AUTHOR INFORMATION	Section 1 of 10
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Author: Christopher Mart, MD, Associate Professor, Pediatric Echocardiography, Department of Pediatrics, Division of Pediatric Cardiology, University of Utah

Christopher Mart, MD, is a member of the following medical societies: American College of Cardiology, American Society of Echocardiography, and Society of Pediatric Echocardiography

Editor(s): Christopher Johnsrude, MD, Associate Professor of Pediatrics, Director of Electrophysiology, University of Louisville School of Medicine; Consulting Staff, Pediatric Cardiology Associates, PSC; Mary L Windle, PharmD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy, Pharmacy Editor, eMedicine.com, Inc; Alvin J Chin, MD, Professor of Pediatrics, University of Pennsylvania School of Medicine, Department of Pediatrics, Division of Cardiology, The Children's Hospital of Philadelphia; Gilbert Herzberg, MD, Assistant Professor, Department of Pediatrics, Section of Pediatric Cardiology, New York Medical College; and Steven R Neish, MD, SM, Director of Pediatric Cardiology Fellowship Program, Department of Pediatrics, Baylor College of Medicine, Clinical Director of Pediatric Cardiology, Texas Children's Heart Center, Director, Brown Foundation Heart Clinic, Texas Children's Hospital

Disclosure

	INTRODUCTION	Section 2 of 10
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Background: Left bundle branch block (LBBB) occurs when transmission of the cardiac electrical impulse is delayed or fails to be conducted along the rapidly conducting fibers of the main left bundle branch or in both left anterior and posterior fascicles. Thus, the left ventricle depolarizes slowly by means of cell-to-cell conduction that spreads from the right ventricle to the left ventricle. This results in the characteristic ECG pattern shown in Image 1.

Pathophysiology: Knowledge of the anatomy and electrophysiology of the cardiac conduction system from the atrioventricular (AV) junction to the distal Purkinje fibers is essential to understanding the pathophysiology of LBBB.

Embryology

The cardiac conduction system develops from rings of specialized tissue found in the embryonic heart tube. One theory describes 4 rings, each located between different segments of the heart tube. With looping and growth of the cardiac septi, the rings are brought together and develop into the sinus node, the AV node, and the penetrating bundle. Another theory describes a single ring of tissue located between the bulbus cordis and the primitive ventricle, which gives rise to the AV node, His bundle, right bundle branch, and left bundle branch.

Anatomy

The specialized conduction system of the heart is composed of cells that conduct electrical impulses faster than the surrounding myocardium. The conduction system can be divided into distinct anatomic segments, and each segment is described in sequence beginning at the AV junction and ending with the Purkinje fibers.

The AV junction can be divided into 3 regions as follows: transitional cell zone, AV node, and penetrating portion of the AV bundle (His bundle, common bundle).

The transitional cell zone is where the right atrium merges with the compact AV node by means of discrete atrial pathways termed the slow and fast pathways.

The next segment is the AV node, which lies anterior and superior to the ostium of the coronary sinus, directly above the insertion of the septal leaflet of the tricuspid valve. This area is located at the apex of the triangle of Koch, which is formed by the tricuspid annulus, the tendon of Todaro, and the ostium of the coronary sinus. Blood supply to the AV node is derived from the AV nodal artery, which is a branch of the right coronary artery in 85-90% of individuals and a branch of the left circumflex coronary artery in 10-15% of individuals.

At the apex of the triangle of Koch, the compact AV node becomes the penetrating bundle of His. It penetrates the central fibrous body at the attachment of the tendon of Todaro, runs between the membranous septum and the muscular septum, and bifurcates at the crest of the muscular ventricular septum. The His bundle is divided into 3 anatomic segments. The proximal, or nonpenetrating, segment lies distal to the AV node and proximal to the central fibrous body. The middle, or penetrating, segment penetrates the central fibrous body and runs posterior to the membranous septum. The distal, or branching, segment bifurcates at the crest of the muscular septum into the right and left bundle branches (see Image 2).

