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Left Anterior 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 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)

 

Locations of the anterior and posterior fascicles within the left ventricle. The curved blue arrows demonstrate the direction of depolarization when the left posterior fascicle fails, leading to left axis deviation. LV, left ventricle; LPF, left posterior fascicle; LAF, left anterior fascicle; LAD, left axis deviation.

Examples

ECG Showing Left Anterior Fascicular Block

Left Anterior Fascicular Block

Left anterior fascicular block. The extreme left axis deviation, qR complexes in aVL, and rS complexes in the inferior leads (II, III, aVF) are diagnostic.
ECG Showing Right Bundle Branch Block with Left Anterior Fascicular Block

Right Bundle Branch Block with Left Anterior Fascicular Block

In addition to the right bundle branch block, the left anterior fascicular block, and atrial fibrillation, the patient was also hyperkalemic.
ECG Showing Bifascicular Block

Bifascicular Block

There is a right bundle branch block and a left anterior fascicular block.
ECG Showing Supraventricular Tachycardia with Aberrancy

Supraventricular Tachycardia with Aberrancy

Post-cardioversion of supraventricular tachycardia with aberrancy in a patient with known right bundle branch and left anterior fascicular blocks.
ECG Showing Trifascicular Block

Trifascicular Block

Right bundle branch block, left anterior fascicular block, and PR prolongation representing trifascicular disease.
ECG Showing Bifascicular Block

Bifascicular Block

Right bundle branch block and left anterior fascicular block.
ECG Showing Second Degree Atrioventricular Block 2:1

Second Degree Atrioventricular Block 2:1

Right bundle branch block, left anterior fascicular block, and 2:1 atrioventricular block. This representing Mobitz II conduction given the extent of infranodal disease. This patient was admitted for a permanent pacemaker.

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.