Cardiorespiratory and neuroendocrine changes induced by methadone in conscious and in isoflurane anaesthetised dogs.

Abstract The aim of this study was to compare the cardiorespiratory and neurohormonal effects of methadone in conscious and in isoflurane anaesthetised dogs. Six mature dogs (28.0 ± 3.8 kg bodyweight) received intravenous (IV) methadone (1 mg/kg) three times, once when conscious and twice during isoflurane anaesthesia (with a wash-out period of 1 week). The vasopressin antagonist relcovaptan (0.1 mg/kg IV) was administered before the methadone either during the first or second (selected randomly) isoflurane anaesthesia to evaluate the contribution of vasopressin to methadone-associated vasoconstriction. Cardiorespiratory data, plasma catecholamines and serum vasopressin were recorded before (baseline) and for 90 min after methadone. Methadone induced dysphoria in all conscious dogs and significantly ( P