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

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.