ECG Basics · Lesson 11 · Leads, axis and conduction
Narrow vs wide QRS: bundle branch block
When one bundle branch is blocked, that ventricle is activated late through ordinary muscle. The QRS widens to 120 ms or more and takes a shape you can read in V1 and V6.
Right bundle branch block (RBBB)
- QRS ≥ 120 ms.
- V1–V2: rsR′ pattern (terminal R′), often called "M-shaped".
- I and V6: wide, slurred S wave (late right ventricular activation).
- T waves opposite to the terminal QRS in V1–V3 are expected (secondary change).
Left bundle branch block (LBBB)
- QRS ≥ 120 ms.
- V1: deep QS or rS.
- I, aVL, V5–V6: broad, often notched R wave with no septal q wave.
- ST and T go opposite to the main QRS deflection (discordant), so ST changes are harder to judge.
Other causes of a wide QRS
A ventricular origin (PVCs, VT, idioventricular rhythm), ventricular pacing, pre-excitation (WPW), hyperkalaemia and sodium-channel-blocking drugs all widen the QRS. A wide-complex tachycardia should be treated as VT until proven otherwise.
In the live simulator, bundle branch block is a layer: you can add RBBB or LBBB on top of sinus rhythm, AF, flutter or PSVT, and the rhythm keeps running. The engine applies it to conducted narrow complexes; ventricular beats are already wide.
References
- Surawicz B, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part III: intraventricular conduction disturbances. J Am Coll Cardiol 2009;53:976–981.