---
title: Bundle branch block
description: What right and left bundle branch block are, why they happen, how they are found on an ECG, when they are harmless, when they point to heart disease and when urgent help is needed.
type: disease
preset: right-bundle-branch-block
morePresets: [left-bundle-branch-block, left-anterior-fascicular-block]
aliases:
  - right bundle branch block
  - RBBB
  - left bundle branch block
  - LBBB
  - BBB
  - fascicular block
  - hemiblock
sources:
  - tier: guideline
    org: minzdrav-rf
    title: Брадиаритмии и нарушения проводимости
    year: 2025
    doi: 10.15829/1560-4071-2025-6669
    url: https://cr.minzdrav.gov.ru/view-cr/160_2
  - tier: guideline
    org: esc
    title: 2021 Рекомендации ESC по электрокардиостимуляции и сердечной ресинхронизирующей терапии
    year: 2022
    doi: 10.15829/1560-4071-2022-5159
    url: https://doi.org/10.15829/1560-4071-2022-5159
  - tier: guideline
    org: minzdrav-rf
    title: Острый инфаркт миокарда с подъемом сегмента ST электрокардиограммы
    year: 2024
    doi: 10.15829/1560-4071-2025-6306
    url: https://cr.minzdrav.gov.ru/view-cr/157_5
  - tier: guideline
    org: nice
    title: Transient loss of consciousness ('blackouts') in over 16s (CG109)
    year: 2023
    url: https://www.nice.org.uk/guidance/cg109
  - tier: reference
    org: msd-manuals
    title: Bundle Branch Block and Fascicular Block
    year: 2024
    url: https://www.msdmanuals.com/professional/cardiovascular-disorders/specific-cardiac-arrhythmias/bundle-branch-block-and-fascicular-block
  - tier: reference
    org: msd-manuals
    title: Bundle Branch Block (Consumer Version)
    year: 2024
    url: https://www.msdmanuals.com/home/heart-and-blood-vessel-disorders/abnormal-heart-rhythms/bundle-branch-block
sources_checked: 2026-10-13
review: null
---

Bundle branch block is a partial or complete interruption of the electrical impulse as it travels along the right or left bundle branch.<Cite n={[5, 6]} /> The same kind of interruption in one of the two fascicles of the left bundle branch is called a fascicular block, or hemiblock; these disorders often occur together.<Cite n={[5, 6]} /> Bundle branch block usually causes no symptoms, but it may point to heart disease.<Cite n={[2, 5]} /> How the impulse normally travels through the heart is explained in the article on the [conduction system of the heart](../../anatomy/conduction-system/).

Bundle branch block should not be confused with [atrioventricular block](../av-block/): that term means a partial or complete interruption of the impulse on its way from the atria to the ventricles.<Cite n={1} />

<Diagram name="bundle-branch-block">
  <p slot="normal">
    Normally the impulse from the <Structure id="sa-node">sinus node</Structure> spreads through the
    atria, is delayed slightly in the <Structure id="av-node">atrioventricular node</Structure> and
    travels down the <Structure id="bundle-of-his">bundle of His</Structure>.<Cite n={5} /> The bundle
    divides into the <Structure id="right-bundle-branch">right bundle branch</Structure> for the right
    ventricle and the <Structure id="left-bundle-branch">left bundle branch</Structure> for the left,
    and the left bundle branch divides again into an anterior and a posterior fascicle.<Cite
      n={[5, 6]}
    /> Both ventricles are activated at the same time, and the QRS complex on the ECG is
    narrow.<Cite n={5} />
  </p>
  <p slot="rbbb">
    In right bundle branch block, conduction along that branch is interrupted, and the ventricles are
    activated one after the other rather than together: the right one later than the left.<Cite
      n={5}
    /> This is why the QRS complex on the ECG becomes wider.<Cite n={5} /> The right ventricle is not
    switched off: it is activated too, only later.<Cite n={5} />
  </p>
  <p slot="lbbb">
    In left bundle branch block, the left ventricle is activated later, and the QRS complex widens
    too.<Cite n={5} /> The ventricles may contract out of step, and the left ventricle then pumps
    blood less efficiently.<Cite n={[2, 5]} />
  </p>
</Diagram>

## Why it happens

<Patient>

Bundle branch block can be caused by many heart diseases, and also by degeneration (wear) of the conduction system itself without any other heart disease.<Cite n={5} />

Right bundle branch block can occur in people who seem healthy, and it is not serious in itself.<Cite n={[5, 6]} /> But it can sometimes point to significant damage to the heart, for example after a previous heart attack.<Cite n={6} /> Left bundle branch block tends to be more serious and is more often linked to heart disease; in older adults it often points to [coronary heart disease](../coronary-heart-disease/) caused by atherosclerosis.<Cite n={[5, 6]} />

