#68 – Bundle Branch Blocks

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Review of the Electrical Conduction System of the Heart

  • Cardiac cells specialized to initiate and distribute electrical impulses in an orderly and sequential manner
  • Sinoatrial Node
    • Located in superior region of the crista terminalis
      • SVC feeds into the right atrium
    • Pacemaker of the heart and initiates the heartbeat
      • Starts in the node, spreads down the walls of the atria, until it reaches the AV node, stimulating contraction of the myocardium
    • Arterial Supply – SA nodal artery via the RCA
  • Atrioventricular Node
    • Located in interatrial septum above the coronary sinus near the attachment of the septal cusp of the tricuspid valve (Triangle of Koch)
    • Passes SA node impulse to the AV bundle (Bundle of His)
    • Arterial Supply – AV nodal artery via the RCA (80-90%) or LCx (10-20%)
  • AV Bundle (Bundle of His)
    • Arises from the AV node and descends along the membranous portion of the interventricular septum, where it divides at the upper border of the muscular portion of the interventricular septum into the left and right bundles suppling their respective ventricles
    • Transmits AV nodal impulses through the interventricular septum to the left and right bundle branches, which gives rise to the Purkinje fibers that ultimately distribute the ventricular myocardium

Bundle Branch Blocks Pearls

  • R-wave = depolarization going TOWARDS the lead
  • S-wave = depolarization going AWAY from the lead
  • RBBB = delay in conduction is oriented to the RIGHT and ANTERIOR
    • QRS = Positive V1 and negative V6
  • LBBB = delay in conduction is oriented to the LEFT and either ANTERIOR or POSTERIOR
    • QRS = Negative V1 and positive V6
  • Wide QRS complex > 120 ms
    • Delay in conduction due to the block
  • Secondary Repolarization (ST-T) Abnormalities
    • T-wave discordance with last deflection of QRS

Causes

  • RBBB
    • More common in patients without structural heart disease
    • Congenital
      • ASD
    • Cardiac
      • Valvulopathies, ischemic heart disease
    • Pulmonary
      • Pulmonary HTN, PTE
  • LBBB
    • 4 main underlying conditions
      • Coronary disease
      • Hypertensive heart disease
      • Aortic valve disease
      • Cardiomyopathies

RBBB EKG Diagnostic Criteria

  • QRS > 120 ms
    • If all other criteria met but QRS < 120 ms, it is termed incomplete RBBB
  • rSR’ pattern in V1 and V2
  • Slurred S-wave in lateral leads (I, aVL, V5, V6)
  • ST depression and T-wave inversion in V1-V3

LBBB EKG Diagnostic Criteria

  • QRS > 120 ms
  • Dominant S-wave in V1-V3
    • Moving away from the leads
  • Broad, monophasic (M-shaped or notched) R-wave in lateral leads (I, aVL, V5, V6)
    • Moving towards the leads
  • Appropriate discordance
    • ST-segment and T-wave are in OPPOSITE direction to the main vector of the QRS complex
  • Left axis deviation
  • Poor R-wave progression

SPECIAL CONSIDERATIONS

Fascicular Blocks

  • Anterior
    • Pathology
      • When blocked, the conduction to the high lateral portion of the ventricle is delayed
        • Spreads to the intact posterior fascicle and RBB
          • Causes left axis deviation
    • Criteria for left anterior fascicular block
      • QRS normal to slightly prolonged
      • Left axis deviation WITHOUT other reasons
      • Small R-wave and large S-wave in inferior leads (II, III, aVF)
      • Small Q-wave with large R-wave in lateral leads (I, aVL)
  • Posterior
    • Pathology
      • When blocked, the conduction to the inferior portion of the ventricle is delayed
        • Spreads to the intact anterior fascicle and RBB
          • Causes right axis deviation
    • Criteria for left posterior fascicular block
      • QRS normal to slightly prolonged
      • Right axis deviation WITHOUT other reasons
      • Small R-wave and large S-wave in lateral leads (I, aVL)
      • Small Q-wave and large R-wave in inferior leads (II, III, aVF)

Sgarbossa’s Criteria

  • Used in the presence of LBBB or paced rhythm to uncover potential ischemia
  • Original (1996)
    • Concordant ST elevation > 1mm in leads with a positive QRS complex (5 points)
    • Concordant ST depression > 1mm in V1-V3 (3 points)
    • Excessively discordant ST elevation > 5mm in leads with a negative QRS complex (2 points)
    • Score ≥ 3 has a specificity of 90% for detecting concomitant ischemia
  • Smith-Modified Sgarbossa Criteria (2012)
    • ≥ 1 lead with ≥ 1mm of concordant ST elevation
    • ≥ 1 lead of V1-V3 with ≥ 1mm of concordant ST depression
    • ≥ 1 lead ANYWHERE with ≥ 1mm ST elevation AND proportionally excessive discordant ST elevation
      • Defined as ≥ 25% of the depth of the preceding S-wave

1893 Cottage Physician

References

  1. Heart. In: Morton DA, Foreman K, Albertine KH. eds. The Big Picture: Gross Anatomy, 2e. McGraw-Hill; Accessed January 17, 2021.
  2. Jaffar A. Anatomical Structure of the Heart. In: Elmoselhi A. eds. Cardiology: An Integrated Approach. McGraw-Hill; Accessed January 17, 2021.
  3. Goldberger AL. Electrocardiography. In: Jameson J, Fauci AS, Kasper DL, Hauser SL, Longo DL, Loscalzo J. eds. Harrison’s Principles of Internal Medicine, 20e. McGraw-Hill; Accessed January 17, 2021.
  4. LITFL. Right bundle branch blocks. https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/
  5. LITFL. Left bundle branch blocks. https://litfl.com/left-bundle-branch-block-lbbb-ecg-library/
  6. REBELEM. Bundle Branch Blocks. https://rebelem.com/bundle-branch-blocks101/
  7. Sgarbossa EB, Pinski SL, Barbagelata A, et al. Electrocardiographic diagnosis of evolving acute myocardial infarction in the presence of left bundle-branch block. GUSTO-1 (Global Utilization of Streptokinase and Tissue Plasminogen Activator for Occluded Coronary Arteries) Investigators. N Engl J Med. 1996; 334(8):481-7. [pubmed]
  8. Smith SW, Dodd KW, Henry TD, Dvorak DM, Pearce LA. Diagnosis of ST-elevation myocardial infarction in the presence of left bundle branch block with the ST-elevation to S-wave ratio in a modified Sgarbossa rule. Ann Emerg Med. 2012; 60(6):766-76. [pubmed]
  9. Meyers HP, Limkakeng AT Jr, Jaffa EJ, et al. Validation of the modified Sgarbossa criteria for acute coronary occlusion in the setting of left bundle branch block: A retrospective case-control study. Am Heart J. 2015; 170(6):1255-64. [pubmed]
  10. CORE EM. Validation of Modified Sgarbossa Criteria. https://coreem.net/journal-reviews/modified-sgarbossa-criteria/

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