What is the normal QRS duration, and what defines a wide QRS?

• A. Normal <100 ms; wide QRS ≥120 ms
• B. Normal <110 ms; wide QRS ≥150 ms
• C. Normal <120 ms; wide QRS ≥140 ms
• D. Normal <120 ms; wide QRS ≥120 ms

Answer: (D)

The main idea here is how long ventricular depolarization takes on the ECG. A normal QRS duration is under 120 milliseconds, because the His-Purkinje system conducts impulses rapidly and the ventricles depolarize in a compact, synchronized way. When the QRS is 120 milliseconds or longer, it’s considered wide, indicating slower or abnormal ventricular conduction or ventricular-origin activation. This broadening can stem from conditions like bundle branch blocks, ventricular rhythms, or pacing. While some texts use tighter normal limits (around 100–110 ms), the widely accepted cutoff for a wide QRS is 120 ms or more, which is why the option stating normal <120 ms and wide ≥120 ms is the best choice. Thresholds like 140–150 ms would miss many true conduction delays, and labeling anything under 120 ms as wide would be incorrect.