Saturday, September 9, 2023

ECG Blog #394 — Is QRS Morphology Disguised?

The ECG in Figure-1 — was obtained from an older man with a history of prior infarction and coronary bypass surgery.
  • How would you interpret this tracing?
  • Clinical implications?

Figure-1: The ECG in today's case. (To improve visualization — I've digitized the original ECG using PMcardio).

MY Thoughts on the ECG in Figure-1:
The rhythm in ECG #1 is sinus at ~65/minute — with an upright P wave in lead II, and a constant and normal PR interval (ie, not more than 0.21 second in duration). The QTc appears to be normal. There is no ECG indication of chamber enlargement — ST-T wave changes do not look acute.
  • The QRS complex is obviously wide (ie, to at least 0.15 second in some leads).

  • In the Chest Leads — The RsR' complex in lead V1, with wide terminal S wave in lead V6 is consistent with an RBBB (Right Bundle Branch Block) pattern.
  • In the Limb Leads — The predominantly positive QRS complex in high-lateral leads I and aVL suggests a LBBB (Left Bundle Branch Block) pattern, albeit with a tiny terminal s wave in lead I — and a small, narrow q wave in lead aVL.

  • Clinical IMPRESSION: Today's ECG shows sinus rhythm with QRS widening in a pattern that is not consistent in all 12 leads with either RBBB or LBBB. Instead — QRS morphology in today's tracing suggests masquerading BBB


What is MBBB? (Masquerading Bundle Branch Block)?
I review my user-friendly approach to the ECG diagnosis of the Bundle Branch Blocks in ECG Blog #282 — and to Hemiblocks and Bifascicular Blocks in ECG Blog #203. In brief — the 3 KEY leads that allow accurate diagnosis of RBBB and LBBB within seconds (!) — are right-sided lead V1 and left-sided leads I and V6.
  • Supraventricular conduction defects not consistent with either RBBB or LBBB in all 3 of these key leads are most easily classified as IVCD (IntraVentricular Conduction Defect) — with this category representing the "end result" of a number of different pathophysiologic processes.

  • MBBB is a special type of IVCD that although uncommon, is important to recognize because it identifies a group of patients with: i) Very severe underlying heart disease; ii) A much higher predisposition for developing complete AV block (and needing a pacemaker); and, iii) An extremely poor longterm prognosis.

ECG Criteria for the diagnosis of MBBB are described by Dhanse et al (J Clin Diag Research: 10(9), 2016) — and Buttner and Cadogan (LITFL, 2021) — and include the following: 
  • An ECG pattern consistent with RBBB in the chest leads (ie, with a widened, predominantly positive QRS in lead V1).
  • An ECG pattern consistent with LBBB in the limb leads (ie, with a widened, monophasic QRS in leads I and aVL).

  • NOTE: Variations on this above "theme" of MBBB are common. Thus, the S wave that is typically associated with RBBB patterns in lateral chest leads V5,V6 may or may not be present. In the limb leads, rather than a strict LBBB pattern — more of an extreme LAHB (Left Anterior HemiBlock) pattern may be seen (ie, with wide and predominantly [if not totally] negative QRS complexes in the inferior leads — and with a smaller [blunted] terminal s wave in leads I and aVL).

  • BOTTOM Line: Knowing the clinical history may aid in recognition of IVCD patterns that are consistent with MBBB (ie, if the patient has a known history of severe, underlying heart disease). Distinction from simple bifascicular block (ie, with RBBB/LAHB) — may be facilitated by seeing one or more of the following: i) More of a monomorphic upright QRS in lead V1 (which lacks the neatly defined, triphasic rsR' with taller right "rabbit ear" seen with typical RBBB); ii) Lack of a wide terminal S wave in lateral chest lead V6; iii) Seeing an all-positive (or at least predominantly positive) widened QRS in leads I and/or aVL, with no more than a tiny, narrow s wave in these leads; and/or, iv) Seeing widened, all-negative (or almost all-negative) QRS complexes in the inferior leads.

What About Today's ECG?
Today's patient has a known history of significant underlying heart disease (ie, he is an older adult with a prior infarction, who underwent coronary bypass surgery). His ECG in Figure-1 shows the following:
  • Sinus rhythm with marked QRS widening.
  • An ECG pattern consistent with RBBB in the chest leads (ie, with an RsR' complex in lead V1 — and wide terminal S waves in lateral chest leads).
  • An ECG pattern consistent with LBBB in the limb leads (ie, with predominantly positive QRS in leads I and aVL, albeit with atypical lbbb features of a narrow, terminal s wave in lead I and an initial q wave in lead aVL).
  • A QRS morphology in the inferior leads that would otherwise suggest an extreme LAHB pattern — if it weren't for the very wide, predominant R waves suggesting lbbb in leads I and aVL.

  • Taken Together: Today's ECG suggests masquerading BBB. It will be important to correlate this tracing clinically — taking into account the increased chance of needing a pacemaker and the poor longterm prognosis.

Acknowledgment: My appreciation to Andrea Röschl (from Neumarkt, Germany) for the case and this tracing.

Related ECG Blog Posts to Today’s Case:

  • ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation. 

  • ECG Blog #282 — reviews a user-friendly approach to the ECG diagnosis of the Bundle Branch Blocks (RBBB, LBBB and IVCD).

  • ECG Blog #203 — reviews ECG diagnosis of Axis, Hemiblocks and Bifascicular Blocks.


  1. hello dear Ken Grauer

    This case has given me many insights.
    Let me ask you about PVC.
    Shouldn't we call it "ventricular parasystole", even though the RR interval is somewhat variable?
    Is it nothing more than AIVR?
    Please let me know.
    sincerely yours

    Naoki Tsukishima

    1. Hello. I am not sure that I understand your question. If you are referring to this case (in this ECG Blog #394) — there are NO PVCs here. Note that there are regular sinus P waves with a constant PR interval — best seen in lead II, but also seen in other leads. So the rhythm is SINUS — with QRS widening due to IVCD (IntraVentricular Conduction Defect) that happens to manifest the unusual pattern of a "Masquerading BBB" (ie, with RRRB-like morphology in the chest leads — and LBBB-like morphology in the limb leads). Let me know if you still have questions about this case! — :)

  2. Great case, New to me...nothing escapes your eagle sharp eyes and your encyclopedic mind. I enjoyed that.

    1. THANKS for the kind words. I bet you start seeing a few more cases of Masquerading BBB — now that you are aware of this entity! It is not common — but it DOES occur (more often than many appreciate!) — :)