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Health condition · Clinically reviewed

Bundle branch block, what the wide QRS really means and when it needs action.

From a benign incidental ECG finding to a marker of serious structural heart disease, bundle branch block spans a wide spectrum. This guide sets out the types, the causes and the modern UK approach.

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Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a UK cardiologist before publication.

  • 02

    Sourced from guidance

    Checked against ESC, NICE and peer-reviewed cardiology sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK practice including CRT, His and left bundle branch area pacing and Sgarbossa criteria.

Key facts

Bundle branch block at a glance.

The essentials, in plain English. What it is, the types you might hear about and how UK teams decide when it needs treatment.

  • What it is

    A delay or block in the His-Purkinje intraventricular conduction system, seen on ECG as a widened QRS with a characteristic shape.

  • Main types

    Right bundle branch block (RBBB), left bundle branch block (LBBB), fascicular blocks (LAFB and LPFB) and bifascicular or trifascicular combinations.

  • Threshold

    Complete block is a QRS of over 120 ms. Incomplete RBBB sits between 100 and 120 ms.

  • When it worries us

    LBBB is usually a marker of structural heart disease. Alternating or trifascicular block suggests impending complete heart block.

  • Acute chest pain

    New LBBB with chest pain is a STEMI equivalent per ESC. Sgarbossa criteria help identify infarction in the presence of LBBB.

  • Modern treatments

    From pacemakers to CRT, His bundle pacing and left bundle branch area pacing (LBBAP) for physiological pacing.

Why this guide matters

Context matters more than the label.

Bundle branch block is a pattern, not a disease. What the ECG actually means depends on the type, the symptoms, and the state of the heart underneath.

  • RBBB is often benign

    In a young, structurally normal heart RBBB is frequently incidental. It still deserves a careful look at the right ventricle and a lookout for PE, ASD or cardiomyopathy.

  • LBBB signals structural disease

    Left bundle branch block usually reflects underlying coronary, valvular or myocardial disease and always warrants an echocardiogram.

  • Advanced patterns need pacing

    Alternating BBB, trifascicular block or syncope with bifascicular block are strong indications for permanent pacing per ESC 2021.

How the diagnosis is made

From an unusual ECG to a clear plan.

The steps a UK cardiologist will normally follow, in order, so you know what to expect and why.

  1. 01

    Assessing

    12-lead ECG

    The starting point. QRS duration, morphology in V1, V6 and I, and axis deviation define the type of block.

  2. 02

    Assessing

    History and symptom review

    Syncope, palpitations, breathlessness, fatigue and chest pain shape how urgently to investigate.

  3. 03

    Assessing

    Bloods and screening

    Troponin, BNP, electrolytes, TSH and Lyme serology as clinically indicated. Selective genetic testing for LMNA, SCN5A and inherited conduction disease.

  4. 04

    Confirming

    Echocardiogram

    Left ventricular function, wall motion, valves and dyssynchrony. The most important next test after the ECG.

  5. 05

    Confirming

    Ambulatory ECG or loop recorder

    Holter, event recorder or implantable loop recorder for intermittent syncope or suspected higher-degree block.

  6. 06

    Confirming

    Ischaemia and structural imaging

    Stress echo, cardiac MRI, SPECT or CT coronary angiography where ischaemia, cardiomyopathy or infiltrative disease is suspected.

  7. 07

    Refining

    Electrophysiology study

    Invasive HV interval measurement in selected patients with syncope or bifascicular block to guide pacing decisions.

Typical timeline: from a first ECG to a settled plan in weeks, faster if there are red flags.

Symptoms

What bundle branch block actually feels like.

Often nothing at all. When symptoms do appear, they range from mild breathlessness to the emergency of syncope with impending complete heart block.

  • Often silent

    Most bundle branch block is asymptomatic and found incidentally on a routine ECG.

  • Syncope or presyncope

    A red flag. Stokes-Adams attacks suggest intermittent complete heart block and need urgent review.

  • Palpitations

    May reflect an associated arrhythmia rather than the block itself. Worth investigating with ambulatory ECG.

  • Breathlessness and fatigue

    LBBB can cause interventricular dyssynchrony and worsen heart failure symptoms in a weakened ventricle.

  • Chest pain with new LBBB

    Treated as a STEMI equivalent per ESC. Sgarbossa criteria help identify infarction in the presence of LBBB.

  • Signs of heart failure

    Peripheral oedema, orthopnoea and reduced exercise tolerance in the context of a wide QRS suggest cardiomyopathy.

  • Incidental finding in a young adult

    Isolated RBBB in a healthy young person is often benign, once structural and inherited causes are excluded.

  • Red flag conduction patterns

    Alternating RBBB and LBBB, or bifascicular block with first-degree AV delay, suggests advanced disease and impending complete block.

Treatment

How bundle branch block is managed in the UK.

Treat the underlying cause first. Reserve pacing and CRT for the situations where they are proven to help, and involve specialist electrophysiology teams early when the pattern is advanced.

  • Treat the underlying cause

    Optimise ischaemic heart disease, control blood pressure, replace diseased valves and treat the specific cardiomyopathy.

  • Reassurance for isolated RBBB

    Asymptomatic isolated RBBB in a structurally normal heart needs no specific treatment. Focus shifts to cardiovascular risk factors.

  • Urgent PCI for new LBBB with ACS

    A new LBBB with ischaemic chest pain is treated as a STEMI equivalent. Urgent primary PCI is the standard of care.

  • Permanent pacemaker

    For symptomatic bradycardia, syncope, trifascicular block or alternating BBB. A dual-chamber DDD device is usually chosen per ESC 2021.

  • Cardiac resynchronisation (CRT)

    Biventricular pacing for heart failure patients with LBBB, a QRS over 130 ms and an ejection fraction of 35% or less. Improves symptoms and mortality.

