Interactions of morphine and isoflurane in horses.
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.


METHODS
6 adult horses.


METHODS
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
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.
Steffey
Eugene P
EP
Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Eisele
John H
JH
Baggot
J Desmond
JD
D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000009:adverse effects / Q000493:pharmacokinetics / Q000494:pharmacology*; D000758:Anesthesia / Q000662:veterinary; D018685:Anesthetics, Inhalation / Q000008:administration & dosage / Q000009:adverse effects / Q000493:pharmacokinetics / Q000494:pharmacology*; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D018592:Cross-Over Studies; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D006207:Half-Life; D006339:Heart Rate / Q000187:drug effects; D006736:Horses; D007530:Isoflurane / Q000008:administration & dosage / Q000009:adverse effects / Q000493:pharmacokinetics* / Q000494:pharmacology*; D008297:Male; D009020:Morphine / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics* / Q000494:pharmacology*
D000701:Analgesics, Opioid; D018685:Anesthetics, Inhalation; D009020:Morphine; D007530:Isoflurane