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 normal 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 halothane MAC for the pelvic and thoracic limbs was redetermined following each treatment. The baseline halothane MAC for the control group was mean +/- s.e. 0.85 +/- 0.02% and no significant change occurred after saline administration. Morphine significantly (P = 0.002) decreased MAC from, mean +/- s.e. 0.90 +/- 0.05% to 0.77 +/- 0.06% in the pelvic limb. 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), respectively. No significant change in MAC occurred following butorphanol. No treatment reduced halothane MAC for the thoracic limb.