Effect of intravenous lidocaine and ketamine on the minimum alveolar concentration of isoflurane in goats

Objective. To evaluate the effects of IV lidocaine (L) and ketamine (K), alone and in combination (LK), on the minimum alveolar concentration (MAC) of isoflurane (ISO) in goats. Study design. Randomized crossover design Animals Eight, adult mixed breed castrated male goats, aged 1-2 years weighing 24-51 kg. Methods. Anesthesia was induced with ISO that was delivered via a mask. The tracheas were intubated and the animals ventilated to maintain an end-tidal carbon dioxide partial pressure between 25 and 30 mmHg (3.3-4 kPa). Baseline MAC (MACB) that prevented purposeful movement in response to clamping a claw was determined in triplicate. After MAC(B) determination, each goat received one of the following treatments, which were administered as a loading (LD) dose followed by a constant rate infusion, IV:L (2.5 mg kg(-1); 100 mu g kg(-1) minute 1), K (1.5 mg kg(-1); 50 mu g kg(-1) minute), L and K combination or saline, and the MAC (MAC(T)) was re-determined in triplicate. Plasma concentrations of L and K were measured around each MAC point and the values averaged. Results. The least-squares mean MACB for all treatments was 1.13 +/- 0.03%. L, K. and LK reduced (p < 0.05) MAC(B) by 18.3%, 49.6%, and 69.4%,, respectively. Plasma concentrations for L, K, and LK were 1617 +/- 385, 1535 +/- 251 and 1865 +/- 317/1467 +/- 185 ng mL(-1) respectively. No change (p > 0.05) occurred with saline. Conclusion. Lidocaine and K caused significant decreases in the MAC of ISO. The combination (LK) had an additive effect. However, the plasma L concentrations were less than predicted, as was the MAC reduction with L. Clinical relevance The use of L, K and the combination, at the doses studied, will allow a clinically important reduction in the concentration of ISO required to maintain general anesthesia in goats.