MIDAZOLAM AND KETAMINE-INDUCTION BEFORE HALOTHANE ANESTHESIA IN PONIES - CARDIORESPIRATORY, ENDOCRINE AND METABOLIC CHANGES

Six Welsh gelding ponies were premedicated with 0.03 mg/kg of aceproma zine intravenously (i.v.) prior to induction of anaesthesia with midaz olam at 0.2 mg/kg and ketamine at 2 mg/kg i.v.. Anaesthesia was mainta ined for 2 h using 1.2% halothane concentration in oxygen. Heart rate, electrocardiograph (EGG), arterial blood pressure, respiratory rate, blood gases, temperature, haematocrit, plasma arginine vasopressin (AV P), dynorphin, beta-endorphin, adrenocorticotropic hormone (ACTH), cor tisol, dopamine, noradrenaline, adrenaline, glucose and lactate concen trations were measured before and after premedication, immediately aft er induction, every 20 min during anaesthesia, and at 20 and 120 min a fter disconnection. Induction was rapid, excitement-free and good musc le relaxation was observed. There were no changes in heart and respira tory rates, Decrease in temperature, hyperoxia and respiratory acidosi s developed during anaesthesia and slight hypotension was observed (mi nimum value 76 +/- 10 mm Hg at 40 mins), No changes were observed in d ynorphin, beta-endorphin, ACTH, catecholamines and glucose, Plasma cor tisol concentration increased from 220 +/- 17 basal to 354 +/- 22 nmol /L at 120 min during anaesthesia; plasma AVP concentration increased f rom 3 +/- 1 basal to 346 +/- 64 pmol/L at 100 min during anaesthesia a nd plasma lactate concentration increased from 1.22 +/- 0.08 basal to 1.76 +/- 0.13 mmol/L at 80 min during anaesthesia, Recovery was rapid and uneventful with ponies taking 46 +/- 6 min to stand. When midazola m/ketamine was compared with thiopentone or detomidine/ketamine for in duction before halothane anaesthesia using an otherwise similar protoc ol in the same ponies, it caused slightly more respiratory depression, but less hypotension. Additionally, midazolam reduced the hormonal st ress response commonly observed during halothane anaesthesia and appea rs to have a good potential for use in horses.