ON VAGAL AND EXTRAVAGAL FACTORS IN CARDIAC SLOWING BY DIGITALIS IN PATIENTS WITH AURICULAR FIBRILLATION

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Abstract
A study was made in 9 patients with auricular fibrillation to determine the effect of atropine upon the ventricular rate after varying doses of digitalis in the same individual. The object was to determine the relative roles of vagal and extravagal factors in cardiac, slowing by digitalis. The intraven. dose of atropine which suffices completely to block the cardiac vagus was about 1/30 grain, since larger doses produced no greater effects upon rate. Atropine cannot prevent cardiac slowing in auricular fibrillation if the dose of digitalis is sufficiently large. Moderate doses of digitalis slow the heart by the vagal mechanism (abolished by atropine). Larger doses in the same individual slow the heart chiefly by an extravagal mechanism (direct action on conduction and not abolished by atropine). Evidently the only factor which determines the mechanism of slowing in the patient with auricular fibrillation is the amt. of digitalis, and either mechanism, vagal or extravagal, may be invoked in the same individual.