When capture beats or fusion beats are observed on an ECG, what is the most likely rhythm?

• A. Atrial fibrillation
• B. Ventricular tachycardia
• C. Sinus tachycardia
• D. Ventricular flutter

Answer: (B)

Capture and fusion beats signal a ventricular origin for the tachycardia. They occur when a ventricular tachycardia is running and a supraventricular impulse briefly conducts to the ventricles (capture) or when a ventricular beat from the VT combines with a conducted beat to produce a hybrid QRS (fusion). Seeing these beats amidst a wide-complex rhythm indicates the ventricle is driving the rhythm, with occasional interaction from normal conduction.

Atrial fibrillation would present as irregular, chaotic atrial activity with no consistent capture or fusion patterns in the ventricular response. Sinus tachycardia usually has a narrow QRS unless aberrancy and lacks true fusion/capture phenomena. Ventricular flutter is a rapid, usually extremely fast ventricular rhythm without discrete capture or fusion beats. Thus, the presence of capture or fusion beats most strongly points to ventricular tachycardia as the rhythm.