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.