Masquerading Bundle Branch Block (MBBB)

Mixed pattern of complete right bundle branch block (RBBB) in precordial leads and complete left bundle branch block (LBBB) in limb leads, indicating severe and diffuse conducting system disease with a poor prognosis

  • Compared with typical bifascicular block, these patients have more extensive fibrosis and degeneration of left bundle pathways
  • Consequent high-grade left anterior fascicular block (LAFB), often associated left ventricular enlargement, manifests as a complete LBBB. This is partially masqueraded by concurrent RBBB
  • Patients have a higher risk of progression to complete AV block than typical bifascicular block
  • A second, precordial type of MBBB has also been described (see below)
Diagnostic criteria

Standard or Precordial Masquerading Bundle Branch Block (MBBB). In both types, RBBB is shown by typical RSR’ pattern in lead V1.

Standard MBBB:
  • RBBB pattern in precordial leads
  • LBBB pattern in limb leads
  • Small or absent S wave in lead I
Standard MBBB. RBBB pattern in precordial leads V1-3
Standard MBBB ECG showing:
Broad QRS complexes with an RBBB pattern (rsR pattern) in the precordial leads V1 to V3
LBBB pattern with near-absence of S waves in lead I in the limb leads.
Left axis deviation and QS complexes in Leads II, III, aVF and M pattern in lead aVL.

Precordial MBBB:
  • RBBB pattern in leads V1-3
  • LBBB pattern in V4-6
  • Absent S wave in leads V5-6
Masquerading Bundle Branch Block (MBBB) 2020
Precordial MBBB ECG showing:
RBBB pattern in right precordial leads with LBBB pattern in left precordial leads.
Absent S waves in V5-6 and lead I.
Clinical significance

Compared with a typical bifascicular block pattern, MBBB indicates more extensive disease of the left bundle pathway and left ventricle, and carries a poorer prognosis:

  • Rate of progression to complete heart block over a four-year period in patients with MBBB was observed at 59% (Barrado et al), compared to 11% over a five-year period in typical bifascicular block (RBBB + LAFB)
  • A large scale review of 600,000 ECGs found the rare MBBB pattern was associated with high mortality (41% over four years) and pacemaker insertion (39% over the same period)
  • These observations were regardless of the presence or absence of trifascicular block
  • Close follow-up of these patients and consideration of PPM insertion is essential even if asymptomatic

Differentiating from typical bifascicular block pattern relies mainly on the absence of prominent S waves in leads I and aVL.

ECG Bifascicular Block (RBBB + LAFB)
Typical bifascicular block pattern (not MBBB):
Note prominent S waves in leads I and aVL
Disease associations
  • Ischaemic heart disease, in particular severe triple vessel disease
  • Lenègre-Lev disease
  • Chaga’s myocarditis
Associated persons

Justin L Richman and Louis Wolff first described this phenomenon in their 1954 publication in the American Heart Journal. They observed four cases of patients with ECGs with signs of LBBB in limb leads, and signs of RBBB in precordial leads. Subsequent vectorcardiograms revealed the nature of conduction deficit to be LBBB

Further examples
Example 1
Lenegre 1964 Atypical Bundle Branch Block
Atypical Bundle Branch Block: ECG of a 71-year-old man with a ventricular aneurysm and cardiac failure, demonstrating LBBB morphology in the limb leads and RBBB in precordial leads. Lenègre 1964
Lenegre Atypical Bundle Branch Block 2
Progression of disease: the same patient in the terminal stage manifesting complete atrioventricular dissociation. Lenègre 1964

Related Topics

Further reading




MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

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