Digitalis acts in atrial fibrillation by ?
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.
Increasing AV node refractoriness
Decreasing atrial contractility
Inhibiting Na+ K+ ATPase pump
Inhibiting Na+ H+ ATPase pump