Bundle Branch Block

Aka: Bundle Branch Block, Left Bundle Branch Block, Right Bundle Branch Block, Left Anterior Hemiblock, Left Anterior Fascicular Block, LAFB, Left Posterior Hemiblock, Left Posterior Fascicular Block, LPFB, Monofascicular Block, Fascicular Block, LBBB, RBBB

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II. Background

  1. Normal bundle branch transmission
    1. Following AV Node and His Bundle, signal divides into the left and right bundles
    2. Results normally in a simultaneous depolarization of each ventricle
  2. Bundle Branch Block
    1. Electrical impulse blocked in the left bundle branch or right bundle branch
    2. Results in a depolarization delay of the affected ventricle
    3. Results in overall widening of the QRS Complex (0.12 or greater meets criteria for BBB)
      1. Left and right bundles signals are separated in time (no longer simultaneous or overlapped)
      2. Shape of QRS is also modified to have two R Waves, with the delayed R Wave referred to as R'
    4. QRS Axis and ventricular hypertrophy are not accurately determined in Bundle Branch Block
      1. Normally each ventricle's depolarization signal is simultaneous and opposes the other
      2. In Bundle Branch Block, these signals are offset resulting in large deflections (positive or negative)
  3. Right Bundle Branch Block
    1. Left ventricle (R) depolarizes before the right ventricle (R')
    2. Best seen in the right sided precordial leads (V1, V2) with characteristic 'M' appearance
  4. Left Bundle Branch Block
    1. Right ventricle (R) depolarizes before the left ventricle (R')
    2. Best seen in the left sided precordial leads (V5, V6) with a concave upward plateau to the top of the QRS Complex
    3. Q Waves absent
      1. Delay in left ventricular depolarization with right ventricle firing first
      2. Q Waves are not seen as the negative depolarization falls in the middle of wide QRS Complex
  5. Incomplete Bundle Branch Block
    1. Pattern of R and R' seen in a patient with a QRS Complex duration less than 0.12 seconds
  6. Critical Rate
    1. Rate at which Bundle Branch Block is seen (may not be evident at slower rates)
  7. Intrinsicoid Deflection (R-Wave Peak Time)
    1. Time from QRS wave onset to peak R Wave (early ventricular depolarization)
  8. Aberrant Conduction
    1. May mimic Bundle Branch Block
    2. Results from a discrepancy between the refractory periods between each ventricle
      1. Refractory period is time in ventricle following depolarization where it will not respond to a new depolarization signal
      2. Refractory periods may be slightly different between the ventricles
      3. At rapid rates, one ventricles depolarization may be delayed (offset) from the other giving the appearance of Bundle Branch Block

III. Findings: Left Bundle Branch Block

  1. EKG findings
    1. Lead V1
      1. QS or rS
    2. Lead V6
      1. Late intrinsicoid deflection
      2. No Q Wave
      3. Monophasic R Wave
    3. Lead I
      1. Monophasic R Wave
      2. No Q Wave
  2. Causes
    1. Chronic Ischemic Heart Disease
    2. Chronic Hypertension (with Left Ventricular Hypertrophy)
    3. Chronic Congestive Heart Failure (abnormal ventricular remodeling)
    4. Valvular heart disease
    5. Old age with a fibrotic conduction system
    6. Massive acute Myocardial Infarction
      1. See New Left Bundle Branch Block
      2. See Sgarbossa Criteria
  3. References
    1. Mattu and Herbert in Majoewksy (2012) EM:Rap 12(11): 4

IV. Findings: Right Bundle Branch Block EKG (RBBB)

  1. EKG Findings
    1. Lead V1
      1. Late Intrinsicoid Deflection (long duration from QRS start to R-wave peak time)
      2. M-shaped QRS Complex ("Rabbit Ears")
      3. Wide R Wave or qR (occasionally)
      4. Tall R Wave in Lead V1
    2. Lead V6
      1. Early Intrinsicoid Deflection (short duration from QRS start to R-wave peak time)
      2. Wide S wave
    3. Lead I
      1. Wide S wave
  2. Precautions: Repolarization Abnormalities (ST Depression and T Wave Inversion)
    1. Unlike Left Bundle Branch Block, RBBB does not significantly interfere with Myocardial Ischemia or infarction detection
    2. Normal findings in RBBB (non-ischemic)
      1. Mild ST segment Depression or T Wave Inversion in right precordial leads (V1 with or without V2 and V3) AND
      2. Affected leads also have an rsR' pattern (initial R Wave may be subtle)
    3. Findings concerning for ischemia or infarction (or non-reassuring)
      1. ST depression or T Wave Inversion in other leads (aside from V1-V3 with rsR')
      2. ST Segment Elevation
      3. Upright T Waves in right precordial leads affected by RBBB (V1-V3 with rsR')
    4. References
      1. Berberian, Brady and Mattu (2023) Crit Dec Emerg Med 37(3): 14-5
  3. Mechanism of RBBB
    1. Most of right bundle branch is subendocardial and susceptible to stretch and other Trauma
  4. Causes: Important
    1. Increased right ventricular pressure
      1. Pulmonary Embolism
      2. Cor Pulmonale (May be accompanied by Right Ventricular Hypertrophy if long standing)
    2. Acute myocardial injury
      1. Myocardial Ischemia or infarction
      2. Inflammation (e.g. Myocarditis)
      3. Chest Trauma
    3. Electrolyte disturbance
      1. Hyperkalemia
  5. Causes: Miscellaneous
    1. Hypertension
    2. Cardiomyopathy
    3. Congenital Heart Disease
    4. Right heart catheterization related injury
    5. Right heart fibrosis (Lev's Disease, Lenegre's Disease)

V. Findings: Left Hemiblocks (left Fascicular Block)

  1. Left Anterior Hemiblock EKG (Left Anterior Fascicular Block or LAFB)
    1. Left Axis Deviation (-45 to -90 degrees)
    2. Small Q Wave in Lead I and aVL may be present (qR pattern)
    3. Small R Wave in Lead II, III and avF (rS pattern)
    4. Normal QRS Duration <120 ms (unless concurrent Right Bundle Branch Block, bifascicular block)
    5. Prolonged R Wave peak time >= 45 ms in lead aVL
    6. No Right Ventricular Hypertrophy
  2. Left Posterior Hemiblock EKG (Left Posterior Fascicular Block or LPFB)
    1. Right Axis Deviation (beyond +120 degrees)
    2. Small R Wave in Lead I
    3. Small Q Wave in Lead III
    4. Normal QRS Duration (unless concurrent Right Bundle Branch Block, bifascicular block)
    5. No Right Ventricular Hypertrophy

VI. References

  1. Dubin (1974) Rapid Interpretation of EKGs, COVER publishing, Tampa, p. 137-47

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