Bundle branch block (BBB)

A delay or block of conduction down one bundle branch, so the ventricles depolarize one after the other rather than together and the QRS widens.

Key ECG features

✓Wide QRS
✓On time, part of the underlying rhythm rather than early
•Sinus P before each complex with a constant PR
±RBBB: wide, deep S waves in leads I, II, III and aVF
±LBBB: broad and predominantly positive in I, II and aVF

✓ required · • typical · ± may be present

ECG characteristics

A wide QRS that is still preceded by a P wave with a constant PR interval, arriving on time rather than early. In right bundle branch block the late forces swing rightward, so leads I, II, III and aVF carry wide, deep S waves. In left bundle branch block they swing leftward, so the complexes stay broad and predominantly positive in leads I, II and aVF.

Wide QRS, still preceded by a P wave and on time

The notched or R prime pattern, where the two ventricles no longer depolarize together

This illustration is schematic. Dashed line is a normal complex. The grid is not drawn to scale.

Common causes

Right bundle branch block can occur in a structurally normal heart as an incidental finding. Left bundle branch block more often accompanies significant left ventricular disease. Either may follow myocardial disease, trauma, or cardiac surgery.

Why it matters

Its importance is that it widens the QRS in a rhythm that is still supraventricular, which is the classic reason a rhythm gets mistaken for ventricular in origin. Finding a P wave in front of every wide complex settles the question.

Differentiate from

Ventricular premature complexesBoth give a wide QRS. The bundle branch block beat is on time and follows a P wave; the premature complex is early and has none.
Ventricular tachycardiaA supraventricular rhythm conducted with bundle branch block can be mistaken for ventricular tachycardia. A P wave in front of every complex settles it when the rhythm is sinus; with atrial fibrillation underneath, the irregularly irregular spacing does instead.
Pathological Q waveIn a wide QRS the initial negative deflection can be deep enough to be read as a pathological Q. Judge the width and shape of the whole complex before attributing it to lost myocardium.