Drug of choice for paroxysmal supraventricular tachycardia is: March 2010, September 2007and 2010, March 2013

Ans. C: Adenosine Initial measures to terminate SVT should include vagal maneuvers. If unsuccessful, termination can be achieved with antiarrhythmic drugs whose primary effects increase refractoriness and/ or decrease conduction (negative dromotropic effect) over the AV node. These drugs can have direct (e.g., verapamil blocks the slow-inward calcium current of the AV node) or indirect effects (e.g., digoxin increases vagal tone to the AV node). In most patients, the drug of choice is either adenosine or verapamil. The advantages of adenosine include its rapid onset of action (usually within 10-25 seconds a peripheral vein), sho half-life (less than 10 seconds) and high degree of efficacy. Its sho half-life minimizes the severity of side effects, such as facial flushing, chest tightness, dyspnea and transient sinus arrest and/or atrioventricular block. In general, adenosine has an excellent safety record, but pause-dependent torsade de pointes may infrequently occur and transient atrial fibrillation may occur due to shoening of the atrial action potential. Adenosine should be given with caution to patients with bronchospasm and is contraindicated in those with sinus node dysfunction or second- or third-degree hea block. Dipyridamole potentates adenosine's effects by blocking its cellular uptake and aminophylline blunts its effects by competitive receptor antagonism. Sequential dosing can be given at 60 second intervals due to adenosine's rapid metabolism. In AV node reentry the most common site of termination is the anterograde slow pathway. Termination may also occur indirectly, i.e., due to adenosine - induced atrial or ventricular premature beats