What are fusion complexes on an electrocardiogram (EKG) in a patient, likely an adult or older individual with a possible history of cardiac issues?

Fusion Complexes on EKG

Definition and Mechanism

Fusion complexes (also called fusion beats) represent a merger between conducted supraventricular impulses and ventricular depolarization occurring simultaneously during atrioventricular dissociation. 1 These complexes occur when two separate electrical wavefronts—one originating from above the ventricles (atria or AV node) and one from within the ventricles—collide and depolarize the ventricles together, creating a hybrid QRS morphology. 1

The resulting QRS complex has an intermediate appearance, falling somewhere between:

• The normal conducted supraventricular beat morphology
• The pure ventricular beat morphology 1

Clinical Significance

Fusion complexes are pathognomonic (definitively diagnostic) for ventricular tachycardia (VT). 1, 2, 3 Their presence proves that:

• AV dissociation exists (the atria and ventricles are beating independently) 1
• A ventricular focus is generating impulses while some supraventricular impulses still occasionally conduct through 1
• The wide-complex tachycardia originates from the ventricles, not from supraventricular tachycardia with aberrant conduction 2

Diagnostic Value in Wide-Complex Tachycardia

When evaluating a patient with wide-complex tachycardia (QRS >120 ms), fusion beats serve as one of the most specific ECG findings for distinguishing VT from supraventricular tachycardia (SVT) with bundle branch block:

• Fusion complexes strongly indicate VT and should prompt immediate treatment as ventricular tachycardia 3
• They are visible in only approximately 30% of VT cases, so their absence does not rule out VT 1
• When present alongside capture beats (fully conducted supraventricular beats during VT), they provide definitive evidence of ventricular origin 2, 3

Recognition on ECG

Look for QRS complexes that appear morphologically intermediate between:

• The patient's baseline QRS during sinus rhythm
• The wide QRS complexes during tachycardia 1

The fusion beat will have:

• A width between normal and tachycardia QRS duration
• A morphology that is neither fully normal nor fully abnormal
• Occasional occurrence during otherwise regular wide-complex tachycardia 1

Critical Clinical Pitfalls

When fusion complexes are identified during wide-complex tachycardia, never assume the rhythm is benign—immediately treat as ventricular tachycardia. 2 Key warnings include:

• Hemodynamic stability does not exclude VT; patients can tolerate VT well, especially younger individuals 2
• Do not use calcium channel blockers (verapamil/diltiazem) for wide-complex tachycardia when the diagnosis is uncertain, as they may precipitate hemodynamic collapse if the rhythm is VT 1
• Always treat wide-complex tachycardia as VT unless you can definitively prove it is SVT with aberrancy 1, 2
• In patients with prior myocardial infarction, wide-complex tachycardia is VT until proven otherwise 2

Context in Electrophysiology Studies

During electrophysiology procedures, fusion can also occur during entrainment maneuvers:

• Fusion during entrainment of atrial flutter or atrial tachycardia appears as partial changes in atrial deflections on ECG 1
• In orthodromic supraventricular tachycardia involving septal accessory pathways, ventricular fusion during entrainment can help differentiate from AV nodal reentry 4
• The presence of fusion is critically dependent on coupling intervals and pacing rates, sometimes having a narrow window for observation 4