Myocardial Sensitization by Thiopental to Arrhythmogenic Action of Epinephrine in Dogs

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Abstract
This study examined the interaction between thiopental and epinephrine in inducing ventricular arrhythmias in dogs. The arrhythmogenic threshold of epinephrine was determined during anesthesia with either halothane alone, thiopental alone, etomidate plus different doses of thiopental, or halothane plus different doses of thiopental. The arrhythmogenic dose and the corresponding plasma concentration of epinephrine during thiopental anesthesia (plasma thiopental concentration: 46-57 .mu.g/ml) were 0.77 .+-. 0.04 .mu.g .cntdot. kg-1 .cntdot. min-1 and 10.7 .+-. 1.5 ng/ml, respectively. During halothane anesthesia (end-tidal: 1.3 MAC) they were 2.59 .+-. 0.49 .mu.g .cntdot. kg-1 .cntdot. min-1 and 45.3 .+-. 9.2 ng/ml, respectively. The dose-effect relationship for the thiopental action was examined during etomidate plus thiopental and halothane plus thiopental anesthesia. The arrhythmogenic plasma concentration of epinephrine was inversely proportional to the plasma thiopental concentration during both anesthetics. During etomidate plus thiopental anesthesia, at plasma thiopental concentrations of 0, 11.2 .+-. 0.83, 20.1 .+-. 1.34, and 33.2 .+-. 1.95 .mu.g/ml, the corresponding epinephrine concentrations were 201.3 .+-. 34.3, 142 .+-. 19.5, 69.1 .+-. 21.2, and 22.7 .+-. 4.5 ng/ml. During halothane plus thiopental anesthesia, at plasma thiopental concentrations of 0, 10 .+-. 0.86, 18.3 .+-. 0.87, and 31.8 .+-. 1.05 .mu.g/ml, the corresponding epinephrine concentrations were 45.3 .+-. 9.2, 34.6 .+-. 8.9, 16.2 .+-. 1.74, and 15.1 .+-. 1.32 .mu.g/ml, respectively. These results suggest that thiopental sensitizes the heart to epinephrine in a dose-dependent manner. This sensitizing action of thiopental would in part explain the thiopental potentiation of hydrocarbon anesthetic-epinephrine arrhythmias.