Left Bundle Branch Block

Etiology

LBBB arises from various factors and conditions that can affect the electrical conduction system. The underlying causes can be broadly categorized into structural heart disease, degenerative changes, and other less common factors.

• Structural heart disease: Coronary artery disease (CAD), hypertension, cardiomyopathies such as dilated, left ventricular noncompaction, stress, and hypertrophic cardiomyopathies, and aortic valve diseases like aortic stenosis and regurgitation
• Degenerative changes: Age-related fibrosis, Lev disease, and Lenègre disease
  • Lev disease is a form of idiopathic, age-related progressive cardiac conduction disorder characterized by extensive calcification and fibrosis of the cardiac skeleton, including the aortic valve annulus, mitral valve annulus, and interventricular septum. This calcification can extend to involve the His bundle and the bundle branches, manifesting as bundle branch blocks, commonly LBBB, and varying degrees of atrioventricular block, which can progress to complete heart block. Lenègre disease involves age-related, progressive degeneration of the conduction fibers, leading to fibrosis and sclerosis. This degeneration typically affects the His bundle and its branches, leading to various conduction blocks, LBBB being the most common. 
• Infiltrative conditions: sarcoidosis or amyloidosis
• Inflammatory cardiomyopathies: Infectious myocarditis or autoimmune disease
• Iatrogenic: Following cardiac surgery involving the aortic valve or interventricular septum or after transcatheter aortic valve replacement (TAVR)
  • At least 30% to 50% of patients develop LBBB after TAVR, which independently predicts mortality in these patients.[5] 
• Electrolyte imbalance: Hyperkalemia
• Medication: Anti-arrhythmic medications
• Congenital structural heart disease
• Idiopathic: Particularly in younger patients or individuals without significant cardiac risk factors   

Epidemiology

The incidence and prevalence of LBBB vary across different populations based on factors such as age, race, gender, and underlying cardiovascular conditions. LBBB is prevalent in about 0.06% to 0.1% of the general population. In general, the incidence is reported to be about 1 to 4 cases per 1000 person-years, increasing with age. In asymptomatic older individuals, particularly men, LBBB is more commonly identified incidentally during routine electrocardiography (ECG) screening. In individuals older than 70, the prevalence can be as high as 1% to 5%. The prevalence further increases in people older than 80, with some studies reporting rates as high as 6% to 7%.[6]

The prevalence of LBBB is generally higher among White individuals compared to other ethnic groups. LBBB is more common in men than women, with prevalence approximately twice as high in some cohorts. Hispanic patients with left ventricular dysfunction have an increased incidence of LBBB.[7] Approximately 33% of patients with heart failure have LBBB; the incidence increases with the severity of left ventricular failure.[8] LBBB is quite common in individuals with dilated cardiomyopathy, with prevalence rates ranging from 10% to 26%.

The Framingham Heart Study results reported that LBBB is more frequently observed in people with a history of hypertension, particularly when the condition is longstanding or poorly controlled. The study reports a significant association between LBBB and CAD. LBBB in individuals with CAD often indicates more extensive myocardial ischemia or infarction and is associated with a worse prognosis. The study also reported that LBBB is associated with a higher risk of developing heart failure and an increased risk of all-cause mortality, cardiovascular mortality, and sudden cardiac death.[9]

Pathophysiology

The septum on the left ventricular side is normally activated first through left-to-right conduction via the small septal branch of the left bundle. In LBBB, conduction shifts to the right bundle, leading to right-to-left activation. The resulting changes include the loss of lateral Q waves and the appearance of tall R waves in lateral leads. After the septum, the remaining left ventricle is activated by functional muscle fibers instead of the His-Purkinje system (HPS), with leftward and posterior conduction, causing broad, notched, or slurred R waves in lateral leads.

Electrical Dyssynchrony

In a normal HPS, the impulse travels down longitudinally with a fast conduction velocity of 1.5 m/s. This speed is facilitated by the high saturation of gap junctions and the longitudinal arrangement of cells. A LBBB results from direct damage to the LBB as a result of conduction delay within the HPS and into the myocardium or a combination of both. With LBBB, the right ventricle is activated first, followed by a right-to-left septal transmission and activation of the left ventricular endocardium at the level of the midseptum. 

Mechanical Dyssynchrony

In LBBB, the septum is activated before aortic valve opening (isovolumetric contraction), stretching the posterior and lateral walls. The posterior and lateral walls are activated later, which stretches the septum. Consequently, dyssynchronous contraction of the left ventricle occurs, causing a mechanical disadvantage. 

