The effects of ketamine on the minimum alveolar concentration of isoflurane in cats

Objectives: To determine the minimum alveolar concentration (MAC) of isoflurane during the infusion of ketamine. Study design: Prospective, experimental trial. Animals: Twelve adult spayed female cats weighing 5.1 ± 0.9 kg Methods: Six cats were anesthetized with isoflurane in oxygen, intubated and attached to a circle-breathing system with mechanical ventilation. Catheters were placed in a peripheral vein for the infusion of fluids and ketamine, and the jugular vein for blood sampling for the measurement of ketamine concentrations. An arterial catheter was placed to allow blood pressure measurement and sampling for the measurement of PaCO_2, PaO ₂ and pH. PaCO₂ was maintained between 29 and 41 mmHg (3.9-5.5 kPa) and body temperature was kept between 37.8 and 39.3°C. Following instrumentation, the MAC of isoflurane was determined in triplicate using a tail clamp method. A loading dose (2 mg kg^-1 over 5 minutes) and an infusion (23 μg kg^-1 minute^-1) of ketamine was started and MAC was redetermined starting 30 minutes later. Two further loading doses and infusions were used, 2 mg kg^-1 and 6 mg kg^-1 with 46 and 115 μg kg^-1 minute^-1, respectively and MAC was redetermined. Cardiopulmonary measurements were taken before application of the noxious stimulus. The second group of six cats was used for the measurement of steady state plasma ketamine concentrations at each of the three infusion rates used in the initial study and the appropriate MAC value determined from the first study. Results: The MAC decreased by 45 ± 17%, 63 ± 18%, and 75 ± 17% at the infusion rates of 23, 46, and 115 μg kg ^-1 minute^-1. These infusion rates corresponded to ketamine plasma concentrations of 1.75 ± 0.21, 2.69 ± 0.40, and 5.36 ± 1.19 μg mL^-1. Arterial blood pressure and heart rate increased significantly with ketamine. Recovery was protracted. Conclusions and clinical relevance: The MAC of isoflurane was significantly decreased by an infusion of ketamine and this was accompanied by an increase in heart rate and blood pressure. Because of the prolonged recovery in our cats, further work needs to be performed before using this in patients.