A 52-year-old man with a history of hypertension and left ventricular hypertrophy presents to the emergency department with palpitations and dyspnea. His ECG shows a regular narrow-complex tachycardia at 180 bpm. Carotid sinus massage fails to terminate the arrhythmia. Intravenous adenosine 6 mg is administered but produces only transient slowing of the heart rate without termination. What is the most appropriate next step in management?

Explanation

## Clinical Scenario Analysis This patient presents with a regular narrow-complex tachycardia refractory to adenosine — a finding highly suggestive of **atrioventricular nodal reentrant tachycardia (AVNRT)** or **atrioventricular reentrant tachycardia (AVRT)** that involves an accessory pathway with properties different from the AV node. ## Mechanism of Adenosine Failure **Key Point:** Adenosine blocks AV nodal conduction by activating adenosine A1 receptors, causing transient AV block. However, some arrhythmias — particularly those using an **accessory pathway (AVRT)** — may conduct anterograde through the accessory pathway and thus escape adenosine's effect if the reentrant circuit does not critically depend on AV nodal conduction. ## Rationale for Verapamil **High-Yield:** Verapamil, a non-dihydropyridine calcium channel blocker, is the **Class IV antiarrhythmic** agent of choice for adenosine-refractory narrow-complex tachycardia. It: 1. Slows AV nodal conduction velocity 2. Prolongs AV nodal refractoriness 3. Terminates reentry by blocking the AV node (the critical node in most AVNRT/AVRT circuits) 4. Has a success rate of 60–80% in adenosine-refractory SVT **Clinical Pearl:** Verapamil is preferred over diltiazem in acute SVT because it has faster onset and higher efficacy in terminating reentrant arrhythmias. ## Why Not Other Options? | Option | Reason | |--------|--------| | Repeat adenosine 12 mg | Adenosine has already failed at 6 mg; higher doses are unlikely to succeed and waste time. | | Amiodarone 150 mg | Amiodarone (Class III) is a second-line agent reserved for hemodynamically unstable patients or when verapamil/diltiazem fail. This patient is hemodynamically stable (no mention of shock or severe distress). | | Synchronized cardioversion | Cardioversion is reserved for hemodynamically unstable SVT or when pharmacologic therapy fails. This patient is stable and has not yet received a trial of verapamil. | ## Treatment Algorithm for Adenosine-Refractory SVT ```mermaid flowchart TD A[Regular narrow-complex tachycardia]:::outcome --> B[Vagal maneuvers + Adenosine 6 mg]:::action B --> C{Terminated?}:::decision C -->|Yes| D[Diagnosis confirmed; observe]:::outcome C -->|No| E{Hemodynamically stable?}:::decision E -->|Yes| F[Verapamil or Diltiazem IV]:::action E -->|No| G[Synchronized cardioversion]:::urgent F --> H{Terminated?}:::decision H -->|Yes| I[Arrhythmia terminated]:::outcome H -->|No| J[Amiodarone or EP consultation]:::action ``` **Key Point:** The stepwise approach is: adenosine → calcium channel blocker → amiodarone/cardioversion. Verapamil is the logical next step after adenosine failure in a stable patient.