Final answer:
A narrow-QRS complex typically signifies that the ventricular depolarization is normal, so any cardiac issues may occur before ventricular depolarization, such as in the atria or AV node.
Step-by-step explanation:
A narrow QRS complex on an ECG usually indicates that the depolarization of the ventricles is occurring normally, which suggests that any issues might be occurring earlier in the conduction pathway, like the atria or the atrioventricular (AV) node. A narrow-QRS complex typically signifies that the ventricular depolarization is not excessively delayed and thus the ventricles themselves are not the primary site of the abnormality. However, although a narrow QRS signifies normal ventricular depolarization, the site of the issue could occur in the atria or AV node as evidenced by conditions like second-degree block where some P waves are not followed by QRS complexes, or atrial fibrillation where the electrical pattern is abnormal prior to the QRS complex.