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More detailed information about left anterior fascicular block

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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  • normal activation of the left ventricle proceeds down the left bundle branch, and on through its two fascicles (left anterior and left posterior) simultaneously
    • left anterior fascicular block (LAFB) is when there is non-conduction of the cardiac impulse down the left anterior fascicle
    • LAFB, also known as left anterior hemiblock, is more common than left posterior fascicular block
    • in LAFB the cardiac impulse spreads first through the left posterior fascicle (inferiorly). As a result of the LAFB there is delayed activation of the anterior and lateral walls of the left ventricle
      • ECG findings of LAFB:
        • QRS interval < 0.12 seconds (usually)
        • left axis deviation
        • qR complex in the lateral limb leads (I and aVl) - preservation of septal Q waves in I and aVl
        • rS pattern in the inferior leads (II, III, and aVf)
    • LAFB is of little to no prognostic significance (1)
      • previous evidence has suggested LAFB carries a normal prognosis (2). Note however that, in the context of patients with suspected CHD who have no history of myocardial infarction, isolated LAFB is an independent predictor of total and cardiac mortality (3)
      • LAFB is found in a wide variety of conditions
        • LAFB may occur in cardiac disease
          • may occur in acute myocardial infarction
            • isolated LAFB occurs in approximately 4% of cases
          • LAFB is the most common type of intraventricular conduction defect following an anterior wall acute myocardial infarction due to ischaemia of the anterior fascicle of the left conduction system
            • LAFB may also be observed following acute inferior wall myocardial infarction
              • this is suggestive that LAFB is a result of disease of the left anterior descending artery (4)
      • other associations of LAFB include:
        • hypertensive heart disease
        • aortic valve disease
        • cardiomyopathy
        • degenerative fibrotic disease of the cardiac skeleton


  1. Harrigan RA et al. Electrocardiographic manifestations: bundle branch blocks and fascicular blocks. The Journal of Emergency Medicine 2003;25(1):67–77
  2. Rowlands DJ.Left and right bundle branch block, left anterior and left posterior hemiblock. Eur Heart J. 1984 Mar;5 Suppl A:99-105.
  3. Biagini E et al. Prognostic significance of left anterior hemiblock in patients with suspected coronary artery disease. J Am Coll Cardiol. 2005 Sep 6;46(5):858-63.
  4. Assali A et al. Importance of left anterior hemiblock development in inferior wall acute myocardial infarction. Am J Cardiol 1997;79: 672– 4

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