What are the causes of a narrow complex electrocardiogram (ECG)?

Causes of Narrow Complex ECG

A narrow QRS complex on ECG (QRS duration <110-120 ms) represents normal ventricular depolarization through the His-Purkinje system and is the physiologic baseline for most individuals, though when associated with tachycardia it indicates supraventricular origin of the rhythm. 1

Normal Narrow Complex QRS

Physiologic Baseline

• QRS duration ≤110 ms in adults >16 years is considered normal 1
• In children 4-16 years: QRS <100 ms is normal 1
• In children <4 years: QRS <90 ms is normal 1
• Normal range in adult males: 74-114 ms (average 95 ms) 1

Key Concept

The narrow complex indicates that ventricular activation proceeds normally through the specialized conduction system (His bundle and bundle branches), rather than through slower myocardial tissue 1

Narrow Complex Tachycardias (When Rate >100 bpm)

Supraventricular Origins

When narrow QRS complexes occur with tachycardia (>100-120 bpm), the rhythm originates above or within the AV node 1, 2, 3:

Sinus-Related:

• Sinus tachycardia 1
• Sinus node reentry 1

Atrial Origins:

• Atrial tachycardia 1, 2
• Atrial flutter 1
• Atrial fibrillation with controlled ventricular response 1
• Multifocal atrial tachycardia 1

AV Nodal/Junctional:

• AV nodal reentrant tachycardia (AVNRT) - most common 1, 4, 5
• AV junctional tachycardia 1

Accessory Pathway-Mediated:

• Orthodromic AV reciprocating tachycardia (AVRT) using concealed accessory pathway 1, 4, 5

Critical Distinction

Narrow complex tachycardias are typically supraventricular, though rarely narrow complex ventricular tachycardias can occur 2. The narrow QRS essentially excludes ventricular tachycardia in most cases, as VT produces wide complexes due to abnormal ventricular activation 1

Normal Variants That Maintain Narrow QRS

Incomplete Bundle Branch Blocks

These maintain relatively narrow complexes compared to complete blocks:

Incomplete RBBB:

• QRS duration 110-119 ms in adults (still may appear "narrow" compared to complete block) 1
• RSR' pattern in V1-V2 with normal QRS duration (<110 ms) is a normal variant, especially in children and athletes 1, 6

Incomplete LBBB:

• QRS duration 110-119 ms in adults 1
• Requires LVH pattern, R peak time >60 ms in V4-V6, and absent Q waves in I, V5, V6 1

Fascicular Blocks

Left anterior fascicular block maintains narrow QRS:

• QRS duration <120 ms (this is a defining criterion) 1
• Frontal axis -45° to -90° 1
• qR pattern in aVL with R-peak time ≥45 ms 1

Clinical Pitfalls

Common Misinterpretations

• An RSR' pattern in V1-V2 with normal QRS duration is NOT pathologic - it's a normal variant particularly in children 1, 6
• Atrial fibrillation with rapid ventricular response appears as irregular narrow complex tachycardia, not wide complex 1
• Pre-excited atrial fibrillation (WPW with AF) produces wide irregular complexes, not narrow 1

When to Investigate Further

• Any complete bundle branch block (QRS ≥120 ms) is NOT a narrow complex and warrants cardiac evaluation 1
• Narrow complex tachycardia in unstable patients requires immediate cardioversion regardless of mechanism 1
• If narrow complex tachycardia mechanism is unclear, adenosine can be both diagnostic and therapeutic 1

Age-Specific Considerations

• Normal QRS duration varies significantly with age - use age-appropriate criteria 1
• In athletes, isolated voltage criteria for hypertrophy with normal QRS duration is physiologic 6