A left posterior fascicular block (LPFB) is a condition where the left posterior fascicle, i.e. the backside part of the left cardiac bundle, does not conduct the electrical impulses from the atrioventricular node. The electricity then has to go through the other fascicle, and thus is a frontal right axis deviation seen on the ECG.
Typically, a LPFB EKG pattern shows a small r wave and a deep s wave in leads I and aVL, and a tall R wave in the inferior leads (II, III, aVF), a slightly widened QRS (100-120ms), and a right axis deviation in the absence of an alternative explanation for the right axis deviation. The deviation should be greater than +120 degrees and it is not a pathognomonic sign; there are other conditions that can give the same picture.