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 targetcontrolled infusions to maintain plasma concentrations of 0.16, 0.31, 0.63, 1.25, 2.5, 5, 10, and 20 ng / 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: MAC(iso.c) = MAC(iso.0) -I-max x C/C + IC50 where MAC(iso.c) is MAC(iso) at plasma dexmedetomidine concentration C, MAC(iso). 0 is MAC(iso) in the absence of dexmedetomidine, Imax is the maximum possible reduction in MAC(iso), and IC50 is the plasma dexmedetomidine concentration producing 50% of Imax. 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 Imax, and IC50 estimated by the model were 1.76 +/- 0.07%, and 1.05 +/- 0.08 ng / 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.