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.