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Objectives: To determine if ketamine administered to bitches at the end of a mastectomy, followed by a six-hour constant rate infusion (CRI), improved postoperative opioid analgesia and feeding behaviour.

Methods: The bitches were randomised into three groups: the placebo group received 0·09 ml/kg isotonic saline intravenously followed by a six-hour CRI of 0·5 ml/kg/hour, the low-dose ketamine received 150 μg/kg ketamine intravenously followed by a six-hour CRI of 2 μg/kg/minute and the high-dose ketamine group received 700 μg/kg ketamine intravenously followed by a six-hour CRI of 10 μg/kg/minute. Any additional opioids given were recorded at the time of extubation and at intervals after extubation. Food intake was evaluated eight (T8) and 20 (T20) hours after extubation by measuring the per cent coverage of basal energy requirements (BER).

Results: No significant difference was observed for opioid requirements between the three groups. The mean percentages of BER coverage did not differ significantly at T8 but the difference between the high-dose and low-dose ketamine groups (P=0·014), and the high-dose ketamine and placebo groups (P=0·038) was significant at T20.

Clinical Significance: This study demonstrated that 700 μg/kg ketamine given intravenously postoperatively followed by a six-hour ketamine CRI of 10 μg/kg/minute improved patient feeding behaviour.