  • His bundle and LBBAP pacing

    Physiological alternatives to conventional CRT. His bundle pacing and left bundle branch area pacing are increasingly used in specialist centres.

  • EP study and ablation

    Selective use for associated tachyarrhythmias, or to measure the HV interval and guide pacing in bifascicular block with syncope.

  • Lifelong specialist follow-up

    Serial ECGs, echocardiograms and pacemaker interrogations. MDT input from electrophysiology, heart failure and inherited cardiac conditions teams.

What this guide is based on

The sources behind every claim on this page.

UK and European guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or cardiologist knows your heart and history and can tell you which parts apply to you. If in doubt, get seen.

  • European Society of Cardiology (ESC). 2021 Guidelines on cardiac pacing and cardiac resynchronisation therapy.

  • European Society of Cardiology (ESC). 2023 Guidelines for the management of acute coronary syndromes.

  • NICE. Chronic heart failure in adults: diagnosis and management (NG106).

  • British Cardiovascular Society and British Heart Rhythm Society. Position statements on pacing and CRT.

  • DVLA. Assessing fitness to drive: cardiovascular disorders (Group 1 and Group 2 rules).

Red flags

When bundle branch block needs urgent attention.

Many BBB patterns are stable. These are the situations that are not, and where hospital or specialist review should not wait.

  • Syncope with bundle branch block

    Any loss of consciousness in a person with a wide QRS deserves urgent cardiology review. Stokes-Adams attacks imply intermittent complete heart block.

  • New LBBB with chest pain

    A STEMI equivalent per ESC. Call 999 and treat with urgent primary PCI pathways.

  • Alternating RBBB and LBBB

    Sometimes called alternating BBB. Considered impending complete heart block and needs immediate admission for pacing.

  • Bifascicular block with first-degree AV delay

    Trifascicular pattern. High risk of progression to complete heart block, especially if the patient has syncope.

  • Rapidly progressive conduction disease

    Progressive PR prolongation, QRS widening or new block in someone with LMNA-related cardiomyopathy is a strong pacing indication.

  • Symptoms suggesting massive PE

    A new RBBB with breathlessness, tachycardia and chest pain in the right context needs immediate assessment for pulmonary embolism.

  • Hyperkalaemia

    A dramatically widened QRS with peaked T waves is a medical emergency and needs urgent treatment.

  • Post-cardiac surgery block

    New or worsening block after aortic valve replacement or TAVI is common. Close monitoring in specialist units is essential.

  • Inherited cardiac conditions

    Family history of sudden death, pacemakers at a young age or unexplained cardiomyopathy warrants inherited cardiac conditions (ICC) review.

Living with it

A manageable diagnosis, with a clear pathway.

Four things that make the biggest difference day to day. Attend your reviews, know the driving rules, report changes promptly and lean on your device team.

A quiet reminder

Small changes on the ECG can matter.

Serial ECGs and echocardiograms track subtle progression that a single snapshot can miss. Consistency of follow-up is what catches trouble early.

  1. 01 Follow-up

    Attend your reviews

    Serial ECGs, echoes and pacemaker or device checks pick up subtle changes early. Regular follow-up is the backbone of good care.

  2. 02 Driving

    Know the DVLA rules

    After syncope, pacing or a new arrhythmia, driving is restricted. Group 1 licences are usually off the road for 4 weeks after pacemaker implantation, Group 2 for 6 weeks.

  3. 03 Symptoms

    Report new symptoms early

    Fresh syncope, chest pain, breathlessness or palpitations after a diagnosis of BBB deserve prompt review, not a wait-and-see approach.

  4. 04 Devices

    Live confidently with a device

    Modern pacemakers, CRT devices and defibrillators are safe with most everyday activities. Your device team will explain what to avoid and when to call.

Frequently asked

Everything we get asked about bundle branch block.

Quick answers on RBBB, LBBB, pacing, CRT and what your ECG really means.

  • What is bundle branch block?

    A delay or block in the electrical wiring that carries impulses through the ventricles. It shows up on an ECG as a widened QRS with a characteristic pattern in the chest leads. It can involve the right bundle (RBBB), the left bundle (LBBB) or the smaller fascicles of the left bundle (fascicular blocks).

  • Is right bundle branch block dangerous?

    Isolated RBBB in a young, otherwise healthy person is often benign. But it can be a marker of pulmonary embolism, right ventricular strain, atrial septal defect, cardiomyopathy or ischaemic heart disease, so an echo and full assessment are always sensible.

  • Is left bundle branch block more serious?

    Usually yes. LBBB is more often associated with structural heart disease including coronary artery disease, hypertension, aortic valve disease and cardiomyopathies. New LBBB with chest pain is treated as a STEMI equivalent per ESC guidance.

  • What is bifascicular or trifascicular block?

    Bifascicular block means two of the three main conduction pathways are blocked, most commonly RBBB with left anterior fascicular block. Add a first-degree AV block and it becomes trifascicular pattern, which carries a higher risk of progressing to complete heart block.

  • When is a pacemaker needed?

    Pacing is recommended for symptomatic bradycardia, syncope with evidence of high-grade block, alternating BBB and selected cases of bifascicular block with unexplained syncope. Cardiac resynchronisation therapy (CRT) is added when there is heart failure with a reduced ejection fraction and a wide QRS with LBBB morphology.

  • Can I drive with bundle branch block?

    Asymptomatic BBB itself does not usually restrict driving. Syncope, arrhythmia or device implantation triggers DVLA rules. Group 1 (car) drivers are typically off the road for 4 weeks after a pacemaker; Group 2 (lorry and bus) drivers for 6 weeks. Always check current DVLA guidance and inform them where required.

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