Bundle branch and fascicular blocks
- Measurement
- Exam priority: medium
When one branch of the heart's wiring is cut, that side is reached slowly through muscle, so the QRS becomes wide and changes shape.
Also called: RBBB, LBBB, Left anterior fascicular block, Hemiblock, Secondary T-wave changes
See it in 3D: Bundle of His · Bundle branches · Interventricular septum · Left ventricle · Right ventricle
What it is
The His bundle splits into a right bundle branch and a left bundle branch with anterior and posterior fascicles. If a branch fails, the ventricle it supplies is activated late by slow muscle-to-muscle spread, widening the QRS to ≥ 0.12 s. A blocked fascicle changes only the axis. Block also alters repolarization, so the T wave turns opposite to the QRS.
Key facts
- Isolated RBBB is often found in healthy people; LBBB almost always means underlying heart disease.
- LBBB delays left ventricular contraction: paradoxical splitting of S2 and dyssynchrony that can worsen heart failure.
- RBBB widens the split of S2 because pulmonary valve closure comes late.
- Bifascicular block (RBBB + a left fascicle) leaves one fascicle; failure of it causes complete heart block.
- Primary T changes come from the cells themselves: ischemia shortens the action potential locally and can invert T; digitalis gives a scooped ST and a short QT.
How it works, step by step
- Normally both ventricles are activated within ≈ 0.08 s through fast Purkinje fibres (≈ 1.5–4 m/s).
- If the right branch fails, the septum and left ventricle fire normally; the right ventricle fires last, through muscle (≈ 0.3–0.5 m/s).
- That late rightward, forward vector adds a terminal R′ in V1 and a broad S in the left leads.
- If the left branch fails, the septum is activated from the right side: the septal vector reverses and the left-lead q disappears.
- The whole left ventricle then fires late and slowly, giving a broad, notched R in the left leads and a deep S in V1.
- Losing the anterior fascicle makes the inferior-posterior LV fire first and the anterosuperior wall last, so the axis swings up and left.
- Because activation now crawls cell to cell, repolarization follows the same order; the T vector points opposite to the late QRS forces.
- Patchy block in scar (old infarcts, fibrosis) breaks activation into irregular fronts: a wide, notched, 'bizarre' QRS that fits no classic pattern.
Electrocardiogram: a normal beat in lead II
HR 75 /minRR 0.80 sPR 0.16 sQRS 0.09 sQT 0.37 sQTc 0.41 s
Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.
- ECG trace (lead II)
- Small box: 0.04 s · 0.1 mV
- Big box: 0.20 s · 0.5 mV
- Selected part
RBBB, LBBB and fascicular block compared
| Feature | RBBB | LBBB | Left anterior fascicular block |
|---|---|---|---|
| QRS width | ≥ 0.12 s | ≥ 0.12 s | < 0.12 s |
| V1 | rSR′, upright terminal | QS or rS, deep and wide | Little change |
| I and V6 | Wide slurred S | Broad notched R, no q | qR in I and aVL |
| Frontal axis | Usually normal | Normal or left | −45° or more |
| Secondary T | Inverted in V1–V3 | Inverted in I, aVL, V5–V6 | Minor |
| Common causes | Normal variant, PE, Chagas, ASD | Hypertension, aortic stenosis, ischemia, dilated cardiomyopathy | Hypertension, ischemia, Chagas |
Clinical notes
- LBBB hides the usual ST signs of infarction; specific criteria and the clinical picture decide whether reperfusion is needed.
- Syncope in a patient with bifascicular block suggests intermittent complete block (Stokes–Adams).
- Acute right heart strain (massive pulmonary embolism) can produce a new RBBB.
Test yourself
Think of the answer, then tap the question to see it.
A QRS of 0.14 s shows rSR′ in V1 and a wide S in V6. Which branch is blocked and why does the R′ appear?
Right bundle branch. The right ventricle is activated last through slow muscle, and that late vector points toward V1, adding the terminal R′.
Why is the small septal q wave missing in V6 in left bundle branch block?
The septum is normally activated from the left; with the left branch blocked it is activated from the right, so the first vector points toward V6 instead of away.
A Colombian farm worker has palpitations, RBBB and an axis of −60°. What disease should be considered first?
Chronic Chagas cardiomyopathy: RBBB with left anterior fascicular block is its classic ECG pattern.
Why are inverted T waves in V1–V3 not by themselves a sign of ischemia in RBBB?
They are secondary changes: repolarization follows the abnormal activation order, so T is discordant to the terminal R′. Only changes beyond this expected pattern suggest a primary cause.
Connections
See it in 3D5
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Atrioventricular Block (StatPearls (NCBI Bookshelf), 2025); Chagas Disease (StatPearls (NCBI Bookshelf), 2025); Myocardial Infarction (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.