Narrow QRS complexes are typical for rhythms arising from which region?

• A. Ventricles
• B. Atrial and junctional
• C. Purkinje fibers
• D. Bundle branches

Answer: (B)

Narrow QRS complexes occur when the ventricular activation happens through the heart’s normal fast conduction system. If the rhythm starts in the atria or at the AV junction, the impulse travels quickly down the His-Purkinje network and the ventricles depolarize in a synchronized, rapid way. This produces a sharp, narrow QRS on the ECG.

In contrast, rhythms that originate in the ventricles—either from ventricular tissue itself or from parts of the conduction system below the AV node—must depolarize the ventricles more slowly, often with some dyssynchrony. That slower, less coordinated activation broadens the QRS complex.

So, rhythms arising from the atrial and junctional regions reliably produce narrow QRS complexes because they utilize the normal ventricular conduction pathways.