Monday, March 15, 2021

ECG Blog #204 — What is Bundle Branch Block?

How would you describe the conduction defect for the ECG shown in Figure-1?

  • Is this RBBB (Right Bundle Branch Block)?
  • Is it LBBB (Left Bundle Branch Block)?
  • Something else?


Unfortunately, no history is available on this patient. That said:

  • Has there been previous infarction?
  • Are you concerned by this ECG about acute infarction?


Figure-1: An ECG with some type of conduction defect ... (See text).



NOTE #1: Some readers may prefer at this point to watch the 13-minute ECG Video PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-22).


Today’s ECG Media Pearl #22 (13:15 minutes Video) — Reviews a user-friendly approach that allows diagnosis of the Bundle Branch Blocks in less than 5 seconds.



NOTE #2: I have excerpted a written summary on the ECG diagnosis of BBB from my ECG-2014-ePub. This appears below in the Addendum (in Figure-2 through Figure-7).

  • CLICK HERE to download a PDF of this 12-page file on BBB.



MY Thoughts on the ECG in Figure-1:

This is a challenging ECG to interpret. The rhythm is sinus with a PAC (the 4th beat occurs early). The PR interval is upper normal at ~0.21 second. The QRS complex is clearly wide (ie, at least 0.14 second).

  • QRS morphology is not consistent with either RBBB or LBBB in each of the 3 KEY leads. That is — the negative QS complex in lead V1 is consistent with LBBB — but if anything, the wide terminal S waves in the tiny complexes seen in leads I and V6 are consistent with RBBB.
  • Since the QRS is wide and the rhythm is sinus — but QRS morphology is not consistent with either RBBB or LBBB — the best way to interpret this conduction defect is IVCD (IntraVentricular Conduction Defect).
  • PEARL #1 — QRS morphology in leads V1, V2 and V3 looks very much like LBBB. But the rest of the ECG looks nothing like LBBB! Instead — there is fragmentation (ie, notching) of the QRS complex in multiple leads (ie, in leads I, II, III; aVL, aVF; and V5, V6). The frontal plane axis is indeterminate (which is unusual with IVCD) — and QRS morphology looks bizarre in lateral leads I and V6. The combination of these findings suggests scarring from either cardiomyopathy, prior infarction(s) and/or some other form of underlying heart disease. Clinically — it is helpful to know from a glance at this ECG that even without the benefit of any history, we KNOW this patient has significant underlying heart disease!
  • PEARL #2  The diagnosis of LVH is clearly more difficult when there is bundle branch block. This is because conduction defects alter the sequence of both ventricular depolarization (during which time the QRS complex is written on the ECG) and repolarization (during which time the ST-T wave is written). That said — the finding of very deep (ie, ≥25-30 mm) S waves in one or more of the anterior leads in a patient with LBBB is highly correlated with LVH. The S wave in lead V2 measures 25 mm — and the S in lead V3 measures more than the 24 mm we see, because the S wave is still descending at the point that the ECG paper runs out. Therefore — it is highly likely that this patient also has LVH.
  • PEARL #3 — Although there are QS complexes in leads V1-thru-V3 — this finding is not specific for prior anterior infarction in the setting of QRS widening. Given the above noted fragmentation in multiple leads — and — the small but wider-than-expected Q wave in lead aVL — it would not be surprising to learn that this patient has had prior infarction. However, this can not be diagnosed with certainty from this single ECG.


Finally, pending clinical correlation — I would interpret the ST-T wave changes in Figure-1 as probably nonspecific. Although there is some J-point ST elevation in leads V1-thru-V3 (and I could not on this single ECG alone rule out the possibility of an acute event early in its course) — I suspect this ST-T wave elevation is not disproportionate, given the deep S waves in these leads. I see little in the way of reciprocal ST depression in other leads.

  • IF the history was not worrisome — I would probably not feel the need for additional ECGs.
  • On the other hand, IF the history suggested new-onset chest pain — then: i) Close clinical monitoring; iiSearching for prior tracings (for comparison); iii) Serial ECGs and troponin in the ED; andiv) Stat Echo (looking for wall motion abnormalities) — would all be indicated.
  • P.S.: It is admittedly difficult to assess ST elevation that occurs in association with QRS widening. That said — the marked fragmentation with ST elevation in the anterior leads here could reflect left ventricular aneurysm. If so — this looks chronic. An Echo would be revealing.

BOTTOM Line: The ECG in Figure-1 shows sinus rhythm, a PAC, IVCD with marked QRS widening, and probable LVH. There is fragmentation in multiple leads, and this patient undoubtedly has significant underlying heart disease. That said — I suspect there are no acute ST-T wave changes. That said — Clinical correlation will be essential in order to know what to do with this tracing.



Relevant ECG Blog Posts to Today’s Post: 

  • ECG Blog #198 — An Irregular WCT (LBBB or IVCD).
  • ECG Blog #162 — LBBB with obvious STEMI.
  • ECG Blog #146 — LBBB with Acute ST-T Wave Changes.
  • ECG Blog #145 — RBBB with Diffuse Subendocardial Ischemia.
  • ECG Blog #144 — AV Block and Alternating BBB.
  • ECG Blog #140 — Atrial Bigeminy with Alternating RBBB/LAHB-LPHB Aberration.
  • ECG Blog #135 — Regular WCT that Looks like Fascicular VT (but is Not!).
  • ECG Blog #130 — 2nd-Degree with Alternating BBB (LBBB-RBBB).
  • ECG Blog #122 — Bifascicular Block in a Child with Congenital Heart Disease.
  • The June 25, 2020 post in Dr. Smith’s ECG Blog — in which I review a case of Sinus Rhythm with Intermittent RBBB.



ADDENDUM (3/15/2021): In the following 6 Figures — I post written summary from my ECG-2014-ePub on the ECG diagnosis of BBB.

  • CLICK HERE — for a PDF of the 12-page file that appears in Figures-2-thru-7.


Figure-2: Intro — Using the Algorithm for QRS Widening to determine the type of conduction defect.


Figure-3: ECG Findings with typical RBBB.

Figure-4: ECG Findings with typical LBBB.


Figure-5: Incomplete LBBB — ECG Findings with IVCD.


Figure-6: Expected ST-T wave changes with RBBB and LBBB.


Figure-7: RBBB-Equivalent Patterns — Incomplete RBBB.


  1. DEAR SIR,

    I would like to ask regarding the entity of "Masquerading Bundle Branch block"(MBBB).

    Can MBBB be used interchangeably with IVCD?

  2. Thank you for your question, which is an excellent one! While one might classify MBBB ( = "Masquerading Bundle Branch Block") as a type of IVCD — it is really a separate entity! I refer you to — — As per Dhanse et al in this article — MBBB is a rare but important finding on ECG — in which the chest leads show a RBBB-like pattern and the limb leads resemble LBBB. To see MBBB suggests severe underlying conduction disease, often associated with a poor prognosis — which is not necessarily the case with other forms of IVCD.