In ventricular tachycardia, what is a fusion beat?

• A. A QRS complex produced by simultaneous activation from the VT focus and the normal conduction system
• B. A QRS complex produced by activation from only the VT focus
• C. A QRS complex with AV nodal conduction delay
• D. A QRS complex appearing after VT termination

Answer: (A)

A fusion beat occurs when two wavefronts activate the ventricles at the same time: the VT focus driving part of the ventricle and the normal conduction system firing as well. The resulting QRS looks like a blend—some features of the wide VT morphology and some of the normal, narrow QRS—because parts of the ventricle are activated by the VT and parts by the normal conduction system. This shows that the VT is ongoing but not completely suppressing the heart’s usual conduction. It’s different from a pure VT beat (activated only by the VT focus) and from a capture beat or other scenarios where normal conduction dominates or occurs after VT ends.