## Clinical Scenario Analysis This patient has WPW syndrome with orthodromic atrioventricular reentrant tachycardia (AVNRT), presenting haemodynamically stable with a regular narrow-complex tachycardia. ## Management of Stable SVT in WPW **Key Point:** In haemodynamically stable patients with SVT due to WPW, adenosine is the first-line pharmacological agent because it blocks AV nodal conduction and terminates the reentrant circuit. **High-Yield:** Adenosine works by: 1. Blocking AV node conduction (the critical part of the reentrant circuit in orthodromic AVNRT) 2. Having a very short half-life (~10 seconds), making it safe if adverse effects occur 3. Achieving termination in >90% of AVNRT cases ## Why Adenosine Is Correct - **Mechanism:** Adenosine causes transient AV block, interrupting the reentrant pathway - **Dosing:** Initial bolus is 6 mg IV rapid push (peripheral line preferred), followed by 12 mg if needed - **Safety:** Short duration of action allows rapid recovery if hypotension or bradycardia occurs - **Efficacy:** Diagnostic and therapeutic—terminates the arrhythmia in stable patients ## Comparison with Other Agents | Agent | Use in WPW | Rationale | |-------|-----------|----------| | Adenosine | First-line (stable) | Safe, rapid onset/offset, high success | | Verapamil | Contraindicated | May accelerate conduction via accessory pathway, risk of VF | | Flecainide | Second-line (if adenosine fails) | Slows accessory pathway conduction; slower onset | | DC cardioversion | Unstable/failed drugs | Reserved for haemodynamic compromise | **Clinical Pearl:** Verapamil and diltiazem are CONTRAINDICATED in WPW with AF or rapid SVT because they slow AV nodal conduction preferentially, allowing the accessory pathway to conduct faster, potentially degenerating into ventricular fibrillation. ## Algorithm for SVT Management ```mermaid flowchart TD A[SVT with delta waves on sinus rhythm<br/>WPW syndrome]:::outcome --> B{Haemodynamically<br/>stable?}:::decision B -->|No| C[Synchronized DC cardioversion<br/>100 J]:::action B -->|Yes| D[Vagal maneuvers<br/>Valsalva, carotid massage]:::action D --> E{Terminated?}:::decision E -->|Yes| F[Observe, arrange EP study]:::outcome E -->|No| G[IV Adenosine 6 mg rapid push]:::action G --> H{Terminated?}:::decision H -->|Yes| I[Successful termination<br/>Arrange EP study/ablation]:::outcome H -->|No| J[Adenosine 12 mg second dose<br/>or IV Flecainide]:::action ``` [cite:Harrison 21e Ch 242]
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