Dr. Mattu walks us through a three step method to make identifying heart blocks easy. Just ask three qestions
1) What is the atrium doing? (regular, irregular rate?)
2) What is the ventricle doing? (morphology and thus location? rate?)
3) How are the two related? (Look at PR interval, is it regular? long then dropped QRS, longer, longer then drop? or randomly related? 1st, 2nd, 3rd degree blocks).
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.
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.
Our favorite ECG Resources