A comparison of two morphine doses on the quality of recovery from general anaesthesia in horses.

Previous studies indicated that perioperative morphine improves recovery quality after general anaesthesia in horses. This clinical trial investigated whether this effect was dose-dependant. Twenty-six horses anaesthetised for surgery were block-randomised to receive one of two intraoperative morphine treatments: Treatment M1 consisted of a constant rate infusion (CRI) of morphine at 0.1mg/kg/h, begun after a loading dose (LD) of 0.15 mg/kg. Treatment M2 was a CRI of 0.2mg/kg/h preceded by an LD of 0.3mg/kg. During recovery, times at the first attempt and at achieving sternal recumbency and standing, and the total recovery duration were not different between groups. Total recovery quality score was not significantly different between groups. Scores for the "sternal phase" were higher (of poorer quality) for M2 but scores in both groups were low. Morphine doses of M2, which are greater than those described previously, do not appear to alter or improve the quality or duration of recovery in horses after surgical anaesthesia.