A 28-year-old woman with no structural heart disease presents with recurrent episodes of palpitations and syncope. ECG during an episode shows a narrow complex tachycardia with a rate of 220 bpm. She is haemodynamically stable. What is the drug of choice for acute termination of this arrhythmia?

Explanation

## Acute Management of Supraventricular Tachycardia (SVT) **Key Point:** Adenosine is the first-line pharmacological agent for acute termination of haemodynamically stable SVT in patients without structural heart disease. ### Mechanism of Action Adenosine acts as an AV nodal blocker by: 1. Binding to adenosine A1 receptors on AV nodal tissue 2. Increasing potassium conductance (hyperpolarization) 3. Decreasing calcium conductance 4. Slowing AV nodal conduction and increasing refractoriness ### Dosing & Administration - **Initial dose:** 6 mg IV rapid bolus (over 1–2 seconds) - **Second dose:** 12 mg IV if first dose fails (after 1–2 min) - **Success rate:** 90–95% in AV nodal reentrant tachycardia (AVNRT) and AV reentrant tachycardia (AVRT) - **Onset:** Within seconds; half-life ~10 seconds ### Advantages Over Alternatives | Feature | Adenosine | Verapamil | Digoxin | Amiodarone | |---------|-----------|-----------|---------|----------| | **Onset** | Seconds | 2–5 min | 30 min–2 hrs | 10–20 min | | **Haemodynamic effect** | Minimal | Significant ↓BP | Minimal | Significant ↓BP | | **Structural heart disease** | Safe | Avoid | Use cautiously | Safe | | **First-line in stable SVT** | Yes | Alternative | No | No | | **Termination rate** | 90–95% | 70–80% | 60–70% | 60–70% | **High-Yield:** Adenosine must be given as a **rapid IV bolus** followed immediately by saline flush to ensure delivery to the AV node before rapid metabolism by red blood cells. ### Side Effects of Adenosine - Transient dyspnoea, chest discomfort, flushing (very common, benign) - Bronchospasm (contraindicated in asthma/COPD) - Atrial fibrillation (rare, usually self-terminating) - Asystole (rare, brief) **Clinical Pearl:** In patients with asthma or COPD, verapamil or diltiazem are preferred alternatives despite slower onset, as adenosine-induced bronchospasm can be severe. ### When Adenosine Fails - Confirm diagnosis (obtain 12-lead ECG) - Exclude atrial flutter (will show AV block with flutter waves visible) - Consider second-line agents: verapamil, diltiazem, or beta-blockers - If haemodynamically unstable → synchronized DC cardioversion (100–200 J)