Left anterior fascicular block electrocardiogram

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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

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)[1]

Exceptions

It is important not to call LAFB in the setting of a prior inferior wall myocardial infarction which may also demonstrate left axis deviation due to the initial forces (Q wave in a Qr complex) in leads II, III, and aVF. As opposed to LAHB, the left axis shift is due to terminal forces (i.e., the S wave in an rS complex) being directed superiorly.

Effects of LAHB on Diagnosing Infarctions and Left Ventricular Hypertrophy

LAHB may be a cause of poor R wave progression across the precordium causing a pseudoinfarction pattern mimicking an anteroseptal infarction. It also makes the electrocardiographic diagnosis of LVH more complicated, because both may cause a large R wave in lead aVL. Therefore to call LVH on an EKG in the setting of an LAHB you should see the presence of a “strain” pattern when you are relying on limb lead criteria to diagnose LVH.[2]

EKG Examples

Shown below is an example of an EKG demonstrating left axis deviation with rS pattern in lead III and QRS complex < 0.12 seconds indicating a left anterior hemiblock. A qR complex is also seen in lead aVL.

Right bundle branch block 4.jpg

Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page


Shown below is an example of an EKG showing rS pattern in lead III, qR complex in lead aVL and QRS complex < 0.12 seconds along with left axis deviation indicating left anterior fascicular block.

LAHB.png

Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page



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.



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