</Patient>

<Student>

In the electrocardiographic classification given by the Russian Ministry of Health guideline, the ventricular conduction system consists of three fascicles: the right bundle branch and the anterior and posterior fascicles of the left bundle branch.<Cite n={1} /> Isolated blocks are right bundle branch block (RBBB), left anterior fascicular block and left posterior fascicular block; combinations of two of them are called bifascicular blocks: RBBB with one of the fascicular blocks, and left bundle branch block (LBBB).<Cite n={1} /> The ESC also defines bifascicular block as LBBB, or RBBB combined with block of the anterior or posterior fascicle of the left bundle branch.<Cite n={2} />

RBBB may occur with anterior myocardial infarction, and then it indicates substantial myocardial injury; transient RBBB may occur after [pulmonary embolism](../pulmonary-embolism/) and indicates significant right ventricular dysfunction.<Cite n={5} /> LBBB is associated with a structural heart disorder more often than RBBB and, unlike RBBB, with a higher risk of future cardiac events and death.<Cite n={5} /> Isolated left posterior hemiblock is rare; isolated left anterior hemiblock is more common and does not appear to be independently associated with increased risk.<Cite n={5} />

After transcatheter aortic valve implantation, LBBB is seen in 19–55% of patients; in half of cases the conduction disorders after this procedure resolve.<Cite n={1} />

</Student>

<HeartModel highlight={['bundle-of-his', 'right-bundle-branch', 'left-bundle-branch']} />

## Symptoms

<Patient>

Bundle branch block usually causes no symptoms at all.<Cite n={[2, 6]} /> Isolated right bundle branch block produces no symptoms, and isolated left bundle branch block rarely does.<Cite n={5} />

In left bundle branch block, the ventricles contracting out of step may make the left ventricle work less efficiently and may contribute to [heart failure](../heart-failure/), particularly if its pumping is already weakened by another heart disease.<Cite n={5} />

If a person with bundle branch block faints, this is a reason for tests: the cause may be that from time to time the impulses stop reaching the ventricles, that is, high-grade [AV block](../av-block/) develops.<Cite n={[1, 2]} /> Sudden dizziness, confusion and feeling faint can also be signs of a sudden slowing of the heart rhythm.<Cite n={1} />

</Patient>

<Student>

According to the ESC, isolated fascicular block and bundle branch block (BBB) are rarely associated with symptoms, but their presence may indicate structural heart disease; management is determined by whether there are symptoms of intermittent bradycardia.<Cite n={2} />

In patients with preserved cardiac function, syncope is not associated with more sudden cardiac death, but mortality is high in patients with BBB and heart failure, previous myocardial infarction and low ejection fraction, and in patients with low ejection fraction syncope is a risk factor for death.<Cite n={2} />

</Student>

## How it is diagnosed

<Patient>

Bundle branch block is found on an [electrocardiogram](../../diagnostic/ecg/) (ECG): each type of block produces a characteristic pattern.<Cite n={6} /> A 12-lead ECG is recommended when a conduction disorder is suspected.<Cite n={1} />

A block that has been found is a reason to look for heart disease: with left bundle branch block it is found more often, while with right bundle branch block often nothing is found.<Cite n={5} /> In left bundle branch block, an ultrasound scan of the heart, [echocardiography](../../diagnostic/echocardiography/), is recommended if heart disease is suspected.<Cite n={1} />

Left bundle branch block can itself cause ST-segment elevation on the ECG, as in a heart attack, and it makes it impossible to use the usual ECG signs of a past heart attack.<Cite n={[3, 5]} /> This is why it is important for the doctor to compare a new ECG with one recorded earlier.<Cite n={3} />

If a person with the block faints, doctors look for transient AV block: they may do an electrophysiology study (EPS) of the conduction system, and if it finds nothing, the rhythm may be recorded for a long time with a recorder implanted under the skin.<Cite n={2} />

</Patient>

<Student>

Main criteria in adults according to the MSD Manual:

- Normally both ventricles are activated simultaneously, and the QRS duration is ≤ 0.11 s.<Cite n={5} />
- RBBB: QRS duration ≥ 0.12 s; rsr′, rsR′ or rSR′ pattern in V1 or V2; S duration greater than R duration, or S duration > 0.04 s, in I and V6.<Cite n={5} /> Incomplete RBBB has the same criteria with a QRS duration > 0.11 s but < 0.12 s.<Cite n={5} />
- LBBB: QRS duration ≥ 0.12 s; broad notched or slurred R wave and absent Q waves in I, aVL, V5 and V6; ST segment and T wave deviated in the direction opposite to the QRS complex.<Cite n={5} />
- Hemiblocks: modest QRS prolongation (< 0.12 s); in left anterior hemiblock, left axis deviation more negative than −45°; in left posterior hemiblock, an axis more positive than +120°.<Cite n={5} />

