{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,9]],"date-time":"2026-03-09T11:28:02Z","timestamp":1773055682016,"version":"3.50.1"},"reference-count":20,"publisher":"American Veterinary Medical Association (AVMA)","issue":"4","content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["ajvr"],"published-print":{"date-parts":[[2004,4,1]]},"abstract":"<jats:title>Abstract<\/jats:title>\n\t\t\t\t<jats:p>\n\t\t\t\t\t<jats:bold>Objective<\/jats:bold>\u2014To quantitate the dose and time-related\neffects of morphine sulfate on the anesthetic sparing\neffect of xylazine hydrochloride in halothane-anesthetized\nhorses and determine the associated plasma\nxylazine and morphine concentration-time profiles.<\/jats:p>\n\t\t\t\t<jats:p>\n\t\t\t\t\t<jats:bold>Animals<\/jats:bold>\u20146 healthy adult horses.<\/jats:p>\n\t\t\t\t<jats:p>\n\t\t\t\t\t<jats:bold>Procedure<\/jats:bold>\u2014Horses were anesthetized 3 times to\ndetermine the minimum alveolar concentration (MAC)\nof halothane in O<jats:sub>2<\/jats:sub> and characterize the anesthetic\nsparing effect (ie, decrease in MAC of halothane) by\nxylazine (0.5 mg\/kg, IV) administration followed\nimmediately by IV administration of saline\n(0.9% NaCl) solution, low-dose morphine (0.1 mg\/kg),\nor high-dose morphine (0.2 mg\/kg). Selected parameters\nof cardiopulmonary function were also determined\nover time to verify consistency of conditions.<\/jats:p>\n\t\t\t\t<jats:p>\n\t\t\t\t\t<jats:bold>Results<\/jats:bold>\u2014Mean (\u00b1 SEM) MAC of halothane was\n1.05 \u00b1 0.02% and was decreased by 20.1 \u00b1 6.6% at\n49 \u00b1 2 minutes following xylazine administration. The\namount of MAC reduction in response to xylazine was\ntime dependent. Addition of morphine to xylazine\nadministration did not contribute further to the\nxylazine-induced decrease in MAC (reductions of\n21.9 \u00b1 1.2 and 20.7 \u00b1 1.5% at 43 \u00b1 4 and 40 \u00b1 4 minutes\nfollowing xylazine-morphine treatments for low-and\nhigh-dose morphine, respectively). Overall, cardiovascular\nand respiratory values varied little among\ntreatments. Kinetic parameters describing plasma\nconcentration-time curves for xylazine were not\naltered by the concurrent administration of morphine.<\/jats:p>\n\t\t\t\t<jats:p>\n\t\t\t\t\t<jats:bold>Conclusions and Clinical Relevance<\/jats:bold>\u2014Administration\nof xylazine decreases the anesthetic requirement\nfor halothane in horses. Concurrent morphine\nadministration to anesthetized horses does not alter\nthe anesthetic sparing effect of xylazine or its plasma\nconcentration-time profile. 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