Step 2 CK First Aid Rapid Practice Test

When stable SVT persists after vagal maneuvers, the drug of choice is adenosine because it acutely and briefly blocks AV nodal conduction, which interrupts the reentrant circuit that most commonly involves the AV node. Adenosine works so quickly and has a very short half-life, so its effects wear off within seconds, allowing a rapid return to sinus rhythm if the SVT is AV nodal dependent. The typical approach is a rapid IV push of 6 mg, followed by a flush; if the rhythm does not convert, a second dose of 12 mg can be given. Mechanistically, adenosine activates A1 receptors in the AV node, opening potassium channels and hyperpolarizing the nodal cells. This transiently halts conduction through the AV node, effectively terminating the reentrant loop that maintains the tachycardia. Other options are less ideal in this immediate context. Atropine is used for symptomatic bradycardia, not for terminating SVT. Metoprolol can slow AV nodal conduction, but it is slower to act and may not terminate the tachycardia as reliably or rapidly as adenosine. Amiodarone has broader antiarrhythmic uses and is more appropriate for refractory tachyarrhythmias or wide-complex tachycardias, rather than the initial pharmacologic step in classic AV nodal–dependent SVT.