RBBB does not significantly interfere with the ECG criteria of past myocardial infarction, whereas LBBB precludes use of the usual criteria.<Cite n={5} /> The Russian Ministry of Health guideline points out that ST elevation is seen not only in myocardial infarction but also in complete LBBB, and calls comparison with an ECG recorded before the attack an important diagnostic step.<Cite n={3} /> Transthoracic echocardiography is recommended in LBBB if structural heart disease is suspected.<Cite n={1} />

In patients with unexplained syncope and bifascicular block, EPS is highly sensitive in identifying intermittent or impending high-grade AV block, but a negative result does not rule it out completely: in some patients with a negative EPS, an implantable loop recorder later detects intermittent or persistent AV block.<Cite n={2} />

</Student>

## How it is treated

<Patient>

Bundle branch block itself usually needs no treatment unless all the bundle branches are blocked.<Cite n={6} /> If it was caused by a heart disease, that disease is treated.<Cite n={6} /> Isolated right bundle branch block without signs of heart disease does not appear to raise the risk of cardiovascular events and needs no specific treatment.<Cite n={5} />

People with bundle branch block and no symptoms are, as a rule, not given a pacemaker: AV block develops in only a few of them, and the risks of implanting the device and of its complications outweigh the benefit.<Cite n={[1, 2]} /> The exception is alternating block, described below. A pacemaker is recommended when a person with bundle branch block faints and an EPS shows a high risk of AV block.<Cite n={[1, 2]} /> It is also recommended in alternating block, when different ECGs show block of the right bundle branch at one time and of the left at another: this points to serious disease of the conduction system and a high chance of sudden complete heart block.<Cite n={[1, 2]} /> The article on [AV block](../av-block/) explains what a pacemaker is.

Some people with left bundle branch block and heart failure with weakened pumping are helped by cardiac resynchronisation therapy (CRT): a special pacemaker stimulates both ventricles so that they contract in step.<Cite n={[2, 5]} /> CRT works better with left bundle branch block than without it, and with right bundle branch block it usually does not help.<Cite n={[1, 2]} /> Whether treatment is needed, and which, is decided by a cardiologist.

</Patient>

<Student>

ESC: permanent pacing is not indicated in asymptomatic patients with BBB or bifascicular block (III, B), except alternating BBB: AV block develops in only a small proportion of patients, and the risks of implantation and of long-term complications of transvenous leads outweigh the benefit.<Cite n={2} /> The Russian Ministry of Health guideline does not recommend permanent pacing in asymptomatic patients with an isolated conduction disorder, 1:1 AV conduction and no other indication.<Cite n={1} />

In syncope with BBB, pacemaker implantation is recommended when the HV interval is 70 ms or longer or block is demonstrated at EPS (Russian Ministry of Health; ESC: for unexplained syncope with bifascicular block, I, B).<Cite n={[1, 2]} /> The ESC allows pacing without EPS in selected patients with unexplained syncope and bifascicular block: elderly, frail, high-risk or with recurrent syncope (IIb, B).<Cite n={2} /> In alternating BBB, pacing is indicated with or without symptoms (ESC: I, C).<Cite n={[1, 2]} />

CRT is recommended for symptomatic patients with heart failure in sinus rhythm with LVEF ≤ 35%, QRS duration ≥ 150 ms and LBBB QRS morphology despite optimal medical therapy (ESC: I, A).<Cite n={[1, 2]} /> Patients with RBBB do not benefit from CRT unless they have so-called masked LBBB.<Cite n={[1, 2]} />

</Student>

## When to call an ambulance

Bundle branch block on its own usually causes nothing and needs no treatment.<Cite n={6} /> Urgent help is needed in two situations.

**Chest pain or pressure**: call your local emergency number (112 in the EU, 911 in the US and Canada, 999 in the UK), as for a suspected [heart attack](../myocardial-infarction/). The Russian Ministry of Health guideline classes new-onset left bundle branch block with chest pain and other signs of ischaemia as ST-elevation myocardial infarction, so help is needed just as urgently.<Cite n={3} />

**Fainting** in a person with bundle branch block: call your local emergency number. Emergency care for fainting caused by a slow heart rhythm is started by the ambulance team.<Cite n={1} /> NICE treats fainting in a person with complete right or left bundle branch block on the ECG as a red flag: the person is referred urgently for cardiovascular assessment, and a specialist reviews the referral within 24 hours (the recommendation covers people over 16).<Cite n={4} />

The site's general safety rule, repeated at the bottom of every page: "If you have chest pain, severe shortness of breath or have fainted, call your local emergency number immediately (112 in the EU, 911 in the US and Canada, 999 in the UK)."

<Quiz />