Digitalis acts in atrial fibrillation by ?
Increasing AV node refractoriness
Decreasing atrial contractility
Inhibiting Na+ K+ ATPase pump
Inhibiting Na+ H+ ATPase pump
Ans. is 'a' i.e., Increase in A.V. node refractoriness Digitalis in arrhvthma o Digitalis is the drug of choice for controlling ventricular rate in atrial fibrillation (AF) with hea failure. o 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 1. It increases effective refractory period-ERP (Le 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. 2. 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 ofA-V node refractory for a fuher period. Thus, any influence which increases atrial rate in AF (e.g. digitalis), by itself reduces ventricular rate. o Inhibition of Na+ K+ ATPase is responsible for inotropic action of digitalis, not for antiarrhythmic action. o Digitalis has two impoant pharmacological actions - I. Increased contractility of hea by inhibiting Na+ ATPase This action is beneficial in cardiac failure. 2. Increased refractoriness of SA node and AV node .1 automaticity and conduction This action is beneficial in arrhythmia.