The right bundle branch, a direct continuation of the penetrating bundle, originates distal to the attachment of the septal leaflet of the tricuspid valve with the membranous septum and surfaces on the right ventricular septum just below the papillary muscle of the conus. It is unbranched and proceeds toward the apex of the right ventricle along the posterior margin of the septal band, courses through the moderator band to the base of the anterior papillary muscle, and proceeds to the right ventricular free wall.

The left bundle branch originates at the crest of the muscular ventricular septum just distal to the membranous septum. It arises in a fanlike fashion and descends inferiorly along the left ventricular septal surface beneath the noncoronary cusp of the aortic valve. The left bundle branch usually branches into 3 major fascicles. The anterior fascicle is directed to the base of the anterolateral papillary muscle, the posterior fascicle is directed to the base of the posteromedial papillary muscle, and, in 60% of hearts, a central fascicle proceeds to the midseptal region. When no central fascicle is present, as in 40% of hearts, the midseptal region is supplied by radiations from the anterior fascicle or the anterior and posterior fascicles.

At the terminal aspect of each bundle branch, Purkinje fibers are interlaced on the endocardial surface of both ventricles and tend to be concentrated at the tips of the papillary muscles.

Electrophysiology of cardiac conduction

The heart is a 2-step mechanical pump coordinated by precisely timed electrical impulses. For the pump to perform optimally, sequential depolarizations of the atria and then the ventricles allow atrial contraction to provide complete diastolic filling of the ventricles (AV synchrony). After the ventricles are filled, rapid activation of the ventricular myocardium permits a synchronized contraction to eject blood most effectively to the great vessels.

Normal cardiac conduction

In normal cardiac conduction, electrical excitation of the heart proceeds in a sequential manner from the atria to the ventricles and is demonstrated on the surface ECG (see Image 3). The electrical impulse generated in the sinus node proceeds through the atria (reflected by the P wave on the ECG) to reach the AV node. As the impulse conducts through the AV node, conduction slows, allowing time for atrial contraction to occur before the ventricle is activated (PR segment). After the impulse passes through the compact AV node, it is rapidly conducted through the crux of the heart to the ventricles by means of the bundle of His (penetrating bundle) to the branching bundle, the bundle branches, the distal Purkinje fibers, and finally the ventricular myocardial cells (narrow QRS complex). When depolarization is complete, the ventricle repolarizes in preparation for conducting another impulse.

Types of LBBB

Complete LBBB occurs when the electrical impulse is delayed or interrupted in either the main left bundle branch or in both the anterior and posterior fascicles. Conduction down the right bundle branch proceeds normally, and the right ventricle depolarizes in the normal fashion. In complete LBBB, conduction from the right ventricle passes first to the interventricular septum, then to the anterior and posterior portions of the left ventricle, and finally to the left lateral free wall. Delayed left ventricular depolarization accounts for the ECG findings in LBBB (see Images 4-5).

Incomplete LBBB occurs in 2 forms, each called hemiblock. In left anterior hemiblock (LAH), transmission of the electrical impulse proceeds normally along the main left bundle branch and the posterior fascicle, but it is blocked or delayed in the anterior fascicle. This blockage results in delayed activation of the anterior portion of the left ventricle. In LAH, the duration of the QRS complex may be of normal or only slightly prolonged duration because of normal rapid conduction down the right and left main bundle and the left posterior fascicle. In addition, the QRS complex is directed superiorly in the frontal plane. This is called left axis deviation, although the term superior axis deviation most accurately describes the finding. Furthermore, QRS axis is normally to the left; therefore, the term left axis deviation makes little semantic sense.

With left posterior hemiblock, transmission of the electrical impulse proceeds normally along the main left bundle branch and the anterior fascicle, but it is blocked in the posterior fascicle. This blockage results in delayed activation of the posterior left ventricle. The QRS complex is again of normal or only slightly prolonged duration, and it inscribes a rightward axis in the frontal plane. Left posterior hemiblock is rarely observed in children, and diagnosis is difficult because of the common association of right axis deviation in children with congenital heart disease and right ventricular hypertrophy.

