Left anterior fascicular block overview

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Overview

Left anterior fascicular block is caused by interruption of the anterior division of the left bundle branch. This fascicle is fragile, easily exposed to damage, and has a single blood supply (the left anterior descending coronary artery).

Pathophysiology

Normal activation of the left ventricle proceeds down the left bundle branch, which consist of two fascicles the left anterior fascicle and left posterior fascicle. Left anterior fascicular block (LAFB), which is also known as left anterior hemiblock (LAHB), occurs when a cardiac impulse spreads first through the left posterior fascicle, causing a delay in activation of the anterior and lateral walls of the left ventricle which are normally activated via the left anterior fascicle.[1] Although there is a delay or block in the activation of the left anterior fascicle there is still preservation of initial left to right septal activation as well as preservation of the inferior activation of the left ventricle (preservation of septal Q waves in I and aVL and small initial R wave in leads II, III, and aVF). The delayed and unopposed activation of the remainder of the left ventricle now results in a shift in the QRS axis leftward and superiorly, causing marked left axis deviation. This delayed activation also results in a widening of the QRS complex, although not to the extent of a complete LBBB. [2]

Causes

Left anterior fascicular block, which is more common than left posterior fascicular block, may be due to damages to the left anterior fascicle as it passes through the left ventricular outflow tract, such causes include aortic stenosis, hypertensive heart disease and cardiomyopathy. It can also be caused by congenital heart diseases such as Atrial septal defect, atrioventricular septal defect, single ventricle, e.t.c However, life threatening conditions such as myocardial infarction must be promptly identified and treated accordingly.

History and Symptoms

The symptoms depends on the degree of blockage of the conduction system of the heart. Patients are usually asymptomatic but when symptoms do occur, they may experience some palpitations, intermittent chest pain, dizziness. A thorough cardiovascular physical examination and an EKG is required in order to make a diagnosis.

Electrocardiogram

Criteria for LAHB

  • Left axis deviation (usually between -45° and -90°), some consider -30° to meet criteria
  • QRS interval < 0.12 seconds
  • qR complex in the lateral limb leads (I and aVL)
  • rS pattern in the inferior leads (II, III, and aVF)
  • Delayed intrinsicoid deflection in lead aVL (> 0.045 s)[3]

Medical therapy

Most isolated fascicular blocks are asymptomatic. Therapy could only be considered in the presence of a persistent bifascicular block or trifascicular block.

References

  1. Mirvis DM, Goldberger AL. Electrocardiography. In: Braunwald E, Zipes DP, Libby P, eds. Heart disease: a textbook of cardiovascular medicine, 6th edn. Philadelphia: WB Saunders; 2001:82–125.
  2. Surawicz B, Knilans TK. Chou’s electrocardiography in clinical practice: adult and pediatric, 5th edn. Philadelphia: W.B. Saunders; 2001.
  3. Mirvis DM, Goldberger AL. Electrocardiography. In: Braunwald E, Zipes DP, Libby P, eds. Heart disease: a textbook of cardiovascular medicine, 6th edn. Philadelphia: WB Saunders; 2001:82–125.



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