Left Bundle Branch Block

Left bundle branch block (LBBB) is an intraventricular conduction abnormality usually caused by ischemic or mechanical factors affecting the cardiac conduction system's left bundle branch. The condition can be seen in association with structural heart diseases, such as ischemic or valvular heart disease, and cardiomyopathies, including dilatation, hypertrophic, fibrotic, or infiltrative cardiomyopathies. LBBB may also be caused by Lev or Lenègre disease. A complete LBBB results in an abnormal left ventricular activation sequence and diffuse slowing of cardiac conduction. LBBB appears as a widened QRS complex with certain characteristic features on electrocardiography, described below.  The atrioventricular bundle is in the triangle of Koch, a space located in the base of the right atrium, which is bounded by the septal leaflet of the tricuspid valve, tendon of Todaro, and coronary sinus orifice. The His bundle is a cylindrical fascicle connecting the atrioventricular node to the bundle branches. The LBB emerges from a triangle at the inferior border of the membranous septum, between the right and noncoronary cusps of the aortic valve. The LBB receives its blood supply from branches of the left anterior descending and posterior descending arteries, with the latter diverging from the right coronary artery in 85% to 90% of people. The LBB trunk has a length of 9 mm and a diameter of 5 mm at the start and 9 mm at the end (reverse trapezoid). This segment then divides into 3 fiber groups: the left anterior, posterior, and septal fascicles.   The clinical significance of LBBB in asymptomatic individuals is controversial. LBBB often appears in the setting of other cardiovascular diseases, such as hypertension, coronary artery disease, or cardiomyopathy. This condition may also occur incidentally in otherwise healthy individuals, particularly those of advanced age. The presence of this rhythm abnormality in middle-aged and older individuals is associated with an increased risk of adverse cardiovascular events, such as heart failure, sudden cardiac death, and cardiovascular mortality. LBBB is a predictor of worse outcomes in patients with heart failure, including recurrent hospitalization and mortality. The presence of LBBB in different cardiomyopathies confers adverse outcomes. The Rotterdam Study, a large population-based investigation, found that asymptomatic, older individuals with LBBB had a higher risk of developing heart failure and experiencing cardiovascular death compared to people without LBBB. LBBB can develop gradually or suddenly. New-onset LBBB in the appropriate clinical context is considered an ST-segment elevation equivalent. In some individuals, particularly those without significant underlying heart disease, LBBB may remain stable for many years without causing symptoms or complications. In cases of structural heart disease, the presence of LBBB can exacerbate dyssynchrony in ventricular contraction, further impairing cardiac function. LBBB is associated with an increased risk of developing arrhythmias, such as atrial fibrillation or ventricular tachycardia. The natural history of LBBB varies widely depending on the presence and progression of underlying cardiovascular conditions. While some individuals with LBBB may remain asymptomatic with a stable course, others may experience worsening cardiac function, arrhythmias, and heart failure, increasing their mortality risk. Regular monitoring and appropriate management, including the potential use of therapies like cardiac resynchronization therapy, are key to improving outcomes in patients with LBBB.