Left Bundle Branch Block

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

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II. Definitions

  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. See Right Bundle Branch Block
    2. Left ventricle (R) depolarizes before the right ventricle (R')
    3. 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
        1. Will not respond to a new depolarization signal
        2. Refractory periods may be slightly different between the ventricles
      2. At rapid rates, one ventricles depolarization may be delayed (offset) from the other
        1. Gives the appearance of Bundle Branch Block

III. Findings: Left Bundle Branch Block

  1. See Sgarbossa Criteria
  2. EKG findings: Diagnostic Criteria
    1. Lead V1
      1. QS or rS
    2. Lead V6
      1. Late intrinsicoid deflection (R-wave peak time >=50 ms from start of R Wave)
      2. No Q Wave
      3. Monophasic R Wave
    3. Lead I
      1. Monophasic R Wave
      2. No Q Wave
  3. EKG: ST and T Waves deviations are NORMALLY, appropriately discordant with the QRS Complex
    1. See Sgarbossa Criteria
    2. ST depression and T Wave Inversion in I, aVL, V5, V6 where QRS is positive
    3. ST Elevation and T Waves upright in V1-V4, where QRS is negative
    4. ST deviation is proportional to the size of the associated QRS Complex (<25%)
  4. Causes of LBBB
    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
  5. Precautions
    1. LBBB significantly obscures EKG interpretation in Acute Coronary Syndrome presentations
    2. LBBB associated ST and T Wave configurations may mimic Acute Coronary Syndrome
    3. LBBB is a marker of significant Cardiovascular Risk
      1. Suggests prior MI, or if new, a massive acute MI
      2. Higher risk of malignant Dysrhythmia, Cardiogenic Shock and sudden death
  6. References
    1. Mattu and Herbert in Majoewksy (2012) EM:Rap 12(11): 4
    2. Brady and Vandersteenhoven (2026) Crit Dec Emerg Med 40(4): 11-4

IV. 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

V. References

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

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