Fascicular Blocks

Fascicular Block Algorithm
LAFB pattern
  • LAD
  • qR in I / aVL
  • rS in II / III / aVF
  • QRS normal or slightly wide
LPFB pattern
  • RAD
  • rS in I / aVL
  • qR in II / III / aVF
  • QRS usually normal
Is RBBB also present? QRS ≥120 ms, rsR′ in V1, broad S in I / V6
Bifascicular block RBBB + LAFB = most common
RBBB + LPFB = less common
Isolated fascicular block Isolated LAFB or isolated LPFB
PR >200 ms?
Trifascicular pattern Bifascicular block + 1° AV block
Bifascicular block only
Pocket Guide
Type EKG Features Notes
LAFB LAD, qR in aVL, rS in II/III/aVF Common, often benign
LPFB RAD, rS in I/aVL, qR in II/III/aVF Rare, usually structural disease
Bifascicular (RBBB + LAFB) RBBB + LAD pattern Most common bifascicular combo
Bifascicular (RBBB + LPFB) RBBB + RAD pattern Less common, consider ischemia
Trifascicular Bifascicular + 1° AV block (PR >200 ms) May progress to complete block

Left Anterior Fascicular Block (LAFB)

  • Most common fascicular block
  • EKG criteria:
    • Left axis deviation (–45° to –90°)
    • Small Q in leads I and aVL
    • Small R in II, III, aVF
    • Normal or slightly widened QRS (≤120ms)
  • Often benign but may indicate underlying structural disease

Left Posterior Fascicular Block (LPFB)

  • Rare (posterior fascicle is broader, dual supply)
  • EKG criteria:
    • Right axis deviation (>+90°) without other cause (RVH, PE)
    • Small R in I and aVL, small Q in II, III, aVF
    • Normal QRS duration
  • Suspect structural heart disease

Bifascicular Block

  • Involves any two of the three fascicles:
    • RBBB + LAFB (most common)
    • RBBB + LPFB

EKG

  • RBBB: QRS ≥120 ms, rsR′ in V1, broad S in I/V6
  • LAFB or LPFB criteria as above (in EKG below there is RBBB, LAD, normal QRS, rS complexes in inferior leads = RBBB + LAFB)

Clinical Significance

  • Marker of conduction system disease
  • Increases risk of progression to complete heart block, especially if:
    • Alternating bundle branch block is present
    • There's associated syncope
  • May need pacemaker if symptomatic

Trifascicular Block

  • Imprecise term; often refers to:
    • RBBB + LAFB/LPFB + prolonged PR interval
  • Not necessarily all 3 fascicles blocked simultaneously
  • Consider EP study or pacing if symptomatic

References

  1. Surawicz, B., Childers, R., Deal, B. J., Gettes, L. S., Bailey, J. J., Gorgels, A., Hancock, E. W., Josephson, M., Kligfield, P., Kors, J. A., Macfarlane, P., Mason, J. W., Mirvis, D. M., Okin, P., Pahlm, O., Rautaharju, P. M., & van Herpen, G. (2009). AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: Part III: Intraventricular conduction disturbances. Journal of the American College of Cardiology, 53(11), 976–981. https://doi.org/10.1016/j.jacc.2008.12.013
  2. Kusumoto, F. M., Schoenfeld, M. H., Barrett, C., Edgerton, J. R., Ellenbogen, K. A., Gold, M. R., Goldschlager, N. F., Hamilton, R. M., Joglar, J. A., Kim, R. J., Lee, R., Marine, J. E., McLeod, C. J., Oken, K. R., Patton, K. K., Pellegrini, C. N., Selzman, K. A., Thompson, A., & Varosy, P. D. (2019). 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Journal of the American College of Cardiology, 74(7), e51–e156. https://doi.org/10.1016/j.jacc.2018.10.044
  3. Pérez-Riera, A. R., Barbosa-Barros, R., Daminello-Raimundo, R., de Abreu, L. C., Baranchuk, A., & Nikus, K. (2018). Left posterior fascicular block, state-of-the-art review: A 2018 update. Indian Pacing and Electrophysiology Journal, 18(6), 217–230. https://doi.org/10.1016/j.ipej.2018.10.001
  4. Milliken, J. A., Macfarlane, P. W., & Lawrie, T. D. V. (1983). Isolated and complicated left anterior fascicular block: A review of suggested electrocardiographic criteria. Journal of Electrocardiology, 16(2), 121–128. https://doi.org/10.1016/S0022-0736(83)80018-1
  5. Elizari, M. V., Acunzo, R. S., & Ferreiro, M. (2007). Hemiblocks revisited. Circulation, 115(9), 1154–1163. https://doi.org/10.1161/CIRCULATIONAHA.106.637389