Bundle Branch & Fascicular Blocks
Key criteria and clinical implications for RBBB, LBBB, and hemiblocks.
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Questions Covered in This Set
10 cards to master
What QRS duration defines a complete bundle branch block?
≥ 0.12 s (3 small boxes). A QRS of 0.10–0.11 s is an incomplete block.
What are the three steps before calling a bundle branch block?
1) Confirm the rhythm is supraventricular (atrial activity present); 2) Check QRS ≥ 0.12 s; 3) Look at V1 and V6 to see which branch is blocked.
Describe the V1 and V6 findings in RBBB.
V1: rSR′ / 'rabbit ears' / M pattern with terminal deflection UP. V6 and lead I: wide, slurred S wave.
Describe the V1 and V6 findings in LBBB.
V1: deep, wide QS or rS (terminal deflection DOWN). V6 and lead I: broad, often notched, monophasic positive R wave with no normal q wave.
What do MaRRoW and WiLLiaM stand for?
MaRRoW = M in V1, W in V6 → Right BBB. WiLLiaM = W in V1, M in V6 → Left BBB.
Clinical significance of RBBB vs LBBB?
RBBB is often benign (healthy older adults) or from right-heart strain (PE, COPD, ASD). LBBB usually signals structural disease; new LBBB with ischemic chest pain is a STEMI equivalent.
What is appropriate discordance?
The ST segment and T wave point opposite the terminal QRS deflection — expected in BBB (e.g., ST elevation V1–V3, ST depression/T inversion V5–V6 in LBBB). Do not call it ischemia.
What are the Sgarbossa criteria used for?
Detecting real infarction in LBBB: concordant ST elevation ≥1 mm, concordant ST depression ≥1 mm in V1–V3, or excessively discordant ST elevation (≥5 mm, or ≥25% of S-wave depth in the modified/Smith rule).
What cannot be diagnosed in the presence of LBBB?
Left ventricular hypertrophy and old anterior MI; axis determination also becomes unreliable.
Distinguish LAFB from LPFB.
LAFB: left axis deviation (−45° to −90°), qR in I/aVL, rS in II/III/aVF, QRS < 0.12 s — common, often benign. LPFB: right axis deviation (+90° to +180°), rS in I/aVL, qR in III/aVF, with no other cause of right axis — rare.
