Masquerading bundle branch block is a rare condition that challenges electrocardiographic interpretation and is associated with abnormal intraventricular conduction of significant clinical impact. The term was first described in 1954 by Richman and Wolff to denote a right bundle branch block (RBBB) pattern in the precordial leads and a left bundle branch block (LBBB) pattern in the frontal plane, reflecting extensive damage to the conduction system with associated cardiac fibrosis.

Electrocardiographic Criteria

The diagnosis of masquerading bundle branch block includes:

  • Prominent R wave in V1 (suggesting RBBB).
  • Left axis deviation in the frontal plane.
  • Absent or < 1 mm S waves in I and aVL, mimicking the LBBB pattern.

Rosenbaum classified masquerading bundle branch block into two types:

  1. Standard type: absence of S waves in I and aVL.
  2. Precordial type: absence of S waves in V5 and V6.
Electrocardiogram with masquerading bundle branch block
ECG | Masquerading Bundle Branch Block

Mechanism of the Block

This block occurs when RBBB is accompanied by advanced block of the left anterior fascicle and structural changes in the left ventricle, such as significant hypertrophy and/or extensive fibrosis. The combination of these factors directs the terminal forces to the left, canceling the typical RBBB forces and eliminating the S waves in leads I and aVL.

Clinical Considerations and Prognosis

The prognosis of patients with masquerading bundle branch block is generally guarded because of the association of this condition with severe structural heart disease and an increased risk of adverse events. This electrocardiographic pattern, which mimics left bundle branch block (LBBB) in some leads, is frequently linked to advanced, extensive damage to the conduction system and is also an indicator of significant fibrosis and left ventricular hypertrophy. These factors contribute to increased mortality and indicate a worse prognosis.

Studies have shown that patients with masquerading bundle branch block have a higher incidence of progression to advanced atrioventricular (AV) block or complete heart block, often requiring a permanent pacemaker. In addition, the condition is usually a marker of poor prognosis because of the high risk of sudden death and of additional complications associated with significant ventricular dysfunction.

References

  1. Scheffer MK, De Marchi MFN, de Alencar Neto JN, Felicioni SP. Eletrocardiograma de A a Z. São Paulo: Manole, 2024.
  2. Scheffer MK, De Marchi MFN, Costa ACM. Right Bundle-Branch Block Pattern in Precordial Leads and Left Bundle-Branch Block Pattern With Left Axis Deviation in Frontal Plane Leads—What Does This Mean? JAMA Intern Med. 2022. 1;182(12):1315.