Bundle Branch Blocks (RBBB vs LBBB)

Conduction system anatomy

The His bundle bifurcates just below the membranous septum into the right and left bundle branches.

• Right bundle (RBB): narrow, single-strand structure running down the right side of the interventricular septum. Vulnerable — even small lesions cause RBBB.
• Left bundle (LBB): broad fan-shaped structure that quickly splits into:
  • Left anterior fascicle (LAF): runs anterosuperiorly toward the anterolateral papillary muscle
  • Left posterior fascicle (LPF): thicker, posteroinferior, more robust blood supply
  • Some describe a third septal fascicle

Damage to any of these produces characteristic delayed activation patterns.

Right bundle branch block (RBBB)

QRS criteria

• QRS duration ≥120 ms (complete) or 100–119 ms (incomplete)
• Terminal R wave in V1 (rsR’ or M-pattern)
• Slurred wide S in lead I, V5, V6
• Normal initial septal activation preserved (small Q in I, V5, V6)

Mechanism

• Initial septal and LV activation proceeds normally
• RV activation delayed via cell-to-cell spread from the LV across the septum
• The terminal R’ in V1 represents this delayed, rightward, anterior RV depolarization

Clinical implications

• Common with aging, structural heart disease, RV pressure overload (PE, COPD, pulmonary hypertension)
• New RBBB with chest pain — concerning for proximal LAD occlusion (septal supply)
• Does NOT obscure ischemia diagnosis significantly — STEMI criteria still apply

Left bundle branch block (LBBB)

QRS criteria

• QRS duration ≥120 ms (complete) or 110–119 ms (incomplete)
• Broad notched or monophasic R in I, aVL, V5, V6
• Absent septal Q waves in I, V5, V6 — septum activates right-to-left
• Deep wide S or QS in V1, V2
• Discordant ST-T changes — ST depression and T inversion opposite to main QRS vector

Strauss criteria for “true” LBBB

• QRS ≥140 ms (men) or ≥130 ms (women)
• Mid-QRS notching or slurring in ≥2 of leads I, aVL, V1, V2, V5, V6
• Predicts CRT response more reliably than older LBBB definitions

Clinical implications

• Almost always pathologic in adults — implies structural disease until proven otherwise
• Associated with cardiomyopathy, hypertension, aortic stenosis, ischemic heart disease
• New LBBB with chest pain is a STEMI equivalent
• Obscures ischemia on the surface ECG — use Sgarbossa criteria:
  • Concordant ST elevation ≥1 mm
  • Concordant ST depression ≥1 mm in V1-V3
  • Discordant ST elevation ≥5 mm (modified: ratio of ST to S wave ≥0.25)

Fascicular blocks

Left anterior fascicular block (LAFB)

• Left axis deviation (-45° to -90°)
• qR in I and aVL
• rS in II, III, aVF
• QRS duration usually normal or only slightly prolonged
• Common and often benign in isolation

Left posterior fascicular block (LPFB)

• Right axis deviation (+90° to +180°)
• rS in I and aVL
• qR in II, III, aVF
• Rare in isolation — must exclude RV hypertrophy, lateral MI, pulmonary disease, normal variant
• When real, suggests significant conduction system disease

Bifascicular and trifascicular block

• Bifascicular: RBBB + LAFB (most common combination) or RBBB + LPFB
• Trifascicular (descriptive label): bifascicular + first-degree AV block — implies disease in the third fascicle but does NOT predict imminent complete heart block
• Progression to complete heart block in asymptomatic bifascicular block is ~1% per year
• Symptomatic patients (syncope) with bifascicular block deserve EP study to measure HV interval; HV >100 ms or pacing-induced infranodal block prompts pacing

Ischemia in the setting of BBB

• RBBB: largely preserved interpretability
• LBBB: relies on Sgarbossa criteria; new LBBB with symptoms = STEMI equivalent
• Ventricular paced rhythm: same approach as LBBB — Sgarbossa criteria apply

CRT and bundle branch blocks

LBBB is the dominant predictor of CRT response.

• LBBB + EF ≤35% + NYHA II-IV + QRS ≥150 ms → strongest CRT indication
• LBBB + QRS 120–149 ms → moderate evidence
• Non-LBBB (RBBB, IVCD) → minimal benefit unless QRS ≥150 ms and other features support
• True LBBB by Strauss criteria predicts response more reliably than non-specific IVCD
• Conduction system pacing (HBP, LBBAP) increasingly used as an alternative to biventricular pacing in CRT candidates, particularly for LBBB

Practical notes

• Document baseline BBB pattern in the chart; rate-related BBB can appear during stress testing or rapid AF
• New BBB in any acute setting deserves explanation — ischemia, PE, electrolyte derangement
• Pre-procedural BBB matters for ablation cases — ablating near the contralateral bundle puts the patient at risk of complete heart block (temporary pacing wire ready)