Ace the Cleveland Clinic EKG Challenge 2026 – Electrify Your Skills and Test Your Heart Smarts!

The key idea is that QRS width shows how the ventricles are activated. When the impulse travels through the normal His–Purkinje system, ventricular depolarization happens quickly, giving a narrow QRS complex (typically under about 120 ms). Normal sinus rhythm originates above the ventricles and uses this fast conduction, so the QRS is narrow. An accelerated junctional rhythm also starts near the AV junction but, like sinus rhythm, it conducts through the His–Purkinje system to activate the ventricles, so the QRS remains narrow as well. A wide QRS would point to a ventricular origin or abnormal conduction, not these supraventricular rhythms. Thus, the QRS complexes are narrow in both normal sinus and accelerated junctional rhythms.