Alpha 1-adrenergic blockade raises epinephrine-arrhythmia threshold in halothane-anesthetized dogs in a dose-dependent fashion.

The authors determined whether increasing alpha 1-adrenergic blockade resulted in progressively less arrhythmic activity in the canine halothane-epinephrine arrhythmia model. Dogs (n = 7) were anesthetized with halothane (1.5%) in oxygen. Stepwise increases in steady-state plasma levels of either of two alpha 1-adrenoceptor antagonists (droperidol, doxazosin) were produced by applying Wagnerian principles to the known pharmacokinetic parameters of these drugs. At each steady state plasma level of these antagonists, the extent of the alpha 1-adrenergic blockade produced was assessed by defining a phenylephrine (PE) dose pressor response curve. The degree of alpha 1-blockade produced was quantitated as the dose of PE that caused a 25-mmHg increase in mean arterial pressure (ED25) as derived by polynomial regression analysis. By analysis of variance (ANOVA) the ED25 increased significantly for each targeted steady state plasma level of either droperidol (P less than 0.001) or doxazosin (P less than 0.001). For an assessment of the antiarrhythmic activity of these alpha 1-antagonists, the arrhythmogenic dose of epinephrine (ADE) was determined at each of the states of alpha 1-adrenergic blockade previously defined. By ANOVA there was a significant increase in the ADE over the range of alpha blockade produced for either droperidol (P less than 0.001) or doxazosin (P less than 0.001). A close correlation (r2) existed between the ED25 and the ADE for the target steady state levels that were achieved for either droperidol (0.99) or doxazosin (0.74).(ABSTRACT TRUNCATED AT 250 WORDS)