Ventricular ectopic foci cause a wide QRS complex on the ECG because which of the following occurs?

• A. Normal conduction pathways are followed
• B. Normal conduction pathways are not followed
• C. impulses originate from the SA node
• D. PR interval shortens

Answer: (B)

Ventricular ectopic foci widen the QRS because the depolarization starts in the ventricular muscle itself and spreads through the ventricles via slow, cell-to-cell conduction rather than the fast His-Purkinje system. This slower, abnormal activation takes longer and occurs in a nonuniform pattern, producing a broad and often bizarre-looking QRS complex. If the impulse followed the normal conduction pathways, the ventricles would depolarize quickly and in a coordinated fashion, yielding a narrow QRS. An origin from the SA node would be a supraventricular source with typical normal-conduction–driven QRS width, not a wide complex. The PR interval reflects atrial-to-ventricular conduction and does not explain the wide QRS seen with ventricular ectopy. Thus, the wide QRS arises because the impulse does not follow normal conduction pathways.