Left posterior fascicular block electrocardiogram

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

The electrocardiogram findings in left posterior fascicular block include right axis deviation of the QRS complex, wide QRS complexes and a S1Q3 pattern.

Electrocardiogram

The diagnosis is often made in the context of previous EKGs. The frontal QRS axis is rightward and there are no other potential causes of right axis deviation such as right ventricular hypertrophy and prior lateral myocardial infarction.

• QRS axis of 100 to 180 degrees
• QRS duration 80-100 msec (usually normal or slightly widened qRS complex)
• Deep S in lead I and aVL (rS in I and aVL)
• qR in II, III, and aVF

EKG Examples

The EKG below shows sinus rhythm, with a some what wide QRS and a right axis deviation. The S1 Q3 pattern suggest an isolated left posterior fasicular block. One should also consider right ventricular hypertrophy but there is no sign of right atrial abnormality, and his echocardiogram was normal.

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