Pharmacokinetics of remifentanil in conscious cats and cats anesthetized with isoflurane

Objective - To characterize the pharmacokinetics of remifentanil in conscious cats and cats anesthetized with isoflurane. Animals - 6 cats. Procedures - Remifentanil (1 μg/kg/min for 5 minutes) was administered IV in conscious cats or cats anesthetized with 1.63% isoflurane in oxygen in a randomized crossover design. Blood samples were obtained immediately prior to remifentanil administration and every minute for 10 minutes, every 2 minutes for 10 minutes, and every 5 minutes for 10 minutes after the beginning of the infusion. Blood was immediately transferred to tubes containing citric acid, flash frozen in liquid nitrogen, and stored at -80°C until analysis. Blood remifentanil concentration was determined by use of liquid chromatography-mass spectrometry. Remifentanil concentration-time data were fitted to compartment models. Results - A 2-compartment model (with zero-order input because of study design) best described the disposition of remifentanil in awake and isoflurane-anesthetized cats. The apparent volume of distribution of the central compartment, the apparent volume of distribution at steady state, the clearance, and the terminal half-life (median [range]) were 1,596 (1,164 to 2,111) and 567 (278 to 641) mL/kg, 7,632 (2,284 to 76,039) and 1,651 (446 to 29,229) mL/kg, 766 (408 to 1,473) and 371 (197 to 472) mL/min/kg, and 17.4 (5.5 to 920.3) and 15.7 (3.8 to 410.3) minutes in conscious and anesthetized cats, respectively. Conclusions and Clinical Relevance - The disposition of remifentanil in cats was characterized by a high clearance. Isoflurane anesthesia significantly decreased the volume of the central compartment, likely by decreasing blood flow to vessel-rich organs.