Dawn's picture

 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. 

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ekgpress@mac.com's picture

     Although suboptimal in quality — this ECG is a wonderful example of 2nd-Degree AV Block, Mobitz Type II. I will highlight a couple of points to complement Dawn’s explanation.

  • The KEY to interpretation lies in the long lead II rhythm strip at the bottom of this tracing — namely, that ALL P waves are on time. Thus, the underlying rhythm is sinus, and we are not dealing with premature beats.
  • As per Dawn, some P waves are not conducted. By definition — this tells us there is 2nd-degree AV block. The 3 types of 2nd-degree AV block are: i) Mobitz I; ii) Mobitz II; and iii) 2:1 AV block, in which we cannot tell whether there is Mobitz I or Mobitz II. By far, the most common type of 2nd-degree AV block is Mobitz I (over 90-95% of cases). The importance of recognizing when Mobitz II is present — is the disturbing tendency for Mobitz II 2nd-degree AV block to sometimes very suddenly develop ventricular standstill. As a result — these patients need pacing.
  • The KEY to recognizing Mobitz II — is that there are consecutively conducted beats on the tracing for which the PR interval remains constant. This is precisely what we see here! Note that the QRS complex is wide (as it almost always is with Mobitz II) — because the defect is much lower down in the conduction system. As per Dawn, QRS morphology here is consistent with underlying LBBB.


NOTE: For those interested, my 60-minute ECG Video on the Basics of AV Blocks reviews in detail key aspects on ECG recognition — GO TO — www.avblockecg.com — If you click on SHOW MORE below the video, you’ll find a detailed linked contents to all in this video.


Ken Grauer, MD  www.kg-ekgpress.com   [email protected] 

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