Genetic Factors

Recent evidence suggests that several genes are implicated in the pathogenesis of LBBB. Affected genes include HCN4, SCN5A, LMNA, GATA4, and ANK2.[10]

History and Physical

LBBB may be silent, especially in its early stages, or present with nonspecific symptoms, such as fatigue, dyspnea, and palpitations. Symptoms often arise from the underlying heart condition.[11] Symptoms due to underlying heart disease can include chest pain, dyspnea, fatigue, palpitations, syncope, or presyncope. Signs of LBBB due to underlying heart disease can include signs of heart failure and cardiomyopathies, valvular heart disease, and systemic diseases with cardiac involvement.

Evaluation

Electrocardiography

According to the American College of Cardiology, American Heart Association, and Heart Rhythm Society, complete LBBB is defined by the following ECG features (see Image. Left Bundle Branch Block on Electrocardiography):

• QRS duration greater than or equal to 120 ms
• Broad notched or slurred R wave in leads I, aVL, V5, and V6 or, occasionally, an RS pattern in leads V5 and V6
• Absence of Q wave in leads I, V5, and V6 (with the possible exception of aVL)
• R wave peak time greater than 60 ms in leads V5 and V6
• ST-T wave changes opposite to the QRS direction [12]

Strauss et al proposed these more stringent criteria:

• QRS duration greater than or equal to 140 ms in men and greater than or equal to 130 ms in women
• Broad notched or slurred R waves in left-sided leads
• Broad notched or slurred QRS in right-sided leads, with QS or rS in V1 and V2 [2]

The European Society of Cardiology provided the following criteria for diagnosis of LBBB in 2021:

• QRS duration greater than or equal to 120 ms
• Notches or slurring in the middle third of the QRS complex in at least 2 of the following leads: V1, V2, V5, V6, I, and aVL, with an R-wave peak time of 60 ms in lead V5 or V6
• The ST segment is slightly opposite to the QRS complex polarity
• In the horizontal plane, QS or rS in V1 with a small "r" slight ST elevation and a positive asymmetrical T wave, and a unique R wave in V6 with a negative asymmetrical T wave. When the QRS duration is less than 140 ms, the T wave in V6 may be positive
• In the frontal plane, an exclusive R wave in I and aVL, often with a negative asymmetrical T wave, slight ST depression, and usually a QS complex in aVR with a positive T wave
• Variable QRS axis [13]

Echocardiography

According to the 2018 guidelines on the evaluation and management of bradycardia and conduction delay from the American Heart Association, American College of Cardiology Foundation, and Heart Rhythm Society, patients with newly diagnosed LBBB should undergo transthoracic echocardiogram (TTE) to rule out structural heart disease.[12] TTE helps evaluate global and regional left ventricular systolic function. TTE, particularly with strain imaging techniques like speckle-tracking echocardiography, can assess the degree of mechanical dyssynchrony in patients with LBBB. This information is valuable for determining the suitability of CRT in patients with symptomatic heart failure and LBBB. 

M-mode echocardiography can show characteristic findings related to interventricular septal motion and the overall left ventricular contraction pattern due to electrical dyssynchrony. The M-mode findings may include "septal flash" or "septal beaking," where the septum moves abruptly toward the left ventricle at the onset of systole and then quickly moves back toward the right ventricle. Dyssynchronous movement between the septum and the posterior wall is seen.

Strain imaging is useful for the evaluation of LBBB. Global longitudinal strain may be reduced overall due to dyssynchronous contraction in patients with LBBB, even in the absence of myocardial infarction or overt heart failure. The basal septal strain is often significantly reduced or abnormal due to the early electrical activation and paradoxical motion of the septum. In contrast, the lateral wall strain may be delayed but preserved, reflecting the dyssynchronous contraction pattern.

The difference in the timing of peak strain across various myocardial segments, known as strain dispersion, is often increased in LBBB. A greater strain dispersion indicates more severe dyssynchrony, which can be predictive of the response to CRT. By identifying the most dyssynchronous segments, strain imaging can guide the optimal placement of the CRT lead, usually in the most delayed region, to achieve maximal resynchronization and improve clinical outcomes. Abnormal strain patterns and high strain dispersion are associated with worse outcomes in patients with LBBB and heart failure. Strain imaging can thus be used for risk stratification and guiding the timing of CRT or other interventions. 

Pulsed-wave Doppler tissue imaging is a valuable tool for assessing mechanical dyssynchrony in LBBB. Peak systolic velocities of the left ventricle's septal and lateral walls at the basal segments are measured. A time difference between peak systolic velocities of the septal and lateral walls (the "septal-to-lateral delay") greater than 60 ms is often considered indicative of significant dyssynchrony.

Stress Imaging

LBBB often complicates the interpretation of noninvasive cardiac tests due to abnormal septal motion and potential perfusion defects, making stress imaging particularly valuable for differentiating between true ischemia and artifacts.

