Effect of intravenous butorphanol infusion on the minimum alveolar concentration of isoflurane in cats

Objective To determine the effect of butorphanol, administered by intravenous (IV) infusion, on the minimum alveolar concentration of isoflurane (MAC(ISO)) in cats and to examine the dosage dependence of this effect.& nbsp;Study design Randomized, placebo-controlled, crossover experimental study. Animals A group of six healthy adult male neutered cats.& nbsp;Methods Cats were anesthetized with isoflurane in oxygen. A venous catheter was placed for fluid and drug administration, and an arterial catheter was placed for measurement of arterial pressure and blood sampling. Four treatments were administered at random with at least 2 week interval between treatments: saline (control), butorphanol low dosage (treatment LD; 0.25 mg kg(-1)& nbsp;IV bolus followed by 85 mg kg(-1) minute -1 for 20 minutes, then 43 mg kg(-1)& nbsp;minute(-1)& nbsp;for 40 minutes, then 19 mg kg(-1)& nbsp;minute(-1)), medium dosage (treatment MD, double the dosages in LD) and high dosage (treatment HD, quadruple the dosages in LD). MACISO was determined in duplicate using the bracketing technique and tail clamping. Pulse rate, arterial pressure, hemoglobin oxygen saturation, end-tidal partial pressure of carbon dioxide and arterial blood gas and pH were measured.& nbsp;Results Butorphanol reduced MAC(ISO) in a dosage-dependent manner, by 23 +/-& nbsp;8%, 37 +/- 12% and 68 +/-& nbsp;10% (mean +/- standard deviation) in treatments LD, MD and HD, respectively. The main cardiopulmonary effect observed was a decrease in pulse rate, significant in treatment HD compared with control.& nbsp;Conclusions and clinical relevance Butorphanol caused a dosage-dependent MAC(ISO) reduction in cats. IV infusion of butorphanol may be of interest for partial IV anesthesia in cats.