A 58-year-old man with a history of hypertension and coronary artery disease presents to the emergency department with palpitations and dyspnea. On examination, his heart rate is 160 bpm, blood pressure is 110/70 mmHg, and he is in acute distress. ECG shows a regular, narrow-complex tachycardia with a rate of 160 bpm. Chest X-ray is normal. Carotid massage and adenosine IV bolus are ineffective. He is haemodynamically stable. Which of the following antiarrhythmic agents would be the most appropriate next step in management?

Explanation

## Clinical Scenario Analysis The patient presents with a **regular, narrow-complex tachycardia** (SVT) that is **haemodynamically stable** and **refractory to adenosine**. This narrows the differential to: - Atrioventricular nodal reentrant tachycardia (AVNRT) — most common - Atrioventricular reentrant tachycardia (AVRT) - Atrial flutter with fixed AV conduction - Atrial tachycardia ## Drug Selection Logic **Key Point:** In haemodynamically stable SVT refractory to adenosine, **calcium channel blockers (verapamil or diltiazem)** are the next-line agents of choice. **High-Yield:** Verapamil is preferred because: 1. **Blocks AV node conduction** — the critical pathway in AVNRT/AVRT 2. **Rapid onset** (1–2 minutes IV) 3. **High efficacy** — converts ~90% of AVNRT when adenosine fails 4. **Haemodynamic stability** maintained in this patient (BP 110/70) ## Why Verapamil Over Alternatives | Agent | Mechanism | Use in SVT | Limitation | |-------|-----------|-----------|------------| | **Verapamil** | L-type Ca²⁺ channel blocker; AV node suppression | First-line after adenosine failure | Contraindicated in WPW + AF | | Amiodarone | Class III (K⁺ channel); also I, II, IV effects | Reserved for haemodynamic instability or refractory cases | Slower onset, more toxicity | | Procainamide | Class IA (Na⁺ channel); prolongs QRS/QT | Atrial arrhythmias; less effective for AVNRT | Risk of hypotension, lupus-like syndrome | | Digoxin | AV node suppression via vagomimetic effect | Chronic rate control; slow onset | Narrow therapeutic index; ineffective acutely | **Clinical Pearl:** Adenosine failure does **not** rule out AVNRT; it may indicate: - Insufficient dose or rapid metabolism - Theophylline/caffeine use blocking adenosine receptors - Operator error (not given as rapid IV bolus) Verapamil is the appropriate escalation in this scenario. ## Mechanism of Action ```mermaid flowchart TD A[SVT refractory to adenosine]:::outcome --> B{Haemodynamically stable?}:::decision B -->|Yes| C[Verapamil IV 5-10 mg bolus]:::action C --> D[Blocks L-type Ca²⁺ channels]:::action D --> E[Slows AV node conduction]:::action E --> F[Breaks reentrant circuit]:::action F --> G[Sinus rhythm restored]:::outcome B -->|No| H[Synchronized cardioversion]:::urgent ``` **Dosing:** Verapamil 5–10 mg IV over 2–3 minutes; may repeat 10 mg after 15–30 minutes if needed. [cite:Harrison 21e Ch 297]