A 58-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 with a visible P wave buried in the T wave of the preceding beat. Blood pressure is 140/90 mmHg, and he is conscious and alert. Carotid massage produces no effect. What is the most appropriate next step in management?

Explanation

## Clinical Scenario Analysis This patient presents with a **regular narrow-complex tachycardia (RNCT)** at 180 bpm with hemodynamic stability (conscious, alert, BP adequate). The ECG finding of a P wave buried in the T wave of the preceding beat is pathognomonic for **atrioventricular reentrant tachycardia (AVNRT)** or **atrioventricular reentrant tachycardia (AVRT)**. ### Diagnostic Clues - Regular narrow-complex tachycardia - P wave buried in or immediately after the QRS (retrograde P wave) - Hemodynamically stable - Failed vagal maneuver (carotid massage) ## Management Algorithm for Stable RNCT ```mermaid flowchart TD A[Regular Narrow-Complex Tachycardia]:::outcome --> B{Hemodynamically Stable?}:::decision B -->|No| C[Synchronized DC Cardioversion]:::urgent B -->|Yes| D[Vagal Maneuvers]:::action D --> E{Converted?}:::decision E -->|Yes| F[Observe, investigate etiology]:::outcome E -->|No| G[IV Adenosine 6 mg rapid bolus]:::action G --> H{Converted?}:::decision H -->|Yes| I[Success - Observe]:::outcome H -->|No| J[IV Calcium Channel Blocker or Beta-blocker]:::action ``` ## Why Adenosine is First-Line **Key Point:** Adenosine is the drug of choice for acute termination of **hemodynamically stable RNCT** because: 1. **Rapid onset** — acts within seconds 2. **High efficacy** — terminates >90% of AVNRT/AVRT 3. **Ultra-short half-life** (~10 seconds) — minimal systemic toxicity 4. **Mechanism** — blocks AV nodal conduction, interrupting the reentrant circuit **High-Yield:** Adenosine must be given as a **rapid intravenous bolus** (6 mg initially, then 12 mg if needed) followed immediately by saline flush to ensure rapid delivery to the heart. ## Why Other Options Are Suboptimal | Option | Why Not First-Line | |--------|-------------------| | **Verapamil 5 mg IV** | Slower onset (1–2 min) than adenosine; reserved for adenosine failure or contraindication | | **DC Cardioversion** | Indicated only for **hemodynamic instability** (hypotension, altered consciousness, chest pain, pulmonary edema); this patient is stable | | **Oral Diltiazem** | Inappropriate route (IV preferred for acute termination); too slow for acute management; used for rate control in chronic SVT | **Clinical Pearl:** In patients with **Wolff-Parkinson-White (WPW) syndrome** with atrial fibrillation, avoid AV nodal blockers (adenosine, verapamil, diltiazem, beta-blockers) as they may accelerate conduction down the accessory pathway, causing ventricular fibrillation. However, this patient has AVNRT/AVRT (not AF), so adenosine is safe. ## Adenosine Dosing & Administration **Mnemonic: "6-12-12" rule** - First dose: **6 mg** IV rapid bolus - If unsuccessful after 1–2 minutes: **12 mg** IV rapid bolus - May repeat 12 mg once more if needed **Warning:** Do NOT give adenosine slowly or as an infusion — it must be a rapid bolus to reach the heart before being metabolized by red blood cells.