During stress echocardiography, the abnormal septal motion can mimic ischemia, leading to false-positive results for CAD. The emphasis should be on the assessment of wall motion abnormalities in nonseptal regions (eg, lateral or posterior walls) to avoid the confounding effects of LBBB on septal motion. Dobutamine stress echocardiography is often preferred over exercise because it can provide clearer differentiation between septal pseudodyskinesis and true ischemia due to graded increment. 

In a myocardial perfusion scan, true ischemic defects tend to be in the territories supplied by the coronary arteries (eg, anterior and lateral walls) rather than just the septum. Scan results should be interpreted in conjunction with clinical findings and other imaging modalities.

Electrophysiological Study

In patients with LBBB, an electrophysiological study (EPS) can provide information about the conduction system, specifically the HPS. This modality can help assess the risk of progression to a more advanced atrioventricular or complete heart block. EPS helps identify patients at high risk for sudden cardiac death who may benefit from implantable cardioverter-defibrillator (ICD) placement, especially in patients with unexplained syncope and evidence of conduction disease.

The inducibility of sustained monomorphic ventricular tachycardia in patients with LBBB, especially those with structural heart disease, is an important finding that may warrant ICD placement. A His-to-ventricular interval of greater than 70 ms is indicative of significant conduction disease in the HPS. A markedly prolonged His-to-ventricular interval (>100 ms) suggests a high risk of progression to a complete heart block. EPS can help differentiate between syncope due to bradyarrhythmia (eg, atrioventricular block) and tachyarrhythmias (eg, ventricular tachycardia), which is crucial for determining the appropriate therapeutic strategy.

Treatment / Management

The management of LBBB depends on the underlying structural heart disease or symptoms. Approaches are based on the impact of LBBB on cardiac function and its relation with specific heart diseases or clinical scenarios.

Asymptomatic Individuals

A TTE is warranted for further evaluation of underlying structural heart disease in these individuals. Cardiac stress tests or anatomical imaging modalities like computed tomography coronary angiography (CTCA) may be indicated in people with CAD risk factors. 

Left Bundle Branch Block in Heart Failure

A thorough medical history should be obtained to determine a patient's New York Heart Association (NYHA) functional class, which helps assess the severity of heart failure symptoms and guides appropriate management strategies. Any history of heart failure exacerbations or hospitalization should be documented. The ECG tracing should be evaluated thoroughly to confirm the presence of LBBB based on known criteria. A TTE should be performed to assess the left ventricular ejection fraction (LVEF).

Standard medical therapy for heart failure should be initiated and optimized as tolerated. Pharmacologic options include angiotensin-converting enzyme inhibitors (or angiotensin receptor blockers), β-blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter-2 inhibitors. Cardiac resynchronization therapy (CRT) is indicated in patients with sinus rhythm, NYHA functional class II, III, or ambulatory IV heart failure despite optimal medical therapy, an LVEF of 35% or less, QRS duration greater than or equal to 150 ms with LBBB morphology, and a meaningful expected survival longer than 1 year. CRT improves symptoms and quality of life and reduces mortality and hospitalization in these patients.[14](A1)

Painful Left Bundle Branch Block Syndrome

This condition is also known as "exercise-induced LBBB." Painful LBBB syndrome is a rare disorder characterized by chest pain or discomfort specifically associated with the onset of LBBB, usually during physical activity or exercise. The pain typically mimics angina, but it occurs without evidence of obstructive CAD. Conducting a treadmill or pharmacologic stress test can reproduce symptoms and document the timing of LBBB onset in relation to exercise. An invasive coronary angiogram or CTCA may be performed if clinical suspicion is high. β-blocker use must be considered to reduce myocardial oxygen demand and potentially minimize the occurrence of LBBB during exercise. Nondihydropyridine calcium channel blockers may be used in individuals who cannot tolerate β-blockers. 

Left Bundle Branch Block in Coronary Artery Disease

LBBB can obscure ST-segment elevations or depressions, which are critical for diagnosing acute myocardial infarction (AMI). The Sgarbossa criteria can aid in diagnosis. This set of criteria includes ST-segment elevation greater than or equal to 1 mm in leads with a positive QRS complex, concordant ST-segment elevation more than or equal to 1 mm in any lead, or ST-segment depression greater than or equal to 1 mm in leads V1 to V3.[15] Immediate reperfusion therapy is indicated if AMI is suspected in a patient with LBBB and ischemic symptoms, especially if they are hemodynamically unstable or demonstrate dynamic ECG changes, positive biomarkers, or ECG changes suggestive of wall motion abnormalities. Management of chronic LBBB in the context of CAD involves a combination of optimal medical therapy, careful diagnostic evaluation, consideration for device therapy, and potential revascularization. (B2)