Frequency:

• In the US: LBBB in children is associated with cardiovascular disease or surgery, and it is not observed in the general population. LBBB may be present in as many as 20% of individuals after aortic valve replacement.

Mortality/Morbidity: LBBB alone may rarely progress to complete heart block and sudden death, but morbidity and mortality rates depend on the associated systemic or cardiovascular disease more than the LBBB itself. Patients with LBBB, left axis deviation, and first-degree heart block or LBBB associated with near-syncope or syncope require close follow-up and/or electrophysiologic study. In adults, LBBB can disturb coronary perfusion of the left anterior descending coronary artery by shortening the duration of diastolic flow.

	CLINICAL	Section 3 of 10
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History: Important points to cover when one obtains a history from a child with LBBB include known congenital heart disease and/or previous cardiac surgery. Questions regarding fatigue, exercise intolerance, presyncope, and syncope may further indicate the clinical significance of the LBBB.

Physical: On physical examination, auscultatory findings in patients with LBBB include an absent or diminished first heart sound and reversed splitting of the second heart sound. That is, the second heart sound is split in expiration and single in inspiration.

Causes: LBBB in children is not a benign entity.

• Risk factors for LBBB

• LBBB is associated with anatomic malformations and abnormalities of the conduction system.

• LBBB has been observed after surgery in the left ventricular outflow tract, septal myomectomy, replacement of the aortic valve, and transcatheter closure of perimembranous ventricular septal defects.

• LBBB is also observed in patients with left ventricular hypertrophy, left ventricular noncompaction and neuromuscular disease, progressive conduction system disease, myocarditis, cardiomyopathy, hemochromatosis, sclerodegenerative diseases, myocardial infarction, aortic valve endocarditis, rheumatic fever with aortic valve involvement, perinatal exposure to HIV type 1, and Wolff-Parkinson-White Syndrome when the abnormal conduction pathway enters the right ventricle. (See also Supraventricular Tachycardia, Wolff-Parkinson-White Syndrome.)

• The LBBB pattern, or rather, left ventricular conduction delay, also occurs in patients with Wolff-Parkinson-White Syndrome in whom the abnormal conduction pathway enters the right ventricle. (See also Supraventricular Tachycardia, Wolff-Parkinson-White Syndrome.)

• Risk factors for LAH

• LAH has been associated with coronary artery disease, left ventricular hypertrophy, cardiomyopathy, tetralogy of Fallot repair, ventricular septal defect repair, septal myomectomy, subvalvar aortic resection, and an anomalous origin of the left coronary artery from the pulmonary artery.

• LAH may be present in patients with autosomal dominant bundle branch disease, and it has been associated with lentiginosis.

• Among congenital heart defects LAH is characteristic of endocardial cushion defects, such as ostium primum atrial septal defect complex and complete AV canal, an abnormality frequently seen in patients with Down syndrome. LAH occurs in endocardial cushion defects because of congenital absence or hypoplasia of the left anterior division.

• Patients with endocardial cushion defects (ostium primum ASD, complete AV canal), tricuspid atresia, double-outlet right ventricle, and certain forms of a functional single ventricle have LAH that may reflect absent, hypoplastic, or abnormally coursing left bundle fascicles rather than an anterior hemiblock per se.

• Risk factors for left posterior hemiblock: Left posterior hemiblock is observed after congenital heart surgery and in patients with congenital aortic stenosis, endocarditis, or diphtheritic myocarditis.

	DIFFERENTIALS	Section 4 of 10
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Bundle Branch Block, Right
Supraventricular Tachycardia, Wolff-Parkinson-White Syndrome
Ventricular Tachycardia

Other Problems to be Considered:

Intraventricular conduction delay
Left ventricular hypertrophy
Wolff-Parkinson-White syndrome
Premature ventricular complexes
Isorhythmic idioventricular rhythm
Supraventricular tachycardia with bundle branch aberrancy
Ventricular paced rhythm