To the Editor:—
We read with interest the study by MacGregor et al. , 1which showed that markedly different dopamine plasma levels may result from similar infusion rates in a homogenous population. Their results bear considerable similarity to a study we performed with epinephrine infusions at a time when arrhythmogenicity of anesthetics was a major concern. We identified that clearance of epinephrine and cardiac output were markedly and variably enhanced as much as 45 min after a large but brief epinephrine infusion in a canine model 2; however, arrhythmias tended to occur at similar plasma levels. Presumably because of this difference in cardiac output and clearance of epinephrine, different infusion rates were necessary to produce similar plasma levels of epinephrine; however, the range was not as dramatic as those determined by MacGregor et al. 1with dopamine. Because dopamine and epinephrine both stimulate a multitude of catecholamine receptors, some of which have opposing hemodynamic effects, one should not find these results surprising. The phenomenon of attaining markedly different plasma levels for a particular infusion rate may be a property shared by catecholamines that stimulate multiple adrenoceptors. The results may be even more variable when subjected to interactions with α- and β-adrenoceptor blocking drugs, which may alter the effects of these catecholamines on circulation and ultimately on their own clearance.