Which ECG features most suggest ventricular tachycardia rather than SVT with aberrant conduction?

• A. Narrow QRS complexes with regular rhythm.
• B. AV dissociation and capture/fusion beats with broad QRS morphology.
• C. P waves clearly conducted 1:1 with QRS complexes.
• D. Regular rhythm with a normal QRS axis.

Answer: (B)

Distinguishing ventricular tachycardia from supraventricular tachycardia with aberrant conduction hinges on signs that the ventricles are pacing independently. AV dissociation means the atria and ventricles beat at different rates, so P waves march through the rhythm without a fixed relationship to the QRS. This independence strongly points to a ventricular source because the ventricle is driving the rhythm rather than following atrial impulses.

Capture beats and fusion beats are additional telltale signs. A capture beat is when a supraventricular impulse successfully conducts to the ventricles during VT, producing a normal narrow QRS for that beat. A fusion beat occurs when a ventricular beat from VT merges with a simultaneous supraventricular impulse, giving a QRS morphology that is intermediate between the typical VT shape and a normal QRS. Both types of beats indicate interaction of both atrial and ventricular impulses in the ventricles, which is characteristic of VT.

Wide QRS morphology is common in VT, reflecting abnormal ventricular conduction, whereas SVT with aberrant conduction can produce wide QRS but typically lacks AV dissociation and the capture/fusion phenomena. The other patterns—narrow QRS with regular rhythm, P waves conducted 1:1 with QRS, or a normal QRS axis—favor SVT with aberrancy rather than VT.

So the combination of AV dissociation plus capture or fusion beats amidst a broad QRS best indicates ventricular tachycardia.