Left Posterior Fascicular Block

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

​Remember it's probably NOT LPFB if you see:

• No evidence of right ventricular hypertrophy
• No evidence of any other cause for right axis deviation
• PE
• TCA overdose
• Lateral MI

Copa Cardiogram #73