Digitalis acts in atrial fibrillation by

Ans. is 'a' i.e., Increase in A.V. node refractoriness Digitalis in arrhythma Digitalis is the drug of choice for controlling ventricular rate in atrial fibrillation (AF) with hea failure. Digitalis reduces the ventricular rate in AF by decreasing the number of impulses that are able to pass down the A-V node and bundle of His. It increases effective refractory period-ERP refractoriness) of A-V node by direct, vagomimetic and antiadrenergic action - the minimum interval between consecutive impulses that can successfully traverse the conducting tissue is increased. A degree of A-V block is naturally established in AF. Because of the relatively long ERP of A-V node, many atrial impulses impinge on it while it is still refractory; others falling early in the relative refractory period get extinguished by decremental conduction - These concealed impulses leave upper margin of A-V node refractory for a fuher period. Thus, any influence which increases atrial rate in AF (e.g. digitalis), by itself reduces ventricular rate.