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Effect of ketamine hydrochloride on the analgesic effects of tramadol hydrochloride in horses with signs of chronic laminitis-associated pain.

OBJECTIVE
To investigate the effects of ketamine hydrochloride on the analgesic effects of tramadol hydrochloride in horses with signs of pain associated with naturally occurring chronic laminitis.


ANIMALS
15 client-owned adult horses with chronic laminitis.


PROCEDURES
Each horse received tramadol alone or tramadol and ketamine in a randomized, crossover study (≥ 2 months between treatments). Tramadol (5 mg/kg) was administered orally every 12 hours for 1 week. When appropriate, ketamine (0.6 mg/kg/h) was administered IV for 6 hours on each of the first 3 days of tramadol administration. Noninvasive systemic blood pressure values, heart and respiratory rates, intestinal sounds, forelimb load and off-loading frequency (determined via force plate system), and plasma tumor necrosis factor-α and thromboxane B(2) concentrations were assessed before (baseline) during (7 days) and after (3 days) each treatment.


RESULTS
Compared with baseline data, arterial blood pressure decreased significantly both during and after tramadol-ketamine treatment but not with tramadol alone. Forelimb off-loading frequency significantly decreased during the first 3 days of treatment with tramadol only, returning to baseline frequency thereafter. The addition of ketamine to tramadol treatment reduced off-loading frequency both during and after treatment. Forelimb load did not change with tramadol alone but increased with tramadol-ketamine treatment. Plasma concentrations of tumor necrosis factor-α and thromboxane B(2) were significantly reduced with tramadol-ketamine treatment but not with tramadol alone.


CONCLUSIONS AND CLINICAL RELEVANCE
In horses with chronic laminitis, tramadol administration induced limited analgesia, but this effect was significantly enhanced by administration of subanesthetic doses of ketamine.