A 68-year-old man with long-standing hypertension shows left axis deviation on ECG (QRS axis −50°) with qR pattern in lead I and rS pattern in leads II and III. Which conduction abnormality best explains this finding?

• A Left posterior fascicular block
• B Left bundle branch block
• C Right bundle branch block
• D Left anterior fascicular block ✓

Explanation

Left axis deviation beyond −30° with a qR pattern in lead I and rS in inferior leads is the classic ECG pattern of left anterior fascicular block. The left anterior fascicle supplies the anterosuperior left ventricular wall; when blocked, the initial vector shifts posteroinferiorly and the main vector goes superiorly and leftward. Left posterior fascicular block causes right axis deviation. RBBB and LBBB produce broad QRS morphology, not isolated axis deviation.

Reference: Goldberger's Clinical Electrocardiography, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.