AUTHOR INFORMATION	Section 1 of 10
Author Information Introduction Clinical Differentials Workup Treatment Follow-up Miscellaneous Pictures Bibliography

Author: Christopher Mart, MD, Associate Professor, Pediatric Echocardiography, Department of Pediatrics, Division of Pediatric Cardiology, University of Utah Coauthor(s): Christopher Zachary, MD, Consulting Pediatric Cardiologist, Private Practice; Kerry Rosen, MD, Assistant Professor, Department of Pediatrics, Pennsylvania State University; Director of Echocardiography, Department of Pediatrics, Milton S Hershey Medical Center

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: Right bundle branch block (RBBB) occurs when transmission of the electrical impulse is delayed or not conducted along the right bundle branch. Thus, the right ventricle depolarizes by means of cell-to-cell conduction that spreads from the interventricular septum and left ventricle to the right ventricle. This results in the characteristic ECG pattern (together with a normal ECG) shown in Image 1.

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

RBBB occurs when the electrical impulse from the bundle of His does not conduct along the right bundle branch. Conduction down the left bundle branch proceeds normally, and the interventricular septum and left ventricle depolarize rapidly in the normal fashion. Depolarization of the right ventricle occurs later and is comparatively slow, accounting for the ECG findings in RBBB (see Images 1, 3-4).

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 of His, 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 that 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.

For a discussion on the anatomy of subtypes, see Types of RBBB.

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 5). The electrical impulse is generated in the sinus node and proceeds along proposed internodal conduction pathways 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 RBBB

Three types of RBBB have been identified in electrophysiologic studies. Proximal, or central, RBBB occurs when a conduction block is present just distal to the bundle of His in the superior aspect of the right bundle branch. This generally occurs when the proximal bundle is injured during surgery for lesions with an inlet or membranous ventricular septal defect (VSD). Another type of RBBB occurs when the impulse is interrupted between the proximal and distal aspects of the right bundle branch; this type is most commonly observed after surgical division of the moderator band. Distal RBBB is observed when distal ramifications of the right bundle are disrupted during right ventriculotomy or resection of muscle bundles in the right ventricular outflow tract. Regardless of the type of RBBB, the ECG patterns remain similar.

Natural history

In general, surgically induced RBBB results in no clinically significant acute hemodynamic consequences and has a benign course over the long term. In rare cases, a progression to complete heart block and sudden death is a concern, particularly if the RBBB pattern is accompanied by additional evidence of substantial injury to the His-Purkinje system (eg, left anterior hemiblock, first-degree AV block). Patients who have undergone repair for tetralogy of Fallot and who have an RBBB pattern with a markedly prolonged QRS duration (>180 ms) may be at increased risk for important ventricular arrhythmias and sudden death. Patients with RBBB from other causes may have diverse natural histories depending on the underlying disease. The outcome may be benign in some forms of familial RBBB, or sudden death may result if the RBBB pattern on ECG is due to Brugada syndrome or Kearns-Sayre Syndrome.

Frequency:

• In the US: The most common cause of RBBB in children is surgery associated with repair of an isolated VSD or another congenital heart disease that includes a VSD (eg, double-chambered right ventricle, AV canal, or tetralogy of Fallot). The incidence of RBBB ranges from 25-81% after repair of a VSD alone to 60-100% after repair of tetralogy of Fallot. The variation of RBBB after surgery is likely due to the proximity of the VSD to the His-Purkinje system, as well as the surgical technique. For example, RBBB is less common with transatrial repair or exclusion ventriculotomy repair of a VSD than with other procedures.

Mortality/Morbidity: Surgically induced RBBB generally results in no clinically significant acute hemodynamic consequences, and it has a benign course over the long term. In rare cases, if RBBB is associated with injury to the His-Purkinje system (eg, left anterior hemiblock, first-degree AV block), it can progress to complete heart block and sudden death.

Patients who have undergone tetralogy of Fallot repair and have a QRS duration >180 ms may be at risk for ventricular arrhythmias and sudden death.

Patients with familial RBBB may have a benign course, whereas those with RBBB and Brugada syndrome or Kearns-Sayre Syndrome are at risk for sudden death.

Age: Surgical repair of tetralogy of Fallot, in addition to closing the VSD, is often associated with a transannular patch of the right ventricular outflow tract. This situation often results in clinically significant pulmonic valve insufficiency and progressive right ventricular dilatation as the patient ages. In addition, some patients have residual stenosis at various levels in the pulmonary circulation. By the time the patient is in his or her late teens or young adulthood, the right ventricle has been subjected to years of abnormal hemodynamics. Patients with RBBB and a markedly prolonged QRS duration (>180 ms) may be at increased risk for ventricular tachycardia and sudden death.

	CLINICAL	Section 3 of 10
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History: The