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

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 leads II, III, and aVF

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

Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org