Narrow QRS Tachycardia: Differential Diagnosis

Narrow QRS Tachycardia: Differential Diagnosis 

Narrow QRS Tachycardia: Differential Diagnosis

Comprehensive Clinical & ECG-Based Approach

Introduction

Narrow QRS tachycardia is one of the most frequently encountered arrhythmias in emergency rooms, CCUs, and electrophysiology labs. A narrow complex (QRS < 120 ms) indicates that ventricular activation is occurring through the normal His–Purkinje system. Therefore, the arrhythmia origin is either:

• Supraventricular (atria or AV junction)

• Ventricular but conducting normally via the conduction system (rare)

Correct diagnosis is critical because management differs dramatically between sinus tachycardia, atrial tachyarrhythmias, and AV reentrant tachycardias.

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Definition

Narrow QRS Tachycardia =

Heart rate >100 bpm

QRS duration <120 ms

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Stepwise Clinical Approach

Before jumping to labels, always analyze systematically:

1. Is it regular or irregular?

2. Are P waves visible?

3. What is the RP interval?

4. Is there AV dissociation?

5. How did it start and terminate?

This structured approach prevents diagnostic errors.

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Classification Overview

Narrow QRS tachycardias are broadly divided into:

1. Sinus tachycardia

2. Atrial tachycardias

3. AV nodal–dependent tachycardias

4. Atrial fibrillation/flutter

5. Rare causes

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1. Sinus Tachycardia

Mechanism

Physiologic increase in sinus node automaticity.

ECG Features

• Upright P in II

• Each P followed by QRS

• Gradual onset and offset

• Rate usually <180 bpm (in adults)

Causes

• Fever

• Anemia

• Hypovolemia

• Pain

• Pulmonary embolism

• Hyperthyroidism

Inappropriate sinus tachycardia is diagnosed only after excluding secondary causes.

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2. Atrial Fibrillation

Key ECG Findings

• Irregularly irregular rhythm

• No discrete P waves

• Fibrillatory baseline

• Narrow QRS (unless aberrancy)

Important Differentials

• Atrial flutter with variable block

• Multifocal atrial tachycardia

Clinical Importance

Rate vs rhythm control strategies were compared in the AFFIRM trial, which showed no survival benefit of rhythm control over rate control.

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3. Atrial Flutter

Mechanism

Macro-reentrant circuit, usually in right atrium.

ECG Clues

• Sawtooth flutter waves (best in II, III, aVF)

• Atrial rate ~300 bpm

• Commonly 2:1 AV block → ventricular rate ~150 bpm

Typical flutter involves the cavotricuspid isthmus.

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4. Multifocal Atrial Tachycardia (MAT)

ECG Features

• Irregular rhythm

• ≥3 different P wave morphologies

• Variable PR intervals

• Rate >100 bpm

Common Setting

• COPD exacerbation

• Hypoxia

• Electrolyte imbalance

Key differentiator from AF: P waves are present.

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5. Focal Atrial Tachycardia

Mechanism

Enhanced automaticity or micro-reentry.

ECG Clues

• Abnormal P morphology

• Long RP tachycardia

• Regular rhythm

• Warm-up and cool-down phenomenon

Seen in digitalis toxicity and post-cardiac surgery.

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6. AV Nodal Reentrant Tachycardia (AVNRT)

Mechanism

Dual AV nodal pathways (slow-fast typical form).

ECG Hallmarks

• Regular narrow complex tachycardia

• Rate 150–250 bpm

• P waves hidden or pseudo r′ in V1

• Short RP interval

Most common paroxysmal SVT in adults.

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7. AV Reentrant Tachycardia (AVRT)

Orthodromic AVRT

Mechanism

Accessory pathway outside AV node.

Associated with Wolff-Parkinson-White syndrome.

ECG

• Narrow QRS

• Retrograde P after QRS

• Short RP tachycardia

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8. Junctional Tachycardia

ECG

• Regular narrow QRS

• Absent or retrograde P waves

• Rate 70–130 bpm (non-paroxysmal)

Seen post cardiac surgery or digoxin toxicity.

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9. Rare Causes

• Sinoatrial reentry tachycardia

• Atypical AVNRT

• Atrial tachycardia with 1:1 conduction

• Concealed bypass tracts

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Regular vs Irregular Narrow QRS Tachycardia

Regular

• Sinus tachycardia

• AVNRT

• AVRT

• Atrial tachycardia

• Atrial flutter with fixed block

Irregular

• Atrial fibrillation

• MAT

• Atrial flutter with variable block

This is the most important initial branching point.

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RP Interval-Based Differentiation

Short RP (<70 ms)

• Typical AVNRT

• Orthodromic AVRT

Long RP

• Atrial tachycardia

• Atypical AVNRT

• Permanent junctional reciprocating tachycardia

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Acute Management Overview

Unstable Patient

Immediate synchronized cardioversion.

Stable Regular Narrow QRS

1. Vagal maneuvers

2. IV adenosine

Adenosine terminates AV node–dependent tachycardias (AVNRT, AVRT).

Irregular Narrow QRS

• Avoid adenosine in irregular wide-complex rhythms

• Manage underlying mechanism

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Diagnostic Pitfalls

1. Mistaking sinus tachycardia for SVT

2. Missing flutter waves at 2:1 block

3. Confusing MAT with AF

4. Ignoring concealed accessory pathways

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Special Situations

Pregnancy

Vagal maneuvers first line.

Structural Heart Disease

Higher suspicion for atrial tachycardias.

Young Patients

Think AVRT and concealed pathways.

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Summary Table

Rhythm Regular? P Waves RP Mechanism

Sinus tachycardia Regular Normal Normal SA node

AF Irregular Absent N/A Chaotic atrial

Flutter Regular/Irregular Sawtooth Fixed Macro-reentry

AVNRT Regular Hidden Short AV node reentry

AVRT Regular Retrograde Short Accessory pathway

MAT Irregular ≥3 morphologies Variable Multiple atrial foci

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Final Clinical Pearls

• Always determine regular vs irregular first

• Adenosine is diagnostic and therapeutic

• Short RP = AV node dependent until proven otherwise

• Long RP = atrial tachycardia unless proven otherwise

• Flutter at 150 bpm is 2:1 block until proven otherwise