Left Posterior Fascicular Block

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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 posterior fascicle is disrupted, current passes along the anterior fascicle and the left ventricle is depolarized in a rightward/posterior direction – producing right axis deviation, Criteria for diagnosing LPFB include:

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

LPFBs rarely occur in isolation, owing to a robust blood supply, and, instead, nearly always coexist with a right bundle branch block (bifascicular block).

 

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 posterior fascicular block (from the anterolateral segment of the left ventricle to the posterobasal segment). LV, left ventricle; LPF, left posterior fascicle; LAF, left anterior fascicle; RAD, right axis deviation.

Examples

ECG Showing Left Posterior Fascicular Block

Left Posterior Fascicular Block

Isolated left posterior fascicular block with right axis deviation, rS complexes in the lateral leads (I, aVL) and qR complexes in the inferior leads (II, III, aVF).
ECG Showing Right Bundle Branch Block & Left Posterior Fascicular Block

Right Bundle Branch Block & Left Posterior Fascicular Block

Right bundle branch blocks do not typically cause an axis deviation because the right ventricle does not have enough myocardium to cause an axis shift (as opposed to a left bundle branch block). When a right axis deviation accompanies a right bundle branch block, there is often an associated left posterior fascicular block, as with this case.
ECG Showing Bifascicular Block

Right Bundle Branch Block & Left Posterior Fascicular Block

The rhythm is irregularly irregular as a result of atrial fibrillation. There is a right bundle branch block and a left posterior fascicular block.

References

  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.