23-22: Left Posterior Fascicular Block

ECG Findings

• QRS duration normal or slightly prolonged

• Right axis deviation (without any other reasons for rightward axis on ECG)

• Small R wave and large S wave in the lateral leads (I, aVL)

• Small Q with a large R wave in the inferior leads (II, III, aVF)

Pearls

1. In left posterior fascicular block (LPFB) the conduction through the left posterior fascicle is blocked, which causes the conduction of the inferior portion of the left ventricle to occur in a delayed fashion, as conduction spreads from the intact left anterior fascicle and the right bundle branch. This causes a typical rightward axis deviation.

2. In contrast to the LAFB, this is rarely a normal variant. The posterior fascicle receives its blood supply from both the left coronary and right coronary arteries; hence, if an LPFB is present, this may indicate multi vessel coronary artery disease.