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

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

RBBB (LITFL)

Note: A incomplete 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)

Incomplete RBBB (LITFL)

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

LBBB (LITFL)

Fascicular 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

LAFB (LITFL)

LAFB Annotated (REBEL EM)

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

LPFB (LITFL)

LPFB Annotated (REBEL EM)

  • 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

Axis Deviation

II, III + aVF

I + aVL

LAFB

Left Axis Deviation

rS Complexes

qR Compexes

LPFB

Right Axis Deviation

qR Complexes

rS Complexes

Read More

REBEL EM: Bundle Branch Blocks: 101

LITFL: Left Anterior Fascicular Block

LITFL: Left Posterior Fascicular Block

LITFL: Right Axis Deviation