Ketamine used for epidural analgesia in surgery can prevent persistent pain. Recently, a levogyral (S[+]) form has become available, which provides 2 to 3 times the analgesia of the racemic (R[-]) form without some of the undesirable side effects. This study evaluated different doses of S(+) ketamine by itself and with morphine. Dogs undergoing ovariohysterectomy were randomly assigned to groups. Group 1 received 1 mg/kg S(+) ketamine, group 2 received 0.5 mg/kg S(+) ketamine plus 0.05 mg/kg morphine, and group 3 received 1 mg/kg S(+) ketamine plus 0.025 mg/kg morphine. Dogs were anesthetized with propofol and maintained with halothane. Variables measured were heart rate, respiratory rate, and systolic blood pressure. During the intraoperative period, an increase of 20% or more was considered a sign of pain. Signs of pain were also assessed at 2, 4, and 8 hours after surgery. None of the dogs had signs of intraoperative pain. There was no significant difference in postoperative pain among the 3 groups. Salivation was the most frequent adverse effect in dogs in groups 1 and 3. This study indicates that epidural administration of S(+) ketamine alone or in combination with morphine provides adequate analgesia for dogs undergoing ovariohysterectomy.

COMMENTARY: Newer understanding of the role of N-methyl-D-aspartate (NMDA) receptors has led to an increased interest in ketamine for pain control. Most ketamine available is a mixture with equal parts of S(+) and R(-) isomers. S(+) ketamine has a greater affinity for the NMDA receptors. A previous study (Vet Surg 33:361-367, 2004) indicated that the racemic form was better for postincisional hyperalgesia. More research needs to be conducted to study the S(+) form as well as the role of ketamine as an adjunct analgesic.

Analgesic effects of epidurally administered levogyral ketamine alone or in combination with morphine on intraoperative and postoperative pain in dogs undergoing ovariohysterectomy. Acosta AD, Gomar C, Correa-Natalini C, et al. AM J VET RES 66:54-61, 2005.