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Bifascicular block

Left Bundle Branch Block With Second-Degree AV Block, Type II

Mon, 11/28/2016 - 18:44 -- Dawn

 This ECG was obtained from an 84-year-old woman who was scheduled for surgery.  When the anesthesiologist did this ECG, the surgery was cancelled. It is a very good example of fascicular-level blocks. 

The underlying rhythm is a regular sinus rhythm at about 95 bpm.  There are some non-conducted P waves which are part of the sinus rhythm (not premature beats).  When the P waves DO conduct, the PR interval is steady at about .15 seconds (148 ms).

In addition, there is a LEFT BUNDLE BRANCH BLOCK.  The ECG criteria for LBBB are:  1) A supraventricular rhythm, 2) A wide QRS, and 3) A negative QRS in Lead V1 and a positive QRS in Leads I and V6.  The QRS duration in this ECG is 136 ms.

There are generally two fascicles (branches) in the left bundle branch, and one main fascicle in the right bundle branch.  So, a LBBB represents a “bi-fascicular block”.  That means that A-V conduction is proceeding down only one fascicle (the right bundle branch).  In that fascicle, there is an “intermittent” block.  When the RBB is not blocked, we see a QRS.  When it is blocked, we see none.  This is then termed an “intermittent tri-fascicular block” – otherwise known as SECOND-DEGREE AV BLOCK, TYPE II.  Type II blocks nearly always have a wide QRS due to the underlying bundle branch pathology.  You may see RBBB, LBBB, or RBBB with left anterior fascicular block (hemiblock).  Very rarely, the combination might include left posterior hemiblock.  The intermittent block in the “healthiest” fascicle(s) is what makes this a second-degree block, and not a complete heart block (third-degree AVB).

The clinical implications of this block are that the heart is operating on only one fascicle, and that fascicle is showing obvious signs of distress.  A third-degree AVB could be imminent.  In addition, LBBB causes a wide QRS, which decreases cardiac output.  Second-degree, Type II AVBs can result in very slow rates, and sometimes cause more hemodynamic instability that some third-degree AV blocks.

This patient was scheduled for pacemaker implantation instead of the originally-scheduled surgery. 

Bifascicular Block and Sinus Bradycardia

Fri, 11/18/2016 - 20:30 -- Dawn

Today’s ECG is from a 75 year old man who has been experiencing syncope. 

Examination of the ECG shows a sinus bradycardia at just under 40 bpm.  There is a first-degree AV block, with a PR interval of about .28 seconds (280 ms).  There is a right bundle branch block.  The ECG criteria for right bundle branch block are:  supraventricular rhythm, wide QRS (120 ms in this case), rSR’ pattern in V1, and  a small, wide S wave in Leads I and V6.  There is actually a “terminal delay”, or extra wave at the end of each QRS complex, reflecting late repolarization of the right ventricle. 

This ECG also shows a left anterior fascicular block, also called left anterior hemiblock.  The left bundle branch usually has two main branches, the anterior-superior and the posterior-inferior.  ECG criteria for left anterior fascicular block are: left axis deviation with a small r wave in Lead III and a small q waves with tall R waves in Leads I and aVL.  There is also a prolonged R wave peak time (> 45 ms) in aVL. There is usually a slightly prolonged QRS, but in this case, there is widening of the QRS due to the RBBB.   Because the right bundle branch is blocked, and one fascicle of the left bundle is blocked, the patient is said to have a “bifascicular block”.  Only one fascicle remains available for conduction from the atria to the ventricles.

We have no information about what caused the conduction block in these two fascicles, but should the third fascicle fail, the patient will be in a complete AV block.  An AV block at the level of the bundle branches will result in an idioventricular escape rhythm – wide QRS complexes with very slow rates – which is a low-output rhythm.  

This patient has also had syncope, which was determined to be related to his bradycardia.  He had an AV sequential pacemaker implanted and did well.

