ESC Guidelines: Adenosine Response in Regular Narrow Complex Tachycardia (NCT)

Adenosine Response in Regular Narrow Complex Tachycardia (NCT) 🫀⚡

Regular narrow complex tachycardia (QRS < 120 ms) is most commonly due to supraventricular tachycardias (SVTs). Adenosine is both a diagnostic and therapeutic agent in this setting because of its transient AV nodal block. Understanding the ECG response to adenosine is crucial for accurate rhythm diagnosis at the bedside, emergency department, and EP lab.

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Mechanism of Action of Adenosine

Adenosine acts primarily on A1 receptors in the AV node, leading to:

Hyperpolarization of AV nodal tissue

Transient complete AV block

Very short half-life (≈ 10 seconds)

This makes adenosine ideal for unmasking atrial activity or terminating AV node–dependent tachycardias.

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Why Adenosine Is Useful in Regular NCT

In regular NCT, the differential diagnosis includes:

AV nodal–dependent tachycardias

Atrial tachyarrhythmias with rapid ventricular response

Adenosine helps by answering one key question:

Is the AV node essential for maintenance of the tachycardia?

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Adenosine Response Patterns in Regular NCT

1. Termination of Tachycardia

Diagnosis:

AVNRT (typical or atypical)

Orthodromic AVRT

ECG Response:

Sudden termination of tachycardia

Return to sinus rhythm

Interpretation:

The arrhythmia is AV node–dependent.

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2. Transient AV Block With Ongoing Atrial Activity

Diagnosis:

Atrial flutter

Atrial tachycardia

ECG Response:

Ventricular rate slows or pauses

Atrial activity continues (flutter waves or P waves become visible)

Interpretation:

The arrhythmia is not AV node–dependent; AV node only conducts impulses.

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3. No Effect or Minimal Effect

Diagnosis:

Sinus tachycardia

Some automatic atrial tachycardias

ECG Response:

Little or no change in rhythm

Possible brief slowing only

Interpretation:

Suggests enhanced automaticity, not reentry.

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4. Transient Termination With Immediate Recurrence

Diagnosis:

Focal atrial tachycardia (adenosine-sensitive)

ECG Response:

Brief termination

Rapid recurrence of tachycardia

Interpretation:

Supports triggered activity or automatic focus, not AV nodal reentry.

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Practical Diagnostic Algorithm (Regular NCT + Adenosine)

1. Regular NCT confirmed (QRS < 120 ms)

2. Vagal maneuvers attempted

3. Adenosine administered (6 mg → 12 mg IV rapid bolus)

4. Observe ECG response:

Terminates → AVNRT / AVRT

AV block unmasks atrial waves → Atrial flutter / AT

No effect → Sinus tachycardia or automatic AT

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Important Clinical Pearls

Always record a continuous ECG strip during adenosine administration

Warn the patient about transient chest discomfort or flushing

Avoid adenosine in:

Irregular wide complex tachycardia

Pre-excited atrial fibrillation

Severe asthma (relative contraindication)

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Summary Table

Adenosine Effect Likely Rhythm

Terminates tachycardia AVNRT / AVRT

AV block, atrial activity visible Atrial flutter / AT

Minimal or no effect Sinus tachycardia

Terminates then recurs Focal atrial tachycardia

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Key Takeaway

Adenosine response is a powerful diagnostic tool in regular narrow complex tachycardia.

Termination implies AV node dependence, while persistence with exposed atrial activity points toward atrial arrhythmias.

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