Role of Adenosine in Wolff-Parkinson-White Syndrome
Adenosine is beneficial and recommended for acute termination of orthodromic AVRT (narrow-complex tachycardia) in WPW syndrome, but is contraindicated in pre-excited atrial fibrillation where it can precipitate life-threatening ventricular fibrillation. 1, 2
Critical Decision Point: QRS Width Determines Safety
The fundamental determinant of adenosine safety in WPW is whether the tachycardia shows a narrow QRS complex (<120 ms) or wide QRS complex (≥120 ms) during the arrhythmia 1, 2:
- Narrow-complex tachycardia indicates anterograde conduction through the AV node (orthodromic AVRT), making adenosine safe and effective 2
- Wide-complex tachycardia indicates anterograde conduction through the accessory pathway (antidromic AVRT or pre-excited AF), making adenosine dangerous 1, 2
When Adenosine is Beneficial: Orthodromic AVRT
For narrow-complex regular tachycardia in WPW patients, adenosine terminates approximately 95% of orthodromic AVRT cases by blocking AV nodal conduction. 2, 3 The mechanism is straightforward: orthodromic AVRT requires the AV node as part of the reentry circuit (anterograde through AV node, retrograde through accessory pathway), and adenosine's transient AV nodal blockade breaks the circuit 1, 3.
Administration Protocol
- Initial dose: 6 mg rapid IV push through a large proximal vein, followed immediately by 20 mL saline flush 2
- If no conversion in 1-2 minutes: 12 mg IV push with saline flush 2
- Termination typically occurs within 20 seconds of administration 3
- A defibrillator must be immediately available at bedside 2
When Adenosine is Contraindicated: Pre-Excited Rhythms
Adenosine is absolutely contraindicated in pre-excited atrial fibrillation because it can trigger ventricular fibrillation. 1, 2 The mechanism of harm involves adenosine's ability to precipitate or accelerate atrial fibrillation while simultaneously blocking the AV node, forcing all atrial impulses to conduct rapidly down the accessory pathway, potentially degenerating into ventricular fibrillation 1, 4.
Alternative Agents for Wide-Complex/Pre-Excited Tachycardia
For hemodynamically stable patients with wide-complex tachycardia or pre-excited AF, use agents that slow accessory pathway conduction 1:
- Procainamide (Class IA) - preferred first-line agent 1, 2
- Ibutilide (Class III) 1, 2
- Flecainide (Class IC) 1, 2
For hemodynamically unstable patients with any pre-excited rhythm: immediate synchronized cardioversion is mandatory 1, 2.
Diagnostic Utility of Adenosine
Beyond therapeutic use, adenosine has diagnostic value in WPW 1, 5:
- During narrow-complex tachycardia, the ECG response to adenosine helps differentiate AVRT from other SVTs 1
- Termination with a P wave after the last QRS favors AVRT or AVNRT 1
- Continuation of tachycardia with AV block excludes AVRT and suggests atrial tachycardia or flutter 1
- In pediatric patients, blocked accessory pathway conduction with adenosine has 100% specificity and positive predictive value for non-rapid baseline antegrade conduction, aiding risk stratification 5
Common Pitfall: AV Nodal Blockers in Undiagnosed WPW
Never administer AV nodal-blocking agents (adenosine, calcium channel blockers, beta-blockers, digoxin) to patients with wide-complex tachycardia of uncertain etiology, as unrecognized WPW with pre-excited AF can result in sudden cardiac death. 1 This represents one of the most dangerous medication errors in emergency arrhythmia management 4.
Special Populations and Dosing Adjustments
Reduce initial adenosine dose to 3 mg in patients 6:
- Taking dipyridamole or carbamazepine
- With transplanted hearts
- Receiving central venous administration
Avoid adenosine entirely in patients with severe asthma due to bronchospasm risk 1, 6.
Definitive Management Context
While adenosine effectively terminates acute episodes of orthodromic AVRT, catheter ablation remains first-line definitive treatment for symptomatic WPW patients, with success rates exceeding 95% 2, 7. Adenosine serves as acute management only, not long-term therapy.