In left posterior fascicular block the current is conducted to the left ventricle via the left anterior fascicle, which makes the current travel upwards and leftwards producing small R waves in the lateral leads and small Q waves in the inferior leads. Depolarisation then spreads down and right, producing tall R waves in the inferior leads and deep S waves in the lateral leads.

Features of LPFB:

left posterior fascicle block



- There is Right Axis Deviation
- There are small R waves with deep S waves in leads I and aVL
- There are small Q waves with tall R waves in leads II, III and aVF
- QRS duration should be essentially normal
- R wave peak time is prolonged (over 45msec) in aVF
- There should be absence of right ventricular hypertrophy, or any other cause of right axis deviation
- Limb lead QRS voltage should be increased

in summary...

left posterior fascicle block summary

Causes of Left Posterior Fascicle Block


- The left posterior fascicle has dual blood supply (LAD + AV nodal artery) so if ischaemic
heart disease is causing this phenomenon, it is SEVERE indeed.
- Could be myocarditis or some sort nof cardiomyopathy
 

Consequences of Left Posterior Fascicle Block

- This is an asymptomatic condition – usually doesn't amount to much.

References

From "the ECG made easy", by Hampton (2003), and ECGs shamelessly stolen from Life in The Fastlane without any sort of permission, but in the non-commercial spirit of free education

2012 ACCF/AHA/HRS Focused Update of the 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines