Left bundle‐branch block—pathophysiology, prognosis, and clinical management

Abstract

Summary: Given its broad use as a screening tool, the electrocardiogram (ECG) has largely become one of the most common diagnostic tests performed in routine clinical practice. As a result, the finding of left bundle-branch block (LBBB) in the absence of a well-defined clinical setting has become relatively frequent and raises questions and often concerns. While in the absence of clinically detectable heart disease LBBB does not necessarily imply poor outcomes, physicians should be aware of the role of LBBB in stratifying risk of cardiovascular events and death in subjects with both ischemic and nonischemic heart disease. This paper reviews historical landmarks, pathophysiologic features, prognostic implications, and clinical management of LBBB in apparently healthy subjects and those with heart disease. Evolving Concepts, Misunderstandings, and Current Appraisal of Left Bundle-Branch BlockAs early as the beginning of the past century, Eppinger and Tothberger, by means of a rudimental but efficient experimental model, performed experiments destroying pieces of dog myocardium by injecting silver nitrate and then observing the induced electrocardiographic (ECG) changes. 1 By means of a single esophageal-anal lead, these and other investigators found that injuring the left and right bundle branches resulted, respectively, in an upward and a downward QRS deflection on ECG. 2 Ironically, the mere extrapolation of data obtained from this experimental canine model resulted in a 25-year misunderstanding of the real electrical abnormalities. Left bundle-branch block (LBBB) pattern was incorrectly identified as right bundle-branch block (RBBB), and vice versa. In fact, since the esophageal-anal lead was erroneously judged to be "vertical" in the dog, the presence in humans of a wide downward deflection in leads II and III was considered to disclose RBBB. 3 Almost 70 years after elucidation of this long-lasting misinterpretation, the electrogenesis and ECG pattern of LBBB appear to be fully clarified. Under normal conditions, the electrical impulse from the His bundle passes through a narrow anterior fascicle, a broader early branching posterior fascicle, and a third septal segment composed of many branches originating from each of the fascicles. The electrical impulse then spreads through a rich peripheral Purkinje network that couples with individual myocardial cells. 4,5 The simultaneous electrical activation of the right ventricle from its own branch results in the QRS complex, which then represents the "sum" of two parallel and independent electrical phenomena. Left bundle-branch block completely modifies the electrical activation of the left ventricle and QRS complex on ECG. The activation of the interventricular septum, which is left-sided in physiologic conditions, originates on its right side. The electrical impulse propagates then inferiorly, to the left, and slightly anteriorly. This results in a nonhomogeneous and delayed depolarization of the left ventricle, which can be only partially preserved in the presen...

Clinical Impact of Persistent Left Bundle-Branch Block After Transcatheter Aortic Valve Implantation With CoreValve Revalving System

“…8 Although definitive explanations are lacking, it has been hypothesized that this effect could be a consequence of subsequent impaired left ventricle contractility, dyssynchrony, worsened mitral regurgitation, and higher risk of progression to complete atrioventricular (AV) block and sudden death. 9 Whether this specific conduction disturbance could have an ominous impact in patients undergoing TAVI is still unclear, because limited data have specifically focused on this topic so far. We thus sought to evaluate, in a multicenter registry of patients treated in high-volume centers, the prognostic significance of persistent LBBB after TAVI with the CoreValve Revalving System (CRS).…”

Cardiac ‘resynchronization’ by a left lateral accessory pathway

“…Clinical and echocardiographic follow-up of LBBB is mainly related to dilated cardiomyopathy and heart failure. However, there are some epidemiological studies that provide evidence of a worse outcome in a population affected with LBBB [2]. Perhaps in this case biventricular depolarization will prevent mechanical dyssynchrony and in the future an evolution to a dilated cardiomyopathy by ventricular remodeling [3][4][5].…”

Relationship between electrocardiographic characteristics of left bundle branch block and echocardiographic findings

“…CLBBB may precede the onset of heart disease for years, leading some investigators to consider it a possible precursor to latent cardiomyopathy [2]. Although some studies have suggested that CLBBB may have a benign prognosis [7,8], most have indicated that CLBBB has poorer prognosis in comparison to normal intraventricular conduction and also compared to right bundle branch block, with a median survival of 5 years from the time of diagnosis [2,4,5,7,[9][10][11][12]. Furthermore, the electrocardiography (ECG) characteristics of CLBBB obscure the application of common criteria for the diagnosis of left ventricular hypertrophy (LVH), myocardial infarction (MI) in the chronic phase, and diseases that cause modifications on the ST-T segment, establishing a need for other studies to assess these heart diseases in the presence of CLBBB [13][14][15][16][17].…”