EFFECT OF HIGH-VOLUME EPIDURAL MORPHINE, KETAMINE AND BUTORPHANOL ON HALOTHANE MINIMUM ALVEOLAR CONCENTRATION IN PONIES

This study determined the effects of epidurally administered morphine, ketamine and butorphanol on halothane minimum alveolar concentration (MAC) in ponies. Seven ponies were anaesthetised with thiopentone and succinylcholine, intubated and anaesthesia maintained with halothane. Ventilation was controlled and blood pressure was maintained within no rmal limits. Following the determination of baseline halothane MAC for the pelvic and thoracic limbs the ponies were given morphine (0.1 mg/ kg bwt), ketamine (0.8 or 1.2 mg/kg bwt), butorphanol (0.05 mg/kg bwt) or saline, epidurally, to a final volume of 0.15 ml/kg bwt. The halot hane MAC for the pelvic and thoracic limbs was redetermined following each treatment. The baseline halothane MAC for the control group was m ean +/- s.e, 0.85 +/-:0.02% and no significant change occurred after s aline administration, Morphine significantly (P = 0.002) decreased MAC from, mean +/- s,e, 0.90 +/- 0.05% to 0.77 +/- 0.06% in the pelvic li mb, Ketamine significantly decreased MAC in the pelvic limb from mean +/- s,e. 0.86 +/- 0.06% to 0.71 +/- 0.04%, and 0.82 +/- 0.03% to 0.71 +/- 0.02%, for the low (P = 0.008) and high dose (P = 0.001), respecti vely. No significant change in MAC occurred following butorphanol, No treatment reduced halothane MAC for the thoracic limb.