THE PHARMACODYNAMICS OF THIOPENTAL, MEDETOMIDINE, BUTORPHANOL AND ATROPINE IN BEAGLE DOGS

This study evaluated the quality of anaesthesia and some of the haemod ynamic effects induced by a combination of thiopental, medetomidine, b utorphanol and atropine in healthy beagle dogs (n = 12). Following pre medication with atropine (ATR, 0.022 mg/kg intravenously (i.v.)) and b utorphanol (BUT, 0.22 mg/kg i.v.), medetomidine (MED, 22 mu g/kg intra muscularly (i.m.)) was administered followed in 5 min by thiopental (T HIO, 2.2 mg/kg i.v.). Heart rate, systolic blood pressure (SBP), diast olic blood pressure (DBP) and mean arterial blood pressure (MBP) were monitored continuously with an ECG and direct arterial blood pressure monitor. Atipamezole (ATI, 110 mu g/kg i.v.) was administered to half of the dogs (n = 6) following surgery to evaluate the speed and qualit y of arousal from anaesthesia. Anaesthesia was characterized by excell ent muscle relaxation, analgesia and absence of purposeful movement in response to surgical castration. Arousal following antagonism of mede tomidine was significantly faster (P < 0.05) than in unantagonized dog s. Recoveries were smooth but recovery times following atipamezole adm inistration were highly variable among dogs (sternal time range 6-38 m in, standing time range 9-56 min). Medetomidine caused a significant ( P < 0.05) increase in SEP, DBP and MBP. Atropine prevented the medetom idine induced bradycardia. In conclusion, this combination provided ad equate surgical anaesthesia in healthy beagle dogs. At the dosages use d in this study, it seems prudent that this combination should be rese rved for dogs free of myocardial disease.