Right Bundle Branch Block With Left Posterior Fascicular Block

Mon, 05/13/2013 - 08:09 -- Dawn

This is a good clear example of right bundle branch block with left posterior fascicular block.  The RBBB is diagnosed by the following criteria:  wide QRS (.12 sec), supraventricular rhythm (NSR), an rsR' pattern in V1, and wide little s waves in I and V6.  The LPFB is inferred by the right axis deviation (Lead III QRS is a bit taller than Lead II and Leads I and aVL are negative), and the fact that there is no other obvious cause for right axis shift noted in this patient.  This constitutes a BIFASCICULAR BLOCK.  The ventricles are being depolarized by way of the anterior fascicle.  In addition, there are slight ST elevations in many leads, with an upward coving in the anterior-septal leads (V1, V2, V3).  Depending upon the patient's history and presentation, this could represent a recent M.I. or pending issues. The borderline first-degree AV block may be of concern in this patient, since first-degree AVB is associated with progression of bifascicular block to complete heart block.  Reference:  Ann Card Anaest, 2010 Jan-Apr;13(1):7-15. doi: 10.4103/0971-9784.58828


Anterior Wall M.I. With Bifascicular Block

Fri, 10/19/2012 - 20:46 -- Dawn

This is a good example of acute anterior wall M.I., with ST elevation in V1 through V6, as well as in Leads I and aVL.  The extensive distribution of ST segment elevations across the anterior and high lateral walls indicates a proximal LAD artery occlusion.  In addition, this ECG shows right bundle branch block, with a QRS width of 144 ms (.14 sec.) and an rsR' pattern in V1. There is also a wide s wave in Lead I which is partly obscurred in V6 by the ST elevation.  The right axis deviation (98 degrees) suggests a left posterior fascicular block which, when coupled with the RBBB, is a bi-fascicular block.  P waves are difficult to see.  Do you think they are found at the end of the QRS complexes, representing a long first-degree AVB?  Look at leads V3 through V6 for clues.

Please feel free to add your comments below.  The more "gurus" the better.

A good ECG to teach your students that a patient facing a life-threatening emergency may have a "normal" rate and regular rhythm.  There is something in this ECG for beginners through advanced students.

Sinus Rhythm With Non-Conducted Atrial Bigeminy

Tue, 06/26/2012 - 20:02 -- Dawn

Jason's Blog: ECG Challenge of the Week, 6-8-12     INTERPRETATION:


1)  In first half of strip: Normal sinus rhythm (rate = 100/min) with . . .

2)  . . . bifascicular block—right bundle-branch block plus left anterior hemiblock 

      (RBBB + LAHB), left axis deviation (LAD) at -57 degrees.

3)   In second half of strip: Sinus rhythm interrupted by a run of nonconducted atrial bigeminy  (arrows (↓); see laddergram).


Jason E. Roediger, CCT, CRAT

jer5150's picture

Jason's Blog: ECG Challenge of the Week for June 10-17. Why did the ventricular rate abruptly decrease?


From June 10, 2012:   As is the case with all practical blogs, I’m encouraging ECG Guru members to engage in active group participation.  Share your thoughts, observations, impressions, findings, and interpretations.  Feel free to compare notes with one another and pick each other’s brains.

Atrial Fib, Bifascicular Block, Pacemaker

Tue, 04/10/2012 - 12:53 -- Dawn

Lots of information in this ECG! The underlying rhythm is atrial fibrillation with a controlled rate. The QRS is .12 seconds in duration, with an rSR' pattern in V1 and a wide s wave in Leads I and V6, indicating right bundle branch block. In addition, the axis is leftward - Leads I and aVL are upright and Leads II, III, and aVF are negative. There is no other obvious reason for the left axis shift, and therefore, the diagnosis by exclusion is left anterior fascicular block. RBBB and LAFB often appear together, as the right bundle branch and the anterior fascicle of the left bundle share the same blood supply from the left coronary artery. ALSO, this patient has a right ventricular pacemaker, and is pacing appropriately when the atrial fib slows. Pacer spikes are not readily seen, but the width of the QRS, the axis of the wide QRS complexes (left), and the timing (after a pause) all support the paced rhythm diagnosis. V5 and V6 actually show a very tiny hint of a spike. The T wave inversions seen in the upright leads are common with RBBB, and are usually considered normal in this setting.

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