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:
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.
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.
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
Some differentials worth reviewing noted below.
Key definition: LAFB + RBBB = Bifascicular block
Our thanks to Dr. Tim Davie for assembling the Copa Cardiogram each week. Please follow this link to get to spreadsheet of all Copa Cardiograms.
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