Left Bundle Branch BlockEdit
Left bundle branch block (LBBB) is a disturbance of the heart’s electrical wiring that delays and disrupts the way the left ventricle is activated. In this condition, impulses that normally travel quickly through the left bundle branch are slowed or blocked, causing asynchronous contraction of the two ventricles. LBBB most often signals underlying heart disease, such as ischemic heart disease or cardiomyopathy, but it can occur in people without obvious prior illness as well. It is typically detected on the electrocardiogram (ECG) and is characterized by a widened QRS complex and distinctive waveform patterns that reflect altered ventricular activation. electrocardiogram The pattern is important not only for diagnosis but also for guiding management and prognosis, because it often accompanies structural heart problems and may influence treatment choices. cardiac conduction system
In contemporary clinical practice, LBBB is more than just an ECG finding; it is a marker that clinicians interpret in the context of symptoms, imaging, and risk factors. New or presumed new LBBB in the setting of chest pain is a situation that can prompt urgent investigation for acute coronary syndromes, while long-standing LBBB with stable symptoms may prompt assessment of heart muscle function and consideration of therapies that address dyssynchrony. The presence of LBBB can complicate the ECG-based diagnosis of myocardial infarction, because the typical patterns used to identify ischemia may be obscured or altered. Management often relies on a combination of clinical evaluation, laboratory biomarkers such as troponin, imaging studies like echocardiography, and, when appropriate, coronary angiography. troponin echocardiography coronary angiography
Pathophysiology
The left bundle branch carries electrical impulses from the His-Purkinje system to activate the left ventricle. When the left bundle is blocked, electrical activation of the left ventricle is delayed relative to the right ventricle, resulting in a longer-than-normal QRS duration and a distinctive pattern on the ECG. The resulting electrical dyssynchrony often translates into mechanical dyssynchrony, meaning the two ventricles do not squeeze together efficiently, which can worsen heart failure symptoms and reduce overall cardiac performance. This pattern tends to be seen in the context of structural heart disease, including prior myocardial injury, pressure or volume overload, or cardiomyopathy. cardiac conduction system cardiomyopathy ischemic heart disease left ventricle
LBBB is typically defined on the ECG by a QRS duration of 120 ms or longer and specific morphology that indicates delayed activation of the left ventricle. In standard ECG leads, features include broad, often notched or slurred R waves in lateral leads (I, aVL, V5–V6) and a lack of initial positive deflections in those leads, with complementary patterns in right precordial leads (V1–V3). ST-T changes are usually secondary and discordant with the QRS complex. These ECG features help distinguish LBBB from other conduction or ventricular conditions such as right bundle branch block or focal myocardial injury. QRS duration electrocardiogram Right bundle branch block
Causes and epidemiology
LBBB most commonly arises in the setting of underlying heart disease. Notable contributors include: - Ischemic heart disease and prior myocardial infarction. ischemic heart disease myocardial infarction - Dilated or hypertrophic cardiomyopathy. cardiomyopathy - Long-standing hypertension and valvular disease, especially aortic stenosis, which can promote progressive left ventricular remodeling. hypertension valvular heart disease aortic stenosis - Infiltrative or inflammatory cardiomyopathies, and post-cardiac surgery or catheter-based interventions - Certain medications or iatrogenic factors that influence conduction
LBBB becomes more common with age and is associated with higher prevalence of risk factors such as coronary disease and heart failure, though it can also appear in people without classic risk factors. The prognosis of LBBB depends heavily on the underlying heart disease and the patient’s functional status, with worse outcomes typically linked to more advanced structural heart disease or reduced left ventricular function. aging heart failure
Clinical presentation
Many individuals with LBBB are asymptomatic and discover the block incidentally on a routine ECG. When symptoms occur, they tend to reflect the underlying heart disease rather than the block itself and can include shortness of breath, fatigue, chest discomfort, or symptoms of heart failure such as edema or reduced exercise tolerance. In people with known coronary disease, new or changing LBBB may raise concern for an acute event and prompt urgent evaluation. chest pain shortness of breath
Diagnosis
Diagnosis starts with the ECG, where the hallmark features of LBBB include QRS duration ≥ 120 ms and characteristic broad, often notched R waves in lateral leads (I, aVL, V5–V6) with secondary ST-T changes discordant to the QRS. Echocardiography is routinely used to assess left ventricular systolic function (ejection fraction) and to identify structural contributors to the block or dyssynchrony. Cardiac imaging such as MRI may be used to characterize cardiomyopathy or prior injury when the diagnosis remains unclear. In the context of acute chest pain, troponin testing and clinical judgment guide the evaluation for myocardial infarction; in the presence of new LBBB, many patients are treated as ST-elevation myocardial infarction (STEMI) until proven otherwise. ejection fraction magnetic resonance imaging ST-elevation myocardial infarction
Differential diagnoses to consider include other conduction abnormalities (such as RBBB) and patterns of ventricular hypertrophy or scar that can mimic LBBB on ECG. The distinction often relies on a combination of ECG morphology across multiple leads and the clinical context. Right bundle branch block electrocardiography ventricular hypertrophy
Management
Management of LBBB centers on addressing the underlying heart disease and, when appropriate, treating dyssynchrony to improve symptoms and outcomes.
General management focuses on aggressive risk-factor modification and guideline-directed medical therapy for heart failure or ischemic heart disease. This includes strategies to optimize blood pressure, cholesterol, diabetes control, smoking cessation, physical activity as tolerated, and vaccination as indicated. guideline-directed medical therapy heart failure ischemic heart disease
In patients with reduced left ventricular ejection fraction (commonly EF ≤ 35%), and a QRS duration of 150 ms or more with LBBB morphology, cardiac resynchronization therapy (CRT) is a well-supported option. CRT uses a specialized biventricular pacemaker to coordinate electrical activation and improve mechanical efficiency of the heart. CRT can be combined with an implantable cardioverter-defibrillator (ICD) in selected patients at higher risk of sudden cardiac death. cardiac resynchronization therapy implantable cardioverter-defibrillator
For ischemic events, standard acute coronary syndrome management applies, and LBBB in this setting may warrant urgent evaluation for revascularization when indicated. In the setting of a new or presumed new LBBB with chest pain, the approach often mirrors STEMI protocols until diagnostic certainty is achieved. ST-elevation myocardial infarction coronary revascularization
Pacemaker therapy aimed at correcting dyssynchrony is not equivalent to treating an intrinsic conduction disease in all contexts; decisions are individualized based on EF, QRS duration and morphology, symptoms, and comorbidity. CRT is specifically targeted at improving synchrony rather than simply “correcting” the block. pacemaker dyssynchrony
Some patients with LBBB and preserved EF may not meet criteria for CRT and are managed with close monitoring and optimization of medical therapy. The balance between potential benefits and risks of device therapy is an area of ongoing clinical judgment and guideline updates. ejection fraction
Prognosis
The presence of LBBB by itself does not determine prognosis; rather, outcomes depend on the underlying cardiac condition and the degree of ventricular dysfunction. LBBB is associated with an increased risk of adverse outcomes in the setting of significant structural heart disease or heart failure, but its presence in isolation with normal LV function can have a relatively favorable course. Regular follow-up with clinical assessment and cardiac imaging helps guide prognosis and treatment adjustments. prognosis heart failure
Controversies and debates
Within clinical practice, several areas of discussion surround LBBB management: - The role of CRT in LBBB with moderately reduced EF or borderline QRS duration remains subject to study and guideline refinement. Some patients derive substantial benefit from resynchronization, while others may have limited or no improvement, underscoring the importance of individualized assessment. cardiac resynchronization therapy - The interpretation of LBBB in acute coronary syndromes has evolved. Historically, new LBBB was treated as a STEMI equivalent, but contemporary practice emphasizes integrating ECG findings with troponin and imaging to determine the need for urgent revascularization. This nuance can affect the urgency and type of intervention. ST-elevation myocardial infarction troponin - There is ongoing discussion about how best to stratify risk and select patients for device therapy, especially as noninvasive imaging and biomarkers improve our ability to phenotype heart failure and dyssynchrony. echocardiography cardiomyopathy
These debates reflect the broader theme in cardiology: matching therapies to the individual's specific pattern of conduction, structure, and function rather than applying a one-size-fits-all approach. The aim is to maximize symptom relief, functional capacity, and long-term outcomes while avoiding over-treatment. heart failure prognosis