noxious stimuli. Adverse effects and horse behavior were also recorded. Plasma concentrations of morphine, morphine-3-glucuronide, and morphine-6-glucuronide were measured via liquid chromatography–mass spectrometry.

Results—No significant differences in any variable were evident after saline solution administration. Intravenous and IM administration of morphine resulted in minimal and short-term cardiorespiratory, intestinal motility, and behavioral changes. A decrease in gastrointestinal motility was detected 1 to 2 hours after IM administration of morphine at doses of 0.05 and 0.1 mg/kg and after IV administration of morphine at a dose of 0.1 mg/kg. Morphine administration yielded no change in any horse's response to noxious stimuli. Both morphine-3-glucuronide and morphine-6-glucuronide were detected in plasma after IV and IM administration of morphine.

Conclusions and Clinical Relevance—Clinically relevant doses of morphine sulfate yielded minimal and short-term behavioral and intestinal motility effects in healthy horses with no signs of pain. Neither dose of morphine affected their response to a noxious stimulus.

Contributor Notes

Supported by The Ohio State University Equine Research Funds.

Presented in abstract form at the 31st Annual Meeting of the American College of Veterinary Anesthesiologists, Chicago, October 2006.

The authors thank John Hubbell, Richard Bednarski, and Phillip Lerche for experimental design advice and Tokiko Kushiro for technical advice.

Address correspondence to Dr. Figueiredo (jfigueir@sgu.edu).