Left bundle branch block

Primary Causes
Left bundle branch block can be primary, meaning it occurs without any identifiable underlying heart disease:

• Isolated degenerative left bundle branch block, related to aging of the heart’s conduction system;
• Congenital abnormality of the conduction system;
• Anatomical variant without functional consequence.

Secondary Causes
More commonly, left bundle branch block is secondary to an underlying heart condition, such as:

• Ischemic heart disease (coronary artery disease, myocardial infarction);
• Dilated or hypertrophic cardiomyopathy;
• Long-standing arterial hypertension;
• Valvular heart disease, especially affecting the aortic valve (aortic stenosis);
• Inflammatory heart diseases (myocarditis, endocarditis);
• Surgical trauma following aortic valve replacement;
• Drug-induced cardiotoxicity;
• Certain forms of congenital heart disease.

Unlike right bundle branch block, left bundle branch block is more often associated with structural heart disease and generally represents a more significant marker of severity. This is why its discovery, even in the absence of symptoms, requires a thorough cardiological evaluation.

How Does Left Bundle Branch Block Present?

Left bundle branch block is most often asymptomatic and is discovered incidentally during a routine electrocardiogram (ECG). However, some patients may experience clinical manifestations, commonly referred to as left bundle branch block symptoms, which are usually related to the underlying heart disease rather than the block itself:

• Shortness of breath during exertion or at rest;
• Unusual fatigue;
• Palpitations;
• Faintness or syncope (rare);
• Signs of heart failure (swelling of the lower limbs, shortness of breath, difficulty breathing when lying down).

The ventricular desynchronization caused by left bundle branch block can, over time, promote or worsen pre-existing heart failure. This is known as pacing-induced cardiomyopathy or desynchronization-induced cardiomyopathy.

Hemodynamic Impact

From a physiological standpoint, left bundle branch block leads to:

• Desynchronized contraction of the left ventricle;
• Reduced efficiency of ventricular filling;
• Decreased stroke volume (the amount of blood ejected with each heartbeat);
• Potential impairment of mitral valve function;
• Increased myocardial oxygen consumption.

These hemodynamic disturbances may be well tolerated in patients without underlying heart disease but can significantly contribute to worsening cardiac function in those with existing heart failure.

How Is Left Bundle Branch Block Diagnosed?

Main Diagnostic Methods
The diagnosis of left bundle branch block (LBBB) is primarily based on a standard electrocardiogram (ECG), which reveals characteristic abnormalities. This pattern, often referred to as LBBB ECG, corresponds to delayed left ventricular activation on the ECG and shows typical left bundle branch abnormalities, such as:

• Widening of the QRS complex (≥ 120 ms);
• Absence of Q waves in leads I and V6;
• Monophasic appearance of the QRS complex in leads V5-V6 with a broad, notched R wave;
• Broad, deep S waves in leads V1-V2;
• Frequent left axis deviation;
• Secondary repolarization abnormalities (ST segment and T wave) with a direction opposite to that of the QRS complex.

A distinction is made between complete left bundle branch block (QRS ≥ 120 ms with all typical morphological criteria) and incomplete LBBB (QRS between 100 and 120 ms with partial morphological features).

Electrocardiograms can be performed at all Rythmopôle Paris locations, including the Pôle Santé Bergère (9th arrondissement of Paris) and the Centre Damrémont (18th arrondissement of Paris).

Additional Tests
Once the diagnosis of LBBB is confirmed, additional tests are often necessary to investigate possible underlying heart disease and assess its functional impact:

• Echocardiography: an essential exam to evaluate heart function and structure, measure left ventricular ejection fraction, and detect possible valve abnormalities or segmental contractility disorders. Available at Cardiopôle Yvart (15th arrondissement of Paris);
• ECG Holter monitor: continuous recording of heart electrical activity for 24 to 48 hours to detect potential associated rhythm disorders;
• Exercise stress test: to assess functional capacity and screen for underlying myocardial ischemia;
• Cardiac MRI: particularly useful for characterizing myocardial tissue (fibrosis, inflammation, ischemia);
• Coronary angiography: indicated in cases of suspected ischemic heart disease.

These tests not only help identify a potential treatable cause but also assess the functional impact of LBBB and guide therapeutic decisions.

Risk Assessment in Left Bundle Branch Block

The risk associated with left bundle branch block (LBBB) depends on several factors:

• Presence of underlying heart disease: this is the main determinant of prognosis;
• Left ventricular function: a reduced ejection fraction (< 40%) in combination with LBBB is a significant risk factor;
• Heart failure symptoms: their presence and severity (NYHA classification) guide the management approach;
• QRS duration: a very wide QRS complex (> 150 ms) is associated with a less favorable prognosis but a potentially better response to resynchronization therapy;
• Associated rhythm disorders: some patients are at increased risk of ventricular arrhythmias;
• Age at onset: LBBB appearing in younger individuals requires closer monitoring.

This risk stratification is carried out by the electrophysiologists at Rythmopôle Paris, who have the expertise to interpret all clinical and diagnostic data and to provide tailored care for each patient.

Main Treatments

The therapeutic management of left bundle branch block (LBBB) depends primarily on two factors: the presence of underlying heart disease and the existence of heart failure symptoms. Life expectancy with left bundle branch block is therefore closely linked to these clinical factors and the response to treatment. In some cases, particularly in the presence of heart failure, the risks associated with LBBB may warrant an advanced therapeutic approach.

• Isolated, asymptomatic LBBB: no specific treatment is required. Regular monitoring is recommended to detect the potential onset of heart disease;
• LBBB with underlying heart disease: treatment is focused on managing the underlying condition (e.g., treating hypertension, coronary revascularization, valve repair);
• LBBB with heart failure and reduced ejection fraction: in addition to optimal medical therapy for heart failure, cardiac resynchronization therapy (CRT) is a major treatment option.

Cardiac resynchronization therapy involves implanting a specialized pacemaker (a biventricular pacemaker or defibrillator with resynchronization function) that helps coordinate the contraction of the heart’s ventricles. This technique is particularly indicated for patients with:

• Symptomatic heart failure despite optimal medical treatment;
• Left ventricular ejection fraction ≤ 35%;
• LBBB with a QRS duration ≥ 130 ms.

This procedure is performed at the Institut Mutualiste Montsouris (Paris 14th arrondissement) by the electrophysiologists of Rythmopôle Paris. It is important to note that this treatment applies only to a small group of patients—those with significant heart failure associated with LBBB.

Recommended Medical Follow-up

Follow-up for patients with LBBB varies depending on the clinical context:

• For patients with isolated LBBB without underlying heart disease: annual clinical and electrocardiographic monitoring;
• For patients with associated heart disease: regular follow-up by a cardiac electrophysiologist (every 3 to 12 months depending on severity), including a clinical exam, ECG, and sometimes echocardiography;
• For patients undergoing cardiac resynchronization therapy: specific monitoring of the implanted device every 3 to 6 months, which can be done at several Rythmopôle Paris centers, including Cardiopôle Peupliers-Trubert.

This personalized follow-up allows for treatment adjustments based on clinical progression and early detection of any deterioration in cardiac function.

Physical Activity

Recommendations regarding physical activity depend primarily on whether an underlying heart condition is present:

• In the absence of detectable heart disease, isolated left bundle branch block generally does not require any specific restrictions. Regular, moderate physical activity is even encouraged;
• If an underlying heart condition is present, recommendations are tailored to the type and severity of the condition;
• For patients who have undergone cardiac resynchronization therapy, a specific rehabilitation program may be offered at Cardiopôle Yvart (Paris 15th arrondissement), where cardiac rehabilitation sessions are available.

Professional Life

The impact of left bundle branch block on professional life is usually limited:

• In the absence of heart disease and symptoms, no professional restrictions are necessary;
• Certain high-risk occupations (pilots, professional drivers, work at height) may require a specific evaluation, especially if a heart condition is also present;
• For patients with heart failure, recommendations depend on the stage of the disease and the effectiveness of treatment.

Follow-up and Precautions

General precautions recommended for patients with left bundle branch block include:

• Informing healthcare professionals about this conduction abnormality, especially before any surgical procedure;
• Carrying a medical card stating the conduction disorder and current treatments;
• Attending regular follow-up appointments with a cardiac electrophysiologist;
• Reporting any new cardiac symptoms promptly (palpitations, fainting, shortness of breath).

In some cases, prolonged stress or intense emotional strain can worsen symptoms associated with left bundle branch block. Therefore, maintaining emotional balance and a calm lifestyle is recommended.

With appropriate care, the vast majority of patients with left bundle branch block can lead an active life without major restrictions.

Rythmopôle Paris offers specialized expertise in the management of cardiac conduction disorders, including left bundle branch block—the most common ventricular conduction disorder identified in clinical practice:

• A team of electrophysiologists specialized in diagnosing and assessing cardiac conduction abnormalities;
• A fully equipped technical platform for all necessary diagnostic and follow-up tests: ECG, echocardiography, stress testing, and ECG Holter monitoring;
• Specific expertise in the indication and implementation of cardiac resynchronization therapy at the Institut Mutualiste Montsouris;
• A tailored cardiac rehabilitation program for heart failure patients at Cardiopôle Yvart;
• Personalized follow-up with the ability to monitor implanted devices at several centers across the Île-de-France region;
• A multidisciplinary approach involving cardiologists, electrophysiologists, and heart failure specialists.