First electrocardiographic documentation of alternating bilateral block in pulmonary embolism.

Massive pulmonary embolism (PE) classically produces sinus tachycardia or right bundle-branch block (RBBB). We report a 46-year-old woman whose initial electrocardiogram showed simultaneous left and right anterior fascicular block-the first description of bilateral anterior fascicular block. Days later the tracing converted to complete RBBB with left posterior fascicular block (LPFB), a recognised bilateral pattern, documenting the first alternation from bilateral anterior fascicular block to the RBBB + LPFB combination. This sequence suggests diffuse, pressure-induced His-Purkinje injury and may herald haemodynamic collapse; the patient died intra-operatively. Alternating bilateral fascicular patterns could serve as early electrocardiographic warnings in massive PE.