Bundle Blocks with Sir WilliaM Marrow
Quick question. When was the last time you immediately recognized a Left Bundle Branch Block (LBBB) on EKG? As in, as soon as the tracing was shown to you, you knew exactly what it was (and of course, what to do)?
If you’re not an EKG aficionado like the next guy, then maybe you weren’t so quick to make the call. Also, left bundle branch blocks aren’t all that common, occurring in less than 1 percent of the general population according to Up To Date (Sauer, 2017). Before we delve into how to recognize an LBBB, let’s revisit some of the basics.
Anatomy and Physiology: Within the interventricular septum of the myocardium, lies a component of the electrical conduction system, called the Bundle of His. The bundle splits into a single Right and main Left branch, the latter of which divides into an anterior and a posterior fascicle. The Left Anterior Descending artery is what supplies the main left bundle, and is the main cause for concern of an acute MI in the setting of a new LBBB. This is why recognizing LBBB is so important!
Pathophysiology: Normally, electrical conduction and activation of the septum follows a left to right pattern, producing the characteristic small q waves in the lateral leads (I, AVL, V5, V6) on EKG. However, in LBBB, electrical impulses are instead relayed to the right bundle first, then to the left bundle via the IV septum. As a consequence, this pattern produces a combination of ANY of the following:
- Widened QRS (>120ms, eliminating q wave) in the lateral leads (I, AVL, V5, V6).
- Tall ‘notched’ R waves in the lateral leads (I, AVL, V5, V6).
- Deep S wave in the precordial right leads (V1, V2, V3).
- ST segment and T wave discordant, compared to the main vector of the QRS complex.
- Increased R wave peak time (>60ms) in left precordial leads.
- Poor R wave progression in the chest leads.
- Left Axis Deviation.
Again, we may not recognize all of these criteria at first glance.
Enter Sir WilliaM, to lessen the burden. Stylistically, his name represents two of the aforementioned characteristic changes:
- Deep S wave (sometimes notched, hence the W) in V1.
- Tall, notched R wave (hence the M) in V6.
Hopefully this ECG tracing, courtesy of Life in The Fast Lane, will help you remember:
PRACTICE: See if you can pick out the characteristic changes in these EKGs!
In another vein of this topic, is how to diagnose acute MI in a patient with pre-existing LBBB. Follow this link on Sgarbossa’s Criteria to learn more about this special case!
Treatment: According to Up To Date, a patient with asymptomatic LBBB, and no underlying cardiac disease, can simply be monitored without intervention. However, if the patient becomes symptomatic, developing a life-threatening conduction abnormality (3rd degree block or Mobitz II 2nd degree AV block) then a permanent pacer is indicated.
Well, I hope this was helpful. And I hope that you also asked yourself about the right bundle. As in, is there also a clever way to recognize RBBB on EKG? Indeed there is! And Sir MarroW can help. Not until next time though. Stay tuned, because he may have some tricks up his sleeve.
References:
Burns, E. (2017, April 16). Left Bundle Branch Block Overview. In Life in The Fast Lane. Retrieved September 13, 2017, from https://lifeinthefastlane.com/ecg-library/basics/left-bundle-branch-block/
Rezaie, S. (2013, June 9). ECG of the Week: LBBB and STEMI. In REBEL EM. Retrieved September 18, 2017, from http://rebelem.com/r-e-b-e-l-ecg-week-lbbb-stemi/
Sauer, W. H. (2017, March 23). Epidemiology. In Left Bundle Branch Block. Retrieved September 13, 2017, from https://www.uptodate.com/contents/left-bundle-branch-block
Smith, S. W. (2012, October 6). Instructive ECGs In Emergency Medicine. In Dr. Smith’s ECG Blog. Retrieved September 18, 2017, from http://hqmeded-ecg.blogspot.com/2012/10/hyperkalemia-in-setting-of-left-bundle.html
Venkatesan, S. (2010, August 23). Left Bundle Branch Block: Infrequently asked questions. In Expressions in Cardiology. Retrieved September 18, 2017, from https://drsvenkatesan.com/2010/08/23/left-bundle-branch-block-ifaqsinfrequently-asked-questions/