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