Acute myocardial infarction and fascicular block.

Left anterior fascicular block may mask an IWMI. On the other hand, a narrow QS deflection in a VF is often misinterpreted as evidence of extensive inferior wall infarction. Vectorcardiographic analysis usually establishes the coexistence of LAFB and IWMI. Standard Lead II of the ECG is the most valuable in differentiating the etiology of ALAD: (1) rS pattern in LAFB, (2) Qr or QR in IWMI, or (3) a QS pattern or a smaller or slurred r wave with a deep S wave in combined IWMI and LAFB.