This question asks to identify the correct match between a specific arrhythmia and its expected response to intravenous (IV) adenosine.

Understanding Adenosine and Arrhythmias

Adenosine is a medication frequently used in clinical practice, particularly in cardiology. It's an endogenous nucleoside that acts primarily on the heart by binding to specific receptors (A1 receptors) predominantly located in the sinoatrial (SA) and atrioventricular (AV) nodes.

• Mechanism of Action: Adenosine slows down the electrical conduction time through the AV node. It also causes a transient block of AV nodal conduction and can prolong the refractory period of the AV node. This effect is crucial for treating certain types of rapid heart rhythms (tachycardias).
• Primary Use: Its main indication is the termination of paroxysmal supraventricular tachycardias (SVTs), especially those involving the AV node, such as AVNRT and AVRT.

Analysis of Arrhythmia Responses to IV Adenosine

Option (a): Atrial fibrillation Response - Termination

Intravenous adenosine is generally not effective in terminating atrial fibrillation. While it might temporarily slow the ventricular rate by affecting the AV node, it does not terminate the underlying disorganized atrial electrical activity.

Option (b): Atrio-ventricular nodal reentrant tachycardia (AVNRT) Response - Termination

This is a correct match. AVNRT is a common type of SVT caused by a reentrant circuit that typically involves the AV node. Adenosine's property of slowing AV nodal conduction and causing transient block effectively interrupts this reentrant pathway, leading to termination of the tachycardia. This is a primary indication for IV adenosine administration.

Option (c): Ventricular tachycardia (VT) Response - Termination and complete recovery

Adenosine is generally not effective in terminating ventricular tachycardia. VT originates from the ventricles, below the His-Purkinje system, and adenosine's primary action is on the AV node. Therefore, it does not typically terminate VT.

Option (d): Atrial flutter Response - Termination and complete recovery

Similar to atrial fibrillation, adenosine is usually not effective in terminating atrial flutter. It may slow the ventricular response by blocking conduction through the AV node, but it does not terminate the organized, rapid atrial rhythm (flutter waves).

Conclusion

Based on the mechanism of action and clinical use of intravenous adenosine, it is most reliably effective in terminating arrhythmias caused by reentrant circuits involving the AV node. Among the given options, Atrio-ventricular nodal reentrant tachycardia (AVNRT) fits this description.