Which ECG clues suggest ventricular tachycardia in a patient with prior myocardial infarction?

• A. Narrow QRS with AV dissociation
• B. Wide QRS with abnormal morphology, AV dissociation, capture/fusion beats
• C. Regular narrow tachycardia with normal morphology
• D. Short PR interval with regular rhythm

Answer: (B)

Distinguishing ventricular tachycardia from other tachycardias on ECG, especially after a myocardial infarction, rests on recognizing that VT begins in the ventricle and often uses scar tissue to reenter. The key clue is a wide QRS tachycardia with abnormal QRS morphology, because the activation spreads through the ventricle rather than the normal His-Purkinje system. AV dissociation is another strong indicator: the atria and ventricles beat independently, showing that the ventricular rhythm is not just a fast atrial rhythm conducted through the AV node. Capture beats and fusion beats are highly specific for VT: occasional normal-looking or hybrid QRS complexes occur when a supraventricular impulse momentarily interacts with the VT, producing a beat that looks different from the ongoing VT morphology. In contrast, a narrow-complex tachycardia or a short PR interval with regular rhythm suggests supraventricular mechanisms or preexcitation rather than VT. Therefore, the combination of wide QRS, abnormal morphology, AV dissociation, and capture/fusion beats best indicates ventricular tachycardia in this setting.