REBEL Core Cast 99.0 – Bundle Branch Blocks

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Definition: Interruption of the normal conduction system leading to aberrant conduction and an abnormal QRS morphology

Anatomy: At the AV node, conduction splits into the right and left bundle branches.  The left bundle branch is composed of anterior and posterior branches.

Right Bundle Branch Block (RBBB)

Physiology

• Normally, the right bundle depolarizes the right ventricle (RV)
• In a RBBB, the right bundle does not activate. The right ventricle is instead depolarized by spread of impulse from the left bundle, through the left ventricle (LV) and then to the RV
• This pattern of electrical spread creates an aberrant QRS morphology
• Depolarization vector produces delayed R waves in leads V1-3 and delayed S waves in the lateral leads.

RBBB Criteria

• Rhythm is supraventricular in origin
• QRS duration > 110 msec (some criteria state > 120 msec)
• Terminal R wave in lead V₁ giving an RSR’ morphology (i.e. “Rabbit Ears”)
• Wide terminal S wave in leads I, aVL, V5 and V₆
• Axis is normal.

Note: A partial RBBB, defined as a RBBB morphology with a QRS < 100 msec, can be pathologic (i.e. in right heart strain) or a normal variant (seen in up to 10% of the general population)

Left Bundle Branch Block (LBBB)

Physiology

• The left bundle is composed of two fascicles (the left anterior and the left posterior fascicle)
• Normally, the left bundle depolarizes the left ventricle
• In a LBBB, the left bundle does not activate. The left ventricle is, instead, depolarized by spread of impulse from the right bundle through the RV and then to the LV.
• This pattern of electrical spread creates an aberrant QRS morphology
• The left bundle has two fascicles (anterior and posterior) and either (or both) can have a conduction abnormality

LBBB Criteria

• Rhythm is supraventricular in origin
• QRS duration > 120 msec
• QS or rS morphology in lead V₁ + V₂
• Broad, dominant monomorphic R wave in lead I, aVL, V5 and V₆

Fascicle Blocks

Left Anterior Fascicular Block (LAFB aka left anterior hemi-block)

• Blocking the left anterior fascicle results in LV depolarization via the left posterior fascicle which inserts into the infero-septal wall of the LV
• Produces small R waves in II, III, aVF
• Produces tall R waves in left-sided leads and deep S waves in the inferior leads
• LAFB EKG Criteria
  • Left axis deviation (QRS up in I, down in II + aVF)
  • Slight prolongation of the QRS complex (but < 120 msec)
  • Small q waves and  large R waves (qR complexes) in leads I and aVL
  • Small r waves and large S waves (rS complexes) in leads II, III + aVF
• Causes of LAD
  • LAFB
  • LBBB
  • LV Hypertrophy
  • Inferior MI
  • Ventricular ectopy
  • Paced rhythm
  • WPW

Left Posterior Fascicular Block (LPFB aka left posterior hemi-block)

• Blocking the left posterior fascicle results in LV depolarization via the left anterior fascicle which inserts into the upper, lateral wall of the LV
• LPFB is much less common than LAFB and the LAFB typically occurs with a RBBB (bifascicular block)
• LPFB EKG Criteria
  • Right axis deviation (RAD) (QRS up in III + aVF, down in I)
  • Slight prolongation of the QRS complex (but < 120 msec)
  • qR complexes in leads II, III + aVF
  • rS complexes in leads I + aVL
  • No evidence of RV hypertrophy
  • No evidence of other cause of right axis deviation
• Causes of RAD
  • LPFB
  • Old lateral myocardial infarction
  • Acute pulmonary hypertension (e.g. pulmonary embolism)
  • Chronic pulmonary hypertension
  • Sodium channel blocking drugs
  • Hyperkalemia
  • Right ventricular hypertrophy
  • Misplaced leads

Post Created By: Anand Swaminathan MD, MPH

Post Peer Reviewed By: Salim R. Rezaie, MD (Twitter: @srrezaie)