Bundle branch and fascicular blocks are frequently seen in those with and without cardiac disease. These patterns are defined by variations in QRS duration and voltage compared to normal. By convention, deflections on the electrocardiogram that are greater than 0.5 mV (ie, greater than 5 mm with standard calibration) are referred to in capital letters; smaller deflections are noted in lower case. Thus, a qRs pattern means that the q and s waves are small and the R wave is large.
LEFT ANTERIOR FASCICULAR BLOCK
A left anterior fascicular block, or hemiblock, is characterized by a pathologic left axis in the frontal limb leads, defined as an axis >-45º or >-60º, depending upon the author, up to -90º. (See "Left anterior fascicular block".) This may be the result of conduction system disease in the anterior fascicle of the left bundle, or may occur when there is disease or fibrosis surrounding the left anterior fascicle (eg, with a myocardial infarction).
The QRS measures <0.12 seconds in duration. The QRS complexes in lead I are upright (waveform 1); in the inferior leads II, III, and aVF they are negative (small R wave, deep S wave). There is a small q wave and tall R wave in lead aVL, and the time to the peak of the R wave in aVL is ≥0.045 seconds. There is often a tall R wave in aVR and poor R wave progression in leads V1-V3.
An inferior wall myocardial infarction also has a left axis as the result of a large Q wave in the inferior leads. This is due to fibrotic tissue rather than conduction system disease. If there is a QR complex due to infarction, the pattern has been called a peri-infarction block. Therefore, a left anterior fascicular block cannot be diagnosed in the presence of an inferior wall myocardial infarction.
LEFT POSTERIOR FASCICULAR BLOCK
A left posterior fascicular block, or hemiblock, occurs when there is conduction system disease involving the posterior fascicle of the left bundle branch. It is defined as a pathologic right axis, >+90º (some authors use an even more rightward axis of 120º) up to 180º. (See "Left posterior fascicular block".) The QRS complex is <0.12 seconds. There are small R and deep S waves in leads I and aVL, and small Q and tall R waves in leads III and aVF (waveform 2).