Objective: To compare the pre-emptive analgesic effects of epidural ketamine or S(+)-ketamine on post-incisional hyperalgesia.
Study design: Prospective randomized study.
Animals: Twenty-four mongrel dogs (1-5 years, weighing 11.9+/-1.8 kg).
Methods: Dogs were anesthetized with propofol (5 mg/kg intravenously) and a lumbosacral epidural catheter was placed. Dogs were randomly allocated to 3 groups, each with 8 dogs. The control group (CG) was administered saline solution (0.3 mL/kg); the ketamine group (KG) ketamine (0.6 mg/kg); and the S(+)-ketamine group (SG) S(+)-ketamine (0.6 mg/kg). The final volume was adjusted to 0.3 mL/kg in all groups. Five minutes after the epidural injection a surgical incision was made in the common pad of the right hind limb and was immediately closed with simple interrupted nylon suture. Respiratory (RR) and heart (HR) rates, rectal temperature (T), sedation (S), lameness score, and mechanical nociceptive threshold by von Frey filaments were evaluated before the propofol anesthesia and at 15, 30, 45, 60, 75, and 90 minutes and then at 2, 4, 6, 8, 12, and 24 hours after epidural injection.
Results: There were no differences in RR, HR, T, or S between groups. Motor blockade of the hind limbs was observed during 20+/-3.6 minutes in KG and during 30.6+/-7.5 minutes in SG (mean+/-SD). Mechanical force applied to obtain an aversive response was higher from 45 minutes to 12 hours in KG and from 60 to 90 minutes in SG, when compared with CG.
Conclusions: Pre-emptive epidural ketamine induced no alterations in RR and HR, and reduced post-incisional hyperalgesia for a longer time than did S(+) ketamine.
Clinical relevance: Although anesthetic and analgesic potency of S(+) ketamine is twice that of ketamine, the racemic form is seemingly better for post-incisional hyperalgesia.