Left Anterior Fascicular Block

The normal infranodal conduction system divides into the right and left bundles, the latter is further subdivided into anterior and posterior divisions or “fascicles.” Disruption of both fascicles produces the familiar left bundle branch block (LBBB) pattern, but each fascicle can be affected independently – resulting in either left anterior fascicular block (LAFB) or left posterior fascicular block (LPFB). When the left anterior fascicle is disrupted, current passes along the posterior fascicle and the left ventricle is depolarized in a leftward/upward direction – producing left axis deviation (and often an extreme left axis deviation, i.e., more than 45 degrees of leftward deviation). Criteria for diagnosing LAFB include:

  1. Left axis deviation (usually > 45 deg)
  2. Small q waves with large R waves (“qR complexes”) in I and aVL
  3. Small r waves with large S waves (“rS complexes”) in II, III, and aVF
  4. Normal or slightly prolonged QRS duration (80-110 ms)


Schematic showing the relative locations of the anterior and posterior fascicles within the left ventricle. The curved blue arrows demonstrate the direction of depolarization with a left anterior fascicular block (from the posterobasal segment of the left ventricle to the anterolateral segment). LV, left ventricle; LPF, left posterior fascicle; LAF, left anterior fascicle; LAD, left axis deviation.



  1. McAnulty JH, Rahimtoola SH, Murphy E, et al. Natural history of “high-risk” bundle-branch block: final report of a prospective study. N Engl J Med. 1982;307(3):137-143.
  2. Surawicz B, Childers R, Deal BJ, et al. AHA/ACCF/HRS Recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances: a scientific statement from the american heart association electrocardiography and arrhythmias committee, council on clinical cardiology; the american college of cardiology foundation; and the heart rhythm society. Endorsed by the international society for computerized electrocardiology. J Am Coll Cardiol. 2009;53(11):976-981.