Effect of dexmedetomidine on the minimum alveolar concentration of isoflurane in cats

This study reports the effects of dexmedetomidine on the minimum alveolar concentration of isoflurane (MAC _iso) in cats. Six healthy adult female cats were used. MAC _iso and dexmedetomidine pharmacokinetics had previously been determined in each individual. Cats were anesthetized with isoflurane in oxygen. Dexmedetomidine was administered intravenously using target-controlled infusions to maintain plasma concentrations of 0.16, 0.31, 0.63, 1.25, 2.5, 5, 10, and 20ng/mL. MAC _iso was determined in triplicate at each target plasma dexmedetomidine concentration. Blood samples were collected and analyzed for dexmedetomidine concentration. The following model was fitted to the concentration-effect data: where MAC _iso.c is MAC _iso at plasma dexmedetomidine concentration C, MAC _iso.0 is MAC _iso in the absence of dexmedetomidine, I _max is the maximum possible reduction in MAC _iso, and IC ₅₀ is the plasma dexmedetomidine concentration producing 50% of I _max. Mean±SE MAC _iso.0, determined in a previous study conducted under conditions identical to those in this study, was 2.07±0.04. Weighted mean±SE I _max, and IC ₅₀ estimated by the model were 1.76±0.07%, and 1.05±0.08ng/mL, respectively. Dexmedetomidine decreased MAC _iso in a concentration-dependent manner. The lowest MAC _iso predicted by the model was 0.38±0.08%, illustrating that dexmedetomidine alone is not expected to result in immobility in response to noxious stimulation in cats at any plasma concentration.