Lesson 17 of 21 · Heart Blocks

Bundle Branch Blocks

When one bundle branch is blocked, that ventricle is depolarized late, through muscle. The QRS widens (≥ 0.12 s) and V1 and V6 tell you which side.

01RBBB vs LBBB

Bundle branch block patterns

RBBB in V1: rSR′ ('rabbit ears').

LBBB in V1: deep, broad QS/rS.

LBBB in V6: broad, notched R with discordant ST-T.

RBBB: may be normal; also PE, RV strain, ASD, Brugada
LBBB: almost always structural heart disease (HTN, CAD, cardiomyopathy, aortic stenosis)
LBBB hides ischemia: use Sgarbossa criteria
Bifascicular block = RBBB + LAFB (or LPFB)

Clinical pearl. Use WiLLiaM MaRRoW: W in V1 + M in V6 = LBBB; M in V1 + W in V6 = RBBB.