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Interactions of morphine and isoflurane in horses
Eugene P Steffey1, John H Eisele, J Desmond Baggot
1Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Objective:
To quantitate dose- and time-related magnitudes of interactive effects of morphine (MOR) and isoflurane (ISO) in horses and to characterize pharmacokinetics of MOR in plasma and the ventilatory response to MOR during administration of ISO.
Animals:
6 adult horses.
Procedure:
Horses were anesthetized 3 times to determine the minimum alveolar concentration (MAC) of ISO in O2 and then to characterize the change in anesthetic requirement as defined by the alteration in ISO MAC following IV administration of saline (0.9% NaCl) solution and 2 doses of MOR (low dose, 0.25 mg/kg; high dose, 2.0 mg/kg). Arterial blood samples were obtained before and after MOR and analyzed.
Results:
Mean +/- SD baseline ISO MAC was 1.43 +/- 0.06%. The ISO MAC did not change with time after administration of saline solution. Effects of MOR on ISO MAC varied. Maximal change in MAC ranged from -20.2 to +28.3% and -18.9 to +56.2% after low and high doses of MOR, respectively. Typical half-life of MOR in plasma was 40 to 60 minutes and related to dose. Mean PaCO2 increased from 70 mm Hg before MOR to 88 to 102 mm Hg for 30 to 240 minutes after the high dose of MOR. Recovery from anesthesia after administration of the high dose of MOR was considered undesirable and dangerous.
Conclusions And Clinical Relevance:
Our results do not support routine clinical use of MOR administered IV at dosages of 0.25 or 2.0 mg/kg as an adjuvant to anesthesia in horses administered ISO.
Insights
Morphine (MOR) and isoflurane (ISO) interactions in horses showed variable effects on anesthetic requirements. High-dose MOR led to dangerous recovery, not supporting its routine use with ISO.
Area of Science:
- Veterinary Anesthesiology
- Equine Pharmacology
Background:
- Isoflurane (ISO) is a common inhalation anesthetic in horses.
- Morphine (MOR) is an opioid analgesic with potential sedative and anesthetic-sparing effects.
Purpose of the Study:
- To quantify dose- and time-dependent interactions between MOR and ISO in horses.
- To characterize MOR pharmacokinetics and ventilatory response during ISO anesthesia.
Main Methods:
- Six adult horses were administered saline or two doses of MOR (0.25 mg/kg or 2.0 mg/kg) intravenously.
- Minimum alveolar concentration (MAC) of ISO was determined to assess anesthetic requirement.
- Arterial blood samples were analyzed for MOR pharmacokinetics and blood gas parameters.
Main Results:
- MOR administration resulted in variable changes in ISO MAC, ranging from -20.2% to +56.2% depending on dose.
- MOR half-life in plasma was 40-60 minutes, dose-dependent.
- High-dose MOR increased PaCO2 and led to undesirable, dangerous recovery from anesthesia.
Conclusions:
- The study does not support the routine clinical use of intravenous MOR at 0.25 or 2.0 mg/kg as an adjuvant to ISO anesthesia in horses.
- The unpredictable effects and adverse recovery profile limit MOR's utility in this context.