Adenosine revealed Pathway on ECG

How Adenosine Unmasks an Accessory Pathway on the ECG: A Detailed Electrophysiology-Focused Article

Adenosine is widely recognized for its use in terminating AVNRT and diagnosing SVT mechanisms, but one of its most powerful—and often under-appreciated—roles is its ability to unmask a concealed accessory pathway (AP) or enhance pre-excitation in patients with Wolff-Parkinson-White (WPW) physiology. Its diagnostic value comes from the way it interacts with AV nodal conduction and refractoriness, allowing latent conduction patterns to suddenly appear on the ECG.

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Electrophysiologic Basis: Why Adenosine Exposes the Pathway

1. Adenosine causes transient AV nodal block

Adenosine hyperpolarizes AV nodal tissue via A1 receptors, creating:

Slowing of AV nodal conduction

Complete but transient AV nodal block

When AV nodal tissue pauses, all conduction from atrium to ventricle must use any available non-nodal route—i.e., an accessory pathway.

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2. Manifest Pre-excitation Appears When AV Node Blocks

If a patient has a concealed or weakly conducting AP, the AV node normally “outcompetes” it because:

Nodal conduction is faster at baseline.

Pathway refractory period is sometimes longer.

When adenosine blocks the AV node, this competition ends. The impulse now travels solely through the AP, producing:

Short PR interval

Delta wave

Widened QRS

Classic WPW pre-excitation pattern

In patients with intermittent WPW, this is often the only way to capture the phenotype on ECG without exercise stress.

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3. Adenosine can also enhance pre-excitation

Even without complete block, slowing the AV node shifts the conduction balance:

More impulses use the AP.

Delta wave becomes more prominent.

QRS widens further.

This is especially useful in:

Poorly pre-excited WPW

Pathways with long antegrade refractory periods

Fasciculoventricular pathways

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4. Revealing Concealed Accessory Pathways During Tachycardia

During supraventricular tachycardia:

Adenosine terminates AVRT by blocking the AV node.

The immediate rhythm after termination may show sudden pre-excitation → diagnostic of an accessory pathway.

In orthodromic AVRT:

Adenosine terminates the circuit.

The first sinus beat after termination may show pre-excitation.

This is strong evidence of a concealed pathway that conducts retrograde during tachycardia.

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Mechanisms by Which Adenosine Reveals a Pathway

Mechanism Effect on ECG Clinical Use

AV nodal block Sudden appearance of delta wave, short PR, wide QRS Diagnose intermittent or latent WPW

Marked AV nodal slowing More pronounced pre-excitation Clarify ambiguous delta wave

Termination of AVRT Post-tachycardia pre-excited beat Confirm presence of concealed AP

Differentiating SVT mechanisms No pre-excitation → favors AVNRT or atrial tachycardia Mechanism identification

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Why Adenosine Is a Powerful Diagnostic Tool in EP

1. It unmasks what would otherwise remain hidden

Some pathways conduct very poorly in sinus rhythm:

Nodally dominant individuals

Pathways with longer refractory periods

Posteroseptal or left-sided pathways with slow conduction

Adenosine reveals them quickly and safely.

2. It differentiates AVNRT from AVRT

AVNRT: No pre-excitation after adenosine; narrow QRS resumes.

AVRT: Post-adenosine beat often shows pre-excitation.

3. It helps clarify ambiguous ECGs

Borderline delta waves or pseudo-delta patterns become diagnostic when AV node is temporarily removed from competition.

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Clinical Scenarios Where Adenosine Is Especially Useful

1. Atypical palpitations with normal baseline ECG

If suspicion for WPW is high despite no delta wave, adenosine challenge is a high-yield test.

2. Intermittent WPW not captured on routine ECG

Adenosine improves diagnostic yield dramatically.

3. Evaluation of wide-complex tachycardia

Adenosine-unmasked pre-excitation helps rule in antidromic AVRT.

4. Post-procedure evaluation

To confirm ablation success, adenosine tests whether the AV node resumes dominance or if residual pathway conduction persists.

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Safety Considerations

Adenosine is generally safe but must be used with caution in:

Pre-excited atrial fibrillation (risk of VF if pathway has very short refractory period)

Severe asthma (bronchospasm)

High-risk WPW with rapid conduction properties

Despite these caveats, when used appropriately under monitoring, it is an excellent diagnostic adjunct.

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Key Takeaways

Adenosine transiently removes the AV node from conduction.

Any surviving conduction must travel via an accessory pathway.

This produces sudden, unmistakable pre-excitation on ECG.

Adenosine is one of the fastest ways to diagnose an accessory pathway—especially concealed or intermittent ones.