What is a fusion complex on an electrocardiogram (ECG)?

Fusion Complex on ECG

A fusion complex on an electrocardiogram (ECG) represents a merger between conducted sinus (or supraventricular) impulses and ventricular depolarization occurring during atrioventricular (AV) dissociation, and is pathognomonic of ventricular tachycardia (VT). 1

Definition and Mechanism

Fusion complexes occur when two different wavefronts of cardiac activation meet and simultaneously depolarize the myocardium. This creates a QRS complex with morphology that is intermediate between two distinct patterns:

• In the context of VT, fusion beats represent a merger between:
  • Conducted supraventricular impulses (from sinus rhythm)
  • Ventricular depolarization originating from the VT focus

The resulting QRS complex shows characteristics of both activation patterns, creating a hybrid or "fusion" morphology.

Clinical Significance

Fusion complexes have critical diagnostic importance:

1. Diagnostic value: They are pathognomonic (definitively diagnostic) of VT 1
2. Differential diagnosis: Help distinguish VT from supraventricular tachycardia (SVT) with aberrancy
3. AV dissociation: Fusion beats indicate the presence of AV dissociation, which is a hallmark of VT

Recognition on ECG

Fusion beats can be identified on ECG by:

• QRS morphology intermediate between:
  • The patient's native QRS complex (during sinus rhythm)
  • The fully paced or ectopic QRS complex (during VT)
• Variable QRS morphology that may change beat-to-beat
• Often seen during the onset or termination of VT episodes
• May appear in lead V1 as an intermediate morphology between LBBB pattern (native) and RBBB pattern (fully paced) 2

Clinical Context

Fusion complexes are most commonly encountered in:

1. Ventricular tachycardia: Where they serve as a diagnostic marker
2. Cardiac resynchronization therapy: Where fusion between LV pacing and intrinsic conduction through the right bundle can be therapeutically beneficial 3, 2
3. Ventricular pacing: Where fusion between paced beats and intrinsic conduction may occur

Importance in Differential Diagnosis

When evaluating wide-complex tachycardias, the presence of fusion complexes is one of the most reliable indicators of VT. According to ACC/AHA/ESC guidelines, if the diagnosis of SVT cannot be proven or easily made, the patient should be treated as if VT were present 1.

Other ECG features that help differentiate VT from SVT with aberrancy include:

• VA dissociation
• QRS width >0.14s (RBBB) or >0.16s (LBBB)
• RS interval >100ms in precordial leads
• QRS concordance in precordial leads

Therapeutic Implications

Recognition of fusion complexes has important treatment implications:

• Confirms VT diagnosis, which requires specific antiarrhythmic management
• Helps avoid inappropriate use of calcium channel blockers (verapamil, diltiazem) which can cause hemodynamic collapse in VT 1
• In cardiac resynchronization therapy, the "fusion band" (range of AV intervals showing fusion) may help optimize device programming 3, 2

Pitfalls and Caveats

• Fusion complexes may be transient and not present throughout the entire tachycardia episode
• P waves can be difficult to recognize during wide-QRS tachycardia, making AV dissociation challenging to identify
• In patients with pre-existing bundle branch block, fusion morphology may be more subtle

When fusion complexes cannot be clearly identified, other physical examination findings of AV dissociation should be sought, including irregular cannon A waves in the jugular venous pulse and variability in the loudness of the first heart sound and systolic blood pressure 1.