bundle branch block pattern

A bundle branch block pattern is the widened, oddly shaped QRS complex seen on an ECG when one of the heart's main electrical cables is not conducting properly. Normally the impulse races down two branches — left and right — so both ventricles activate together almost instantly. If one branch is blocked, that side has to be reached the slow way, through ordinary muscle, and the QRS broadens.

Because activation now spreads in two stages instead of one, the QRS not only widens but takes on a characteristic shape that depends on which branch is blocked. A right bundle branch block and a left bundle branch block each produce their own recognisable QRS contour in particular leads, which lets a reader tell them apart.

A bundle branch block can be a long-standing, harmless finding or a sign of underlying heart disease, depending on the person and the setting. Its importance also lies in how it changes the ECG: a left bundle branch block, in particular, can mask or mimic the signs of a heart attack, so a new one in the right clinical context is taken seriously and prompts further assessment.