The pre-emptive effect of epidural ketamine on wound sensitivity in horses tested by using von Frey filaments

To evaluate the pre-emptive analgesic effect of pre-incisional epidural ketamine.

A blinded, randomized experimental study.

Sixteen mixed breed mares, 7.6 ± 2.8 years old, weighing 352 ± 32 kg.

In a pilot study, an incision was made on one lateral thigh using a lidocaine block and no further analgesics, and it was verified that the nociceptive threshold was lower on the incised side than nonincised side (p ≤ 0.05), and that von Frey filaments evoked a pain response. The 16 animals were divided into group A (ketamine, n = 9) and B (saline, n = 7). An epidural catheter was inserted 24 hours before the trials. The thigh was shaved bilaterally, and the right side was blocked (incised side) using lidocaine. Twenty-five minutes later, ketamine (A) or saline (B) was administered epidurally. Five minutes later, a 10-cm skin incision was made on the right side, and then sutured. Nociceptive threshold was determined with von Frey filaments at 1, 3, and 5 cm around the incision at 15-minute intervals for 2 hours, then at 4, 6, and 8 hours. Behavioral alterations, heart and respiratory rates were recorded. Nociceptive thresholds from these points were averaged to obtain mean values at each time, converted to a logarithmic scale, and submitted to a nonparametric analysis (Mann-Whitney and one-way repeated measures anova test, p ≤ 0.05).

After 8 hours, the global range score revealed reduced hyperalgesia (p < 0.01) around the incision in 92% (4.65-4.27) of evaluated intervals in group A (ketamine). There were no significant changes in behavior, heart and respiratory rates.

It was concluded that pre-emptive epidural ketamine reduced post-incisional pain in the horse, and that von Frey filaments were able to quantify cutaneous sensitivity after tissue damage.

Epidural ketamine injection can reduce post-incisional sensitivity in the horse.