S-ketamine versus racemic ketamine in dogs: their relative potency as induction agents.

To determine the potency ratio between S-ketamine and racemic ketamine as inductive agents for achieving tracheal intubation in dogs. Prospective, randomized, 'blinded', clinical trial conducted in two consecutive phases. 112 client-owned dogs (ASA I or II). All animals were premedicated with intramuscular acepromazine (0.02 mg kg(-1)) and methadone (0.2 mg kg(-1)). In phase 1, midazolam (0.2 mg kg(-1)) with either 3 mg kg(-1) of racemic ketamine (group K) or 1.5 mg kg(-1) of S-ketamine (group S) was administered IV, for induction of anaesthesia and intubation. Up to two additional doses of racemic (1.5 mg kg(-1)) or S-ketamine (0.75 mg kg(-1)) were administered if required. In phase 2, midazolam (0.2 mg kg(-1)) with 1 mg kg(-1) of either racemic ketamine (group K) or S-ketamine (group S) was injected and followed by a continuous infusion (1 mg kg minute(-1)) of each respective drug. Differences between groups were statistically analyzed via t-test, Fisher exact test and ANOVA for repeated measures. Demographics and quality and duration of premedication, induction and intubation were comparable among groups. During phase 1 it was possible to achieve tracheal intubation after a single dose in more dogs in group K (n = 25) than in group S (n = 16) (p = 0.046). A dose of 3 mg kg(-1) S-ketamine allowed tracheal intubation in the same number of dogs as 4.5 mg kg(-1) of racemic ketamine. The estimated potency ratio was 1.5:1. During phase 2, the total dose (mean ± SD) of S-ketamine (4.02 ±1.56 mg kg(-1)) and racemic ketamine (4.01 ± 1.42) required for tracheal intubation was similar. Racemic and S-ketamine provide a similar quality of anaesthetic induction and intubation. S-ketamine is not twice as potent as racemic ketamine and, if infused, the potency ratio is 1:1.