The posterior fascicle is much large and less susceptible to injury.  As such it is not likely to occur in isolation, and more commonly seen with some AV node dysfunction as a Bifascicular block (LPFB + RBBB).  Dr, Burns walks us through some of expected ECG findings and also reminds that you should not dx LPFB without first considering more worrisome causes of right axis deviation: PE, TCA overdose, lateral MI, or RVH.  Check out his post on LITFL.

Conduction goes down functioning anterior fascicle, from endocardium to epicardium (initial small voltage) to remaining myocardium from the anterior/superior LV towards the inferior/posterior LV resulting in the following changes:

• Right axis deviation (> +90 degrees)
• Small R waves with deep S waves in leads I and aVL
• Small Q waves with tall R waves in inferior leads (II, III, aVF)

This altered conduction takes longer than usual and the delay can be seen on ECG:

• QRS duration normal or slightly prolonged (80-110ms)
• Prolonged R wave peak time in aVF
• Increased QRS voltage in the limb leads

It helps to remember how the current is conducted.  The posterior fascicle originates inferiorly and posteriorly on the endocardium.  Thus the initial flow of current in LV will be rightward and down, as current moves to the epicardium.  Additionally, depolarization of the right ventricle is progressing normally.  As depolarization progresses, it moves from inferior, posterior to anterior, superior giving you the  patter noted below.  Check out LITFL's page on LAFB.
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• Left axis deviation
• Small q waves with tall R waves in leads I and aVL
• Small r waves with deep S waves in leads II, III, aVF
• QRS duration is normal or slightly prolonged (80-110 ms)
• Prolonged R wave peak time in aVL > 45 ms (measure from start of Q to peak of R)
• Increased QRS voltage in the limb leads

Dr. Burns also provides a beautiful pearl that in LAFB you might see LVH voltage criteria in aVL (R wave height > 11 mm), but there will be no LV strain pattern.

This week take a chance to walk step by step through an initially complex appearing EKG.  Dr. Mattu shows how a step by step approach, stopping at each abnormality found to analyze the differential.  A reminder of his systematic approach: Click the Read More link to see some of the diferentials discussed this week.

1. Rate and rhythm
2. Axis
3. Intervals
4. Enlargement
5. Ischemia/infarction

Dr. Mattu walks us through how to read an abnormal appear ECG and not miss the subtle MI.  He recommends a logical, stepwise approach.  By considering the differential of each abnormality seen you can begin to understand what is happening with the pt's heart.  Finally, apply the filter of your final diagnosis and look for the subtle clues to ischemia.

1) Rate and Rhythm
2) Axis
3) Intervals
4) Enlargement
5) Ischemia

Some differentials worth reviewing noted below.

Key definition: LAFB + RBBB = Bifascicular block

Dr. Amal Mattu discuss ED differential for Rightward axis.  
BLUF: think about

• PE
  
• Na channel blocker toxicity 
• Lead misplacement

Check out the link for full details and quiz.