Left anterior fascicular block
| Left anterior fascicular block | |
 | |
|---|---|
| Example of left anterior hemiblock |
For patient information click here
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]
Synonyms and keywords: LAFB; LAHB; left anterior hemiblock
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
Life Threatening Causes
Life-threatening causes include conditions which may result in death or permanent disability within 24 hours if left untreated.
Common Causes
Causes by Organ System
| Cardiovascular | Alcoholic cardiomyopathy, anomalous origin of the left coronary artery, aortic stenosis, atrial septal defect, atrioventricular septal defect, Coronary artery disease, double outlet right ventricle, hypertensive heart disease, hypertrophic cardiomyopathy, ischemic cardiomyopathy, left ventricular hypertrophy, Lev's disease, postpartum cardiomyopathy, restrictive cardiomyopathy, single ventricle, transposition of the great vessels, tricuspid atresia |
| Chemical/Poisoning | No underlying causes |
| Dental | No underlying causes |
| Dermatologic | Lentiginosis |
| Drug Side Effect | No underlying causes |
| Ear Nose Throat | No underlying causes |
| Endocrine | No underlying causes |
| Environmental | No underlying causes |
| Gastroenterologic | No underlying causes |
| Genetic | Atrial septal defect, Charcot-Marie-Tooth disease, double outlet right ventricle, Kearns-Sayre syndrome, lentiginosis, limb-girdle muscular dystrophy |
| Hematologic | No underlying causes |
| Iatrogenic | Aortic valve replacement, septal myomectomy, subvalvar aortic resection, Tetralogy of Fallot repair, ventricular septal defect surgery |
| Infectious Disease | Chagas disease |
| Musculoskeletal/Orthopedic | Limb-girdle muscular dystrophy |
| Neurologic | Charcot-Marie-Tooth disease, Kearns-Sayre syndrome |
| Nutritional/Metabolic | No underlying causes |
| Obstetric/Gynecologic | Postpartum cardiomyopathy |
| Oncologic | No underlying causes |
| Ophthalmologic | No underlying causes |
| Overdose/Toxicity | No underlying causes |
| Psychiatric | No underlying causes |
| Pulmonary | Obstructive sleep apnea |
| Renal/Electrolyte | No underlying causes |
| Rheumatology/Immunology/Allergy | No underlying causes |
| Sexual | No underlying causes |
| Trauma | No underlying causes |
| Urologic | No underlying causes |
| Miscellaneous | Ageing |
Causes in Alphabetical Order
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]
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.[4]
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.
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.
Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page
References
- ↑ 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.
- ↑ Surawicz B, Knilans TK. Chou’s electrocardiography in clinical practice: adult and pediatric, 5th edn. Philadelphia: W.B. Saunders; 2001.
- ↑ 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.
- ↑ Surawicz B, Knilans TK. Chou’s electrocardiography in clinical practice: adult and pediatric, 5th edn. Philadelphia: W.B. Saunders; 2001.