Left Posterior Fascicular Block:
- Frontal Plane Axis of 90 degrees to 180 degrees
- Small R Wave in Leads I and aVL
- Tall R Wave in Leads II, III and aVF
- Q Wave in Inferior Leads less than 0.04 seconds
Left posterior fascicular block is characterized by an initial vector which is directed superiorly and to the left, resulting in a small R wave in leads I and aVL and a small Q in III and aVF which measures less than 0.04 seconds. The main QRS vector in the frontal plane is directed inferiorly and to the right with a value between +90 and +180 degrees, i.e. abnormal right axis deviation. This is expressed on the body surface electrocardiogram by a tall R wave in leads II, III and aVF and an S wave in leads 1 and aVL. The total QRS duration, as with left anterior fascicular block should be less than 0.120 seconds (120ms). However, unlike the situation with left anterior fascicular block which frequently occurs in the absence of an associated conduction abnormality, isolated left posterior fascicular block is unusual. It occurs most frequently in association with right bundle branch block.