Left Bundle Branch Block After Transcatheter Aortic Valve Replacement and Surgical Myomectomy

The presence of LBBB post-TAVR can increase the risk of developing high-grade atrioventricular block, requiring permanent pacemaker implantation. Surgical myomectomy for hypertrophic obstructive cardiomyopathy may result in LBBB due to the disruption of the conduction system during septal muscle resection. Patients with new-onset LBBB post-TAVR or surgical myomectomy should have continuous telemetry monitoring for at least 48 to 72 hours to detect any episodes of high-grade atrioventricular block or symptomatic bradycardia. Longer monitoring may be warranted in patients with other risk factors for conduction abnormalities, such as preexisting right bundle branch block or prolonged PR interval.

Patients with isolated, asymptomatic LBBB without other conduction disturbances may not require immediate pacemaker implantation. These individuals should have a close follow-up with repeated ECGs and possibly ambulatory monitoring (eg, Holter monitoring) to detect any delayed progression to high-grade atrioventricular block. High-risk patients, especially those with symptomatic conduction abnormalities or evidence of high-grade atrioventricular block, should receive a permanent pacemaker. 

Differential Diagnosis

Incomplete Left Bundle Branch Block

In incomplete LBBB, the LBB is partially impaired, causing a delay in the initial depolarization of the left ventricle. However, the impulse passes through the block and normally travels to the rest of the left ventricle. Thus, on ECG, the QRS complex is widened but not as wide as in complete LBBB. The QRS duration in incomplete LBBB is typically between 110 and 120 ms. The characteristic broad or notched R waves seen in lead I, aVL, V5, and V6 may still be present but are less marked. The ECG does not show the full pattern of complete LBBB, such as the deep S waves in leads V1 and V2, which are more typical of complete LBBB. The R-wave peak time in left-sided leads measures greater than 60 ms.

Ventricular-Paced Rhythm

Ventricular-paced rhythm can produce a wide QRS complex with a morphology similar to LBBB. Identification of pacemaker spikes preceding the QRS complexes or known pacemaker placement can help distinguish this clinical entity from true LBBB. A paced rhythm often does not have an R wave in V6. 

Left Ventricular Hypertrophy

Left ventricular hypertrophy can sometimes produce a wide QRS complex with delayed intrinsicoid deflection in the lateral leads, which can resemble LBBB. The Sokolow-Lyon criteria for increased QRS voltage, along with ST-segment and T-wave changes characteristic of left ventricular hypertrophy rather than typical LBBB patterns, can aid in distinguishing between the 2 conditions.

Premature Ventricular Contractions

Premature ventricular contractions (PVCs) originating from the right ventricle or septal area can produce a QRS morphology that mimics LBBB. PVCs are usually premature, and the QRS complex is wider and different from the native QRS complexes. An irregular rhythm due to ectopy helps differentiate PVCs from a consistent LBBB pattern.

Prognosis

The prognosis of LBBB depends on patient characteristics and underlying heart disease. LBBB generally carries a benign prognosis in asymptomatic individuals without underlying heart disease. However, studies have reported an increased risk of incident heart failure and cardiovascular mortality in asymptomatic individuals with LBBB over long-term follow-up.[16] 

In patients with underlying conditions like CAD, hypertension, or cardiomyopathy, asymptomatic LBBB is generally associated with a worse prognosis compared to those without LBBB, and LBBB can indicate an increased risk of progression to heart failure, arrhythmias, and increased mortality. Patients with LBBB and CAD have more adverse outcomes than patients with CAD without LBBB.

LBBB has a 2.9 times increased risk of cardiovascular mortality and a 1.4 times increased risk for all-cause mortality. Patients with LBBB who undergo nuclear testing for suspected CAD have a 50% increased risk of all-cause mortality. In patients with heart failure, the presence of an LBBB is associated with increased cardiovascular outcomes and mortality.

However, a recent study that attempted to isolate LBBB's sole contribution to outcomes found that, if contributions of confounders were excluded, LBBB contributed far more modestly to poor outcomes, likely because this rhythm disturbance is more of a symptom of dilated cardiomyopathy as opposed to a causative agent in the progression of the disease.[17] Patients with LBBB are restricted from piloting aircraft in the United States and the United Kingdom, as LBBB is a possible precursor to a complete atrioventricular block.[18]

Complications

Complications of LBBB include:

• Worsening heart failure
• Left ventricular dysfunction
• Tachyarrhythmias, such as atrial fibrillation or ventricular tachycardia
• Progression to complete heart block
• Delayed diagnosis in AMI

Prompt treatment and regular monitoring of affected individuals can reduce the risk of these complications.