A 35-year-old man with Wolff–Parkinson–White (WPW) syndrome presents with a wide-complex tachycardia at 240 bpm. He is haemodynamically stable. Which of the following is the most appropriate acute pharmacological management?

Explanation

## Acute Management of Antidromic SVT in WPW Syndrome **Key Point:** In WPW syndrome with antidromic SVT (wide-complex tachycardia), **procainamide** is the drug of choice because it slows conduction in the accessory pathway without enhancing AV nodal conduction. ### Pathophysiology of WPW-Related Tachycardia WPW syndrome has two main tachycardia presentations: | Type | Circuit | QRS | Drug of Choice | Why | |------|---------|-----|-----------------|-----| | **Orthodromic SVT** (90%) | Antegrade AV node → Retrograde AP | Narrow | Adenosine, verapamil | AV nodal block terminates circuit | | **Antidromic SVT** (10%) | Antegrade AP → Retrograde AV node | Wide | Procainamide, flecainide | Slows AP conduction | **High-Yield:** The **critical distinction** is that adenosine and verapamil block the AV node but NOT the accessory pathway. In antidromic SVT, blocking the AV node may actually accelerate conduction down the accessory pathway, increasing the ventricular rate and risking degeneration to atrial fibrillation with rapid ventricular response. ### Why Procainamide Is Correct 1. **Class IA antiarrhythmic** — slows conduction in both the AV node AND the accessory pathway 2. **Mechanism:** Blocks fast sodium channels, increasing refractory period of the accessory pathway 3. **Dosing:** 10–15 mg/kg IV at 25–50 mg/min (max 500 mg) 4. **Onset:** 5–10 minutes 5. **Efficacy:** Slows conduction in the accessory pathway, terminating the circuit **Clinical Pearl:** Flecainide (Class IC) is an equally effective alternative with similar mechanism. Both are preferred over Class II and III agents in WPW antidromic SVT. ### Why Other Agents Are Wrong **Adenosine & Verapamil — DANGEROUS in Antidromic SVT:** - Block the AV node, removing the retrograde limb of the circuit - Unopposed conduction down the accessory pathway accelerates - Ventricular rate may increase paradoxically - Risk of degeneration to atrial fibrillation with 1:1 conduction down the AP → ventricular fibrillation **Digoxin — CONTRAINDICATED:** - Enhances conduction down the accessory pathway (via vagomimetic effect and AV nodal block) - Increases risk of rapid ventricular response and AF with RVR - Narrow therapeutic window ### Distinguishing Orthodromic vs. Antidromic SVT ```mermaid flowchart TD A[WPW-related SVT]:::outcome --> B{QRS width?}:::decision B -->|Narrow| C[Orthodromic SVT]:::outcome C --> D[Antegrade AV node<br/>Retrograde AP]:::outcome D --> E[Adenosine or Verapamil]:::action B -->|Wide| F[Antidromic SVT]:::outcome F --> G[Antegrade AP<br/>Retrograde AV node]:::outcome G --> H[Procainamide or Flecainide]:::action H --> I[Slows AP conduction]:::outcome ``` **Mnemonic:** **WAVE** — **W**ide-complex in **WPW** → **A**void **V**erapamil/**A**denosine, use **E**ffective AP blockers (Procainamide, Flecainide)