Measuring QRS Duration on ECG: Steps, Normal Range, and Prolongation Criteria

QRS duration = time from onset of Q wave (or R wave if no Q) to end of S wave (return to baseline).

Step-by-Step

1. Identify QRS onset: first deflection from baseline (Q if present, else R).
2. Identify J-point: where S wave meets baseline (start of ST segment).
3. Measure horizontal distance (ms) on calibrated ECG (25 mm/s → 1 mm = 40 ms; 50 mm/s → 1 mm = 20 ms).
4. Use lead with widest QRS (usually V1–V3 or lead II).

Reference Values (Adults)

• Normal: 80–100 ms (≤0.10 s / ≤2.5 small boxes at 25 mm/s).
• Incomplete bundle branch block: 100–120 ms.
• Complete BBB / IVCD: >120 ms.

Clinical Significance

Prolonged QRS → intraventricular conduction delay (BBB, fascicular block), ventricular pre-excitation (WPW), ventricular pacing, hyperkalemia, Na⁺ channel blocker toxicity. Correlate with clinical context.