MK

7. ECG Bundle Branch Block Notes

Ventricular Conduction System

  • The ventricular conduction system components:
    • Sinoatrial node (SA node)
    • Atrioventricular node (AV node)
    • His bundle
    • Left bundle branch
    • Anterior fascicle
    • Posterior fascicle
    • Right bundle branch
    • Purkinje fibers
  • Note: The SA node and AV node are located in the atria and are not part of the ventricular conduction system.
  • The AV node continues into the bundle of His, which branches into the right and left bundle branches. The left bundle branch divides into anterior and posterior fascicles. The right bundle branch and the fascicles further branch into the Purkinje network spanning the ventricles. The ventricular conduction system is the His–Purkinje system.

Bundle Branch Block (BBB) – Definition and General Features

  • BBB is indicated by a wide QRS complex (> 0.10 seconds).
  • Block can occur in either the left or right bundle branch.
  • BBB causes prolongation in the depolarisation of the ventricle.
  • Left vs Right BBB implies different conduction pathways; this matters for interpreting the ECG and the resultant depolarisation pattern.

Steps to Identify BBB

  1. Detect a wide QRS complex (QRS > 0.10\text{ s}).
    • Ensure ALL QRS complexes that follow a P-wave are wide; do not mistake for PVCs, VT, etc.
    • If a bundle branch is blocked, it’s blocked, and the blockage will be beat-to-beat consistent.
  2. Use Leads V1, V2 and V5, V6 to determine whether the blockage is in the left or right bundle branch.

RBBB vs LBBB – Criteria

  • Right Bundle Branch Block (RBBB) criteria:
    • QRS duration > 0.10\text{ s}
    • RSR′ pattern in Lead V1
    • Wide S wave in V6 (and V5 when present)
  • Left Bundle Branch Block (LBBB) criteria:
    • QRS duration > 0.10\text{ s}
    • Large, wide positive R wave in V6 (often notched)
    • QS in V1
    • Abnormal ST segment and T-wave morphology

Right Bundle Branch Block (RBBB) – Visuals and Features

  • Classic ECG hallmarks:
    • In V1, R′ is present (the second R wave in the QRS complex).
    • In V6 (and V5 when present), S waves are wide.
  • The figure description notes:
    • Paper speed is 50 mm/s.
    • One large box equals 100 ms.
  • The hallmark of both RBBB and LBBB is the QRS duration of at least 120\text{ ms} (or longer).
  • Notes on interpretation from the visual example: RBBB vs LBBB differentiation can be demonstrated via the pattern in V1, V6 and related leads.

Right Bundle Branch Block – Electrophysiology (RBBB)

  • Normally, both ventricles depolarize nearly simultaneously, with the left ventricle contributing most of the ECG vector due to its larger mass.
  • In RBBB, the right ventricle is depolarized after the left ventricle, so its vector appears later in the QRS, manifesting as an R′ wave that is directed anteriorly and rightward.
  • Clinical significance:
    • In asymptomatic individuals, RBBB is not typically associated with adverse outcomes.
    • New RBBB in patients with chest pain may indicate occlusion of the left anterior descending artery (LAD).
    • New RBBB in patients with acute dyspnea may indicate pulmonary embolism.
    • In most cases, RBBB is a benign finding with little impact on prognosis.

Left Bundle Branch Block – Key Features and Pathophysiology

  • How LBBB occurs:
    • The impulse first travels down the right bundle branch.
    • Then the impulse activates the interventricular septum from right to left (opposite of normal activation).
    • Finally, the impulse activates the left ventricle.
  • Electrophysiology of LBBB:
    • Ventricular depolarization normally starts in the interventricular septum, which gets Purkinje fibers from the left bundle branch.
    • In LBBB, septal depolarization occurs via impulses spreading from the right ventricle, diminishing the small septal r-waves in V1–V2 and the small q-waves in V5–V6 (the septal q-waves may be diminished or disappear).
    • Depolarization then continues slowly toward the left ventricular free wall, producing a vector directed leftward.
    • ECG manifestations include:
    • Wide S-wave in V1–V2 (referred to as QS complex if the r-wave is absent)
    • Broad and clumsy R-wave in V5–V6
    • R-wave may be notched at the apex
  • Clinical implications of LBBB:
    • LBBB is always pathological and affects left ventricular contractility and pumping function.
    • LBBB is associated with adverse cardiovascular outcomes and commonly accompanies:
    • Hypertension
    • Ventricular hypertrophy (LVH)
    • Valvular heart disease
    • Myocarditis
    • Ischemic heart disease
    • Heart failure
    • Cardiomyopathies
    • Framingham Heart Study findings (acquired LBBB):
    • ~7-fold increased risk of heart failure
    • ~2-fold increased risk of coronary artery disease
    • Higher risk of developing right ventricular hypertrophy
    • LBBB is rare in young individuals and tends to have less impact on prognosis in this group.

Clinical Flags and Practical Considerations

  • Normal vs abnormal progression:
    • LBBB is a clear abnormal conduction pattern with significant clinical implications.
  • Safety and exercise considerations (from red/amber flags in slides):
    • In LBBB cases or when red/amber flags appear, exercise testing and activity considerations may be guided by clinical flags (e.g., “Red Flag: Do not Exercise”; “Amber Flag: Get medical clearance; Exercise with caution”).

Summary of Key Concepts and Practical Implications

  • BBB is diagnosed by a wide QRS complex (> 0.10\text{ s}) indicating delayed ventricular depolarization.
  • Differentiation between RBBB and LBBB relies on QRS morphology in specific leads:
    • RBBB: RSR′ in V1 and wide S in V6
    • LBBB: Wide R in V6 (often notched), QS in V1, with broad, notched R in V5–V6 and abnormal ST-T.
  • RBBB physiology leads to delayed right ventricular activation with an R′ wave on the right side of the QRS; often benign but can signal pathology if new or associated with symptoms.
  • LBBB physiology reflects altered septal activation (right-to-left) and broad changes across the left ventricle, with strong associations to structural heart disease and worse prognosis in many settings.
  • In both BBBs, consideration of leads V1, V2, V5, V6 is essential for determination of block location and QRS morphology.
  • The diagnostic thresholds and descriptions in the slides include both a general cutoff (> 0.10\text{ s}) and an explicit note of 120 ms or longer as a hallmark in the provided figure, underscoring some variability in teaching materials.
  • Understanding the conduction system anatomy helps explain the ECG patterns observed in RBBB and LBBB and their clinical consequences.

Equations and Numerical References

  • QRS duration threshold for BBB:
    • QRS > 0.10\text{ s} (100 ms) [general criterion]
    • QRS \ge 120\text{ ms} (120 ms) [defined in figure notes]
  • Cardiac cycle timing reference in figure:
    • One large box on standard paper speed (50 mm/s) equals 100\text{ ms}.
  • Paper speed reference from the figure:
    • Paper speed = 50\text{ mm/s}

Notes on Interpretation and Real-World Relevance

  • Recognize that RBBB in asymptomatic individuals is commonly a benign variant but warrants clinical context.
  • New RBBB in the setting of chest pain or dyspnea should prompt evaluation for LAD occlusion or pulmonary embolism, respectively.
  • LBBB generally signals underlying disease and complicates interpretation of other ischemic changes on the ECG; it may mask or mimic ischemic patterns, affecting diagnostic assessment in acute settings.
  • Familiarity with the leads V1, V2, V5, V6 is essential for accurate BBB identification and distinguishing between RBBB and LBBB in clinical practice.