Right and left bundle branch block and fascicular blocks

What it is

Below the His bundle, the conduction system divides into the right bundle branch and the left bundle branch, and the left divides into an anterior and a posterior fascicle. When a branch fails, the ventricle it serves is depolarised slowly, muscle cell to muscle cell, from the other side, so the QRS widens to 120 ms or more. In right bundle branch block (RBBB) the septum and left ventricle activate normally and the right ventricle late, which adds a terminal rightward force: an rSR' («M») pattern in V1–V2 and a broad, slurred S wave in I, aVL, V5 and V6. RBBB can be a normal finding, but a new one may point to pulmonary embolism, right ventricular strain, an atrial septal defect, ischaemia or, in Latin America, Chagas cardiomyopathy. In left bundle branch block (LBBB) the septum is activated from right to left, so the normal small septal q waves in the lateral leads disappear; V1 shows a deep QS or rS and the lateral leads a broad, notched R. LBBB almost always reflects heart disease (hypertension, coronary disease, cardiomyopathy, aortic stenosis or valve procedures) and hides ischaemia and hypertrophy. A fascicular block shifts the axis without widening the QRS much: left anterior fascicular block gives left axis deviation and left posterior fascicular block right axis deviation. RBBB with a fascicular block is bifascicular block; with a long PR as well, it raises the risk of complete heart block.

How to recognise it

• RBBB: QRS 120 ms or more; rsr', rsR' or rSR' in V1 (sometimes only a broad notched R); wide S (40 ms or more) in I and V6.
• RBBB does not change the axis by itself: a marked axis shift means an added fascicular block.
• LBBB: QRS 120 ms or more; broad notched or slurred R in I, aVL, V5–V6 without q waves; QS or rS in V1–V3; R-peak time in V5–V6 above 60 ms.
• LBBB normally has ST and T opposite to the main QRS deflection (appropriate discordance) and poor R-wave progression.
• Left anterior fascicular block: axis −45° to −90°, qR in I and aVL, rS in II, III and aVF, QRS under 120 ms.
• Left posterior fascicular block: right axis deviation (about +90° to +180°), rS in I and aVL, qR in III and aVF, once right ventricular strain and lateral infarct are excluded.
• Bifascicular block: RBBB plus left anterior or posterior fascicular block; add a PR above 200 ms and the risk of complete block rises.

Why it happens

• The right bundle is long and thin, so it is easily blocked by stretch or pressure of the right ventricle (PE, strain, catheter passage).
• In RBBB the left ventricle depolarises first; the late right ventricle gives the terminal R' in V1 and the terminal S in the lateral leads.
• In LBBB the septum depolarises right to left, which removes septal q waves; the whole left ventricle activates late and slowly.
• Repolarisation follows the abnormal depolarisation, so secondary ST–T changes point away from the terminal QRS deflection.
• A fascicular block reroutes activation within the left ventricle: the axis swings away from the blocked fascicle.

Step by step

1. Measure QRS width in the widest lead — 120 ms or more means a block, pre-excitation, pacing or a ventricular rhythm.
2. Look at V1 — a terminal positive R' suggests RBBB; a deep negative QS or rS suggests LBBB.
3. Check I, aVL, V5 and V6 — a wide slurred S confirms RBBB; a broad notched R without q confirms LBBB.
4. Work out the axis — left deviation with qR in aVL is anterior fascicular block; added to RBBB it is bifascicular.
5. Check the PR and the ST segments — a long PR adds to the risk; concordant ST change in LBBB suggests ischaemia.
6. Compare with an old ECG — a new block with symptoms is acute until proved otherwise.

Bundle branch and fascicular blocks at a glance

Differential and pitfalls

• Other causes of a wide QRS: ventricular tachycardia, pre-excitation (WPW), ventricular pacing, hyperkalaemia and sodium-channel blocker toxicity.
• Brugada type 1 pattern mimics RBBB in V1–V2 (coved ST elevation) but lacks the wide S wave in I and V6.
• In LBBB, ST elevation discordant with the QRS is expected; concordant ST elevation of 1 mm or more, or ST depression of 1 mm or more in V1–V3, suggests infarction.
• LBBB makes the voltage criteria for left ventricular hypertrophy, Q waves and T inversion unreliable; the QT looks long because the QRS is wide.
• A broad complex tachycardia in a patient with known bundle branch block can be SVT with aberrancy, but treat it as VT if there is any doubt.
• Rate-dependent block appears only at faster rates; incomplete RBBB with right axis in a young adult suggests an atrial septal defect.

Next step

• New LBBB or RBBB with chest pain or instability: call the senior and cardiology; apply the Sgarbossa criteria and treat as possible infarction if they are positive or the patient is unstable.
• New RBBB with breathlessness or hypoxia: consider pulmonary embolism and assess for it.
• LBBB at any time: request an echocardiogram to look for structural heart disease and reduced ejection fraction.
• Bifascicular block with syncope: admit for monitoring and cardiology review; a pacemaker may be needed.
• Isolated RBBB or fascicular block in a well patient: no treatment; record it so the next ECG can be compared.

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Elsewhere in DurgiPedia

• ECG basics: waves, intervals and leads
• The cardiac electrical axis
• The cardiac conduction system
• The ECG in ischemia and infarction

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Right Bundle Branch Block (StatPearls (NCBI Bookshelf), 2023); Left Bundle Branch Block (StatPearls (NCBI Bookshelf), 2024).

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