Which of the following drugs is used for termination as well as prophylaxis of paroxysmal supraventricular tachycardia?
**Core Concept**
Adenosine is a purine nucleoside that selectively blocks AV nodal conduction, thus terminating and preventing paroxysmal supraventricular tachycardia (PSVT). It works by activating adenosine receptors in the AV node, leading to a decrease in AV nodal conduction velocity.
**Why the Correct Answer is Right**
Adenosine is administered intravenously for the rapid termination of PSVT. Its mechanism of action involves binding to adenosine receptors (A1 and A3) on the AV node, leading to a decrease in AV nodal conduction velocity and refractoriness. This results in the termination of the re-entrant circuit that maintains PSVT. Additionally, adenosine can be used for prophylaxis in patients with a history of PSVT, as it can help prevent the initiation of the re-entrant circuit.
**Why Each Wrong Option is Incorrect**
**Option A:** Digoxin is a cardiac glycoside that increases the force of contraction and slows AV nodal conduction, but it is not the first-line treatment for PSVT termination or prophylaxis.
**Option B:** Verapamil is a calcium channel blocker that can slow AV nodal conduction, but it is not used for rapid termination of PSVT due to its slower onset of action.
**Option C:** Quinidine is an antiarrhythmic drug that can prolong the QT interval and is not typically used for the termination or prophylaxis of PSVT.
**Clinical Pearl / High-Yield Fact**
Adenosine is the preferred initial treatment for PSVT due to its rapid onset of action and high efficacy in terminating the arrhythmia. A common side effect of adenosine is transient facial flushing and a sensation of shortness of breath.
**Correct Answer: D. Adenosine**