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Fascicular block

Left Anterior Fascicular Block (Hemiblock)

Sun, 09/13/2015 - 22:10 -- Dawn

This ECG provides an example of LEFT ANTERIOR FASCICULAR BLOCK (LAFB).  It is from a 71-year-old woman for whom we have no other history.  She also has first-degree AV block and right bundle branch block.  RBBB and LAFB together are called bifascicular block.  It is not uncommon to see this type of bifascicular block, as the right bundle branch and the  anterior fascicle of the left bundle share a blood supply. 

The conduction system below the AV node consists of the Bundle of His, the left bundle branch, and the right bundle branch.  While there is some variation among individuals, most of us have two main fascicles, or branches, of the left bundle.  The ANTERIOR-SUPERIOR fascicle carries the electrical impulse to the anterior wall of the left ventricle, and the POSTERIOR - INFERIOR fascicle carries the impulse to the inferior area of the left ventricle.

Blocks can occur at any level in the conduction system, including left bundle branch block, right bundle branch block, left anterior fascicular block, left posterior block, and bi-fascicular blocks. LAFB can have many causes, including myocardial infarction, cardiomyopathies, fibrosis of the cartilagenous ring, and aortic valve disease.  Left anterior fascicular block is much more common than left posterior fascicular block. Both are also called hemiblocks.

When LAFB is present, the initial septal depolarization forces are still left to right, providing a small initial q wave in Lead I and a small r wave in Lead III.  After septal depolarization is complete, the activation vector moves inferiorly and to the right as the electrical wavefront moves through the left posterior hemifascicle and right bundle branch. The impulse finally makes its way to the left and superiorly via slow conduction through myocardium normally depolarized by the left anterior hemifascicle, which is blocked.  It is because the terminal left ventricular activation moves upward and toward the left that the  inferior leads have negative deflections.

Left Anterior Fascicular Block

Sun, 09/28/2014 - 23:39 -- Dawn

This ECG provides an example of LEFT ANTERIOR FASCICULAR BLOCK (LAFB).  It is from an elderly woman for whom we have no other history.

The conduction system below the AV node consists of the Bundle of His, the left bundle branch, and the right bundle branch.  While there is some variation among individuals, most of us have two main fascicles, or branches, of the left bundle.  The ANTERIOR-SUPERIOR fascicle carries the electrical impulse to the anterior wall of the left ventricle, and the POSTERIOR - INFERIOR fascicle carries the impulse to the inferior area of the left ventricle.

Blocks can occur at any level in the conduction system, including left bundle branch block, right bundle branch block, left anterior fascicular block, left posterior block, and bi-fascicular blocks. LAFB can have many causes, including myocardial infarction, cardiomyopathies, fibrosis of the cartilagenous ring, and aortic valve disease.  Left anterior fascicular block is much more common than left posterior fascicular block. Both are also called hemiblocks.

When LAFB is present, the initial septal depolarization forces are still left to right, providing a small initial q wave in Lead I and a small r wave in Lead III.  After septal depolarization is complete, the activation vector moves inferiorly and to the right as the electrical wavefront moves through the left posterior hemifascicle and right bundle branch. The impulse finally makes its way to the left and superiorly via slow conduction through myocardium normally depolarized by the left anterior hemifascicle, which is blocked.  It is because the terminal left ventricular activation moves upward and toward the left that the  inferior leads have negative deflections.

The diagnostic criteria for LAFB are:  LEFT AXIS DEVIATION (QRS axis between -45 degrees and -90 degrees); qR pattern in Lead I; rS pattern in Lead III; delayed activation time evident in Lead aVL - the time from onset of the QRS to the peak of the R wave is 45 ms or more. (This example barely makes that criteria); QRS duration normal or slightly wide, but not 120 ms or more (unless there is also RBBB).  LAFB also causes poor R wave progression in the precordial leads, with late transition and S wave present in V6.

Before deciding on a diagnosis of LAFB, you must rule out previous or acute INFERIOR WALL M.I.  The pathological Q waves that can occur with necrosis can cause a left axis deviation in the frontal plane.  The presence of a small r wave in Lead III rules out pathological Q wave in that lead.  If any fascicular block (hemiblock or bundle branch block) occurs during the course of an M.I., the patient should be watched carefully for progression of the block.  Be prepared to pace if necessary in that situation. 

Thanks to our Consulting Expert, Dr. Ken Grauer, for his editing assistance.

Right Bundle Branch Block With Atypical QRS in V1 and LAFB or Ventricular Rhythm???

Wed, 03/27/2013 - 19:28 -- Dawn

This is quite an interesting ECG, and the ECG Guru would love to hear what you think about it.  What we do know is that it is a wide-complex bradycardia in a patient for whom we have no clinical data, except that she is a 51 year old female.  The rhythm is probably junctional, as no P waves are seen and the rhythm is regular.  The rate of 63 per minute would be consistent with that. Interestingly, no disassociated sinus P waves are seen.  All slow wide-complex rhythms should be evaluated for idioventricular origin, or AIVR.  The QRS in V1 shows an atypical right bundle branch block pattern.  We usually look for rSR', or "bunny ears",  but  this ECG shows an upright R wave with a smaller, slurred r wave before it.  What makes this look like RBBB is the prominent wide little S wave in V6 and in Lead I.  We question the R wave progression, too.  Do you think it is possible that the electrodes for V2 and V3 are switched?  The axis is leftward, causing Lead II to be nearly biphasic - it represents a synthesis of what is seen in Leads I and III.  This is enough left axis shift to diagnose a left anterior fascicular block (with RBBB = bifascicular block). 

This is a great ECG, and we can't wait to hear from all you ECG Gurus out there. Maybe we will need to adjust our diagnosis after we hear from you.

How Do You Teach the Hemiblocks?

Wed, 11/23/2011 - 17:01 -- Dawn

Today's Expert is Dr. KEN GRAUER, MD   Dr. Grauer is Professor Emeritus (Dept. Community Health/Family Medicine, College of Medicine, University of Florida in Gainesville).
Dr. Grauer has been a leading family physician educator for over 30 years. During that time he has published (as principal author) more than 10 books and numerous study aids on the topics of ECG interpretation, cardiac arrhythmias, and ACLS (including an ongoing Educational ECG Blog) . For more information about Dr. Grauer, see his website: https://www.kg-ekgpress.com/
 
ANSWER:

Teaching the hemiblocks has often been an area that leads to confusion among those learning ECG interpretation. It is easy to understand why ... - even expert electrocardiographers don't agree on the definition as to what constitutes a hemiblock. I've always felt that when many equally correct answers exist to a question - Why not choose one of the answers that is easy to apply and easy to remember? So it is with the hemiblocks. All that a "hemiblock" is - is failed conduction down one of the two major fascicles of the left bundle branch.
Although there are millions of fibers within the conduction fascicles - for practical purposes, there are 2 main divisions to the left bundle branch (See accompanying PDF in RESOURCES  for explanatory figure and description). These 2 divisions of the left bundle branch are the left anterior and left posterior hemifascicles. A hemiblock entails failed conduction in one of these hemifascicles. If conduction fails in both hemifascicles (or if the defect in conduction is proximal to the level where the main left bundle branch divides into these 2 hemibranches) - then complete left bundle branch block (LBBB) will arise. For practical purposes - LPHB (left posterior hemiblock) is rare. This is because the left posterior hemifascicle is both much thicker as well as enjoying of dual blood supply from both left and right coronary arteries - vs the much thinner and singly supplied left anterior hemidivision. Although I've never seen a study quantifying the relative frequency of LAHB vs LPHB - in my experience LPHB is very rare (probably less than 1-2% of the hemiblocks). Even cardiologists often do not agree on whether or not LPBH is present - such that most noncardiologists would be none the worse if they never in their life diagnosed LPHB (suggestions for how to diagnose LPHB are included in the attached PDF). Thus, IF a hemiblock is present - it will almost always be LAHB.
 
How then to diagnose LAHB? For practical purposes - one can equate the diagnosis of LAHB with that of a "pathologic" left axis. LAD (left axis deviation) is defined as an axis that falls in the upper left quadrant (ie, between -1 to -90 degrees). We define "pathologic LAD" as an axis more negative than -30 degrees. Fortunately - this is EASY to determine on the ECG. We know that the axis lies perpendicular (90 degrees away) from a lead that is isoelectric (equal parts positive and negative). Therefore, assuming lead I is positive (so that the axis lies in the left hemisphere) - IF the QRS complex is isoelectric in lead II (at +60 degrees) - then the axis must lie 90 degrees away from lead II, or at -30 degrees. All one has to do to determine IF there is a pathologic left axis is look at lead II. If the net QRS deflection in lead II is more positive than negative - then the axis lies LESS than 90 degrees away from lead II, or between -1 and -30 degrees. On the other hand - IF the net QRS deflection in lead II is more negative than positive - then the axis must lie MORE than 90 degrees away = a "pathologic left axis" = LAHB.
 
Reasons to consider teaching the above approach for the hemiblocks is that it is equally accurate and far simpler than worrying about complex morphologic criteria or axis deviations exceeding other amounts. The beauty of the above approach is that it allows accurate determination of whether LAD is sufficiently negative to satisfy criteria for LAHB in less than 5 seconds.
 
 
 
Ken Grauer, MD
 

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