Wolff-Parkinson-White (WPW) & Pre-Excited Atrial Fibrillation
Comprehensive emergency evaluation and protocolized resuscitation of Wolff-Parkinson-White (WPW) syndrome and accessory pathway conduction emergencies: embryological Kent bundle accessory pathway, baseline triad (short PR < 120 ms, delta wave, wide QRS > 120 ms), distinguishing narrow-complex Orthodromic AVRT from wide-complex Antidromic AVRT, recognition of Pre-Excited Atrial Fibrillation (irregular, wide-complex, polymorphic tachycardia with rates > 250–300 bpm), immediate synchronized electrical cardioversion, IV Procainamide protocols, and absolute contraindications to AV nodal blocking agents ('ABCD').
Resuscitation Quick Actions • First 2 Minutes
Pre-Excited AF Hallmark
Irregularly irregular, wide-complex tachycardia with polymorphic QRS and rates frequently > 250–300 bpm (FBI: Fast, Broad, Irregular)
Unstable = Shock Stat
Synchronized electrical cardioversion immediately (biphasic 100–200J); do NOT delay for medications if hypotensive or altered
Stable First-Line Drug
Procainamide 10 to 17 mg/kg IV infused at 20–50 mg/min (or Ibutilide 1 mg IV over 10 min); slows accessory pathway conduction
Procainamide Stop Rules
Stop procainamide if: arrhythmia terminates, QRS widens by > 50%, SBP drops < 90 mmHg, or max dose 17 mg/kg reached
ABSOLUTELY AVOID 'ABCD'
NO Adenosine, NO Beta-blockers, NO Calcium channel blockers (Diltiazem/Verapamil), NO Digoxin/Amiodarone -> CAUSES FATAL VF ARREST
Orthodromic AVRT
Narrow QRS (conducts down AV node, up accessory pathway); treat like standard SVT with vagal maneuvers and Adenosine
Bottom-Line Clinical Pearl
Pre-Excited Atrial Fibrillation is a lethal emergency that occurs when atrial fibrillation conducts down an uninhibited, non-decremental accessory pathway (Bundle of Kent) directly into the ventricles at rates exceeding 250–300 bpm, predisposing to immediate degeneration into ventricular fibrillation. The ECG shows an irregularly irregular, wide-complex polymorphic tachycardia with variable QRS morphology and beat-to-beat cycle lengths. Hemodynamically unstable patients require immediate Synchronized Electrical Cardioversion (100–200J). In stable patients, the drug of choice is IV Procainamide (10–17 mg/kg at 20–50 mg/min). AV nodal blocking agents (Adenosine, Beta-blockers, Calcium channel blockers, Digoxin — 'ABCD') are ABSOLUTELY CONTRAINDICATED; blocking the AV node shunts 100% of atrial fibrillatory impulses down the accessory pathway, causing immediate ventricular fibrillation and death!
Wolff-Parkinson-White (WPW) syndrome arises from an anomalous congenital myocardial band—the accessory pathway (Bundle of Kent)—that bridges the atria and ventricles across the fibrous atrioventricular ring, bypassing the physiological conduction delay of the AV node.
Unlike the AV node, which exhibits decremental conduction (slowing conduction velocity as atrial heart rate increases to protect the ventricles), accessory pathways typically conduct with fixed, ultra-fast velocities and short refractory periods. On baseline resting 12-lead ECG, fusion of early ventricular pre-excitation via the accessory pathway with delayed normal conduction down the His-Purkinje system creates the classic triad: (1) Short PR interval ($< 120\text{ ms}$), (2) Slurred, notched initial QRS upstroke (Delta wave), and (3) Secondary QRS prolongation ($> 120\text{ ms}$) with discordant ST-T wave changes.
| Dysrhythmia Type | Conduction Circuit & Direction | ECG Characteristics | First-Line Therapeutic Regimen |
|---|---|---|---|
| Orthodromic AVRT (90–95% of AVRT) | Down the AV node, up the accessory pathway (retrograde) | Regular, narrow QRS ($< 120\text{ ms}$), rate 160–220 bpm; retrograde P waves visible in ST segment | Modified Valsalva maneuver, IV Adenosine (6 mg then 12 mg), or IV Diltiazem/Metoprolol (safe because antegrade conduction utilizes the normal AV node). |
| Antidromic AVRT (5% of AVRT) | Down the accessory pathway (antegrade), retrograde up the AV node | Regular, wide QRS ($> 120\text{ ms}$) with classic delta wave morphology, rate 160–220 bpm; perfectly regular | Procainamide (10–17 mg/kg IV) or Synchronized Cardioversion. Treat as presumed Ventricular Tachycardia; avoid AV nodal blockers. |
| Pre-Excited Atrial Fibrillation (FBI) | Chaotic atrial fibrillatory impulses (300–600 bpm) conduct directly antegrade down the accessory pathway | FBI: Fast, Broad, Irregular! Irregularly irregular, wide-complex polymorphic QRS; rates frequently 250 to 350+ bpm; bizarre changing QRS shapes | EMERGENCY: If unstable: Immediate Synchronized Cardioversion (100–200J). If stable: IV Procainamide (17 mg/kg at 20–50 mg/min). ALL NODAL BLOCKERS CONTRAINDICATED! |
Critical Pitfall / Contraindication
THE 'ABCD' CONTRAINDICATION IN PRE-EXCITED AF: Never administer AV nodal blocking drugs—Adenosine, Beta-blockers, Calcium channel blockers (Diltiazem, Verapamil), or Digoxin ('ABCD')—to a patient with Pre-Excited Atrial Fibrillation. IV Amiodarone is also contraindicated in acute pre-excited AF because it possesses intrinsic beta-blocking and calcium-blocking activity. Selectively paralyzing the AV node eliminates decremental competition, shunting 100% of atrial impulses (at 400–600 bpm) directly down the Kent bundle into ventricular myocardium, inducing immediate Ventricular Fibrillation and cardiac arrest within seconds.
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Related Emergency Protocols & Differentials
Narrow Complex Tachycardias: SVT & Flutter
Adenosine and nodal blocking in standard SVT.
Open Protocol Related EM ProtocolWide Complex Tachycardias: VT vs SVT
Distinguishing ventricular tachycardia from pre-excitation.
Open Protocol Related EM ProtocolTachydysrhythmias & Electrical Cardioversion
Synchronized cardioversion parameters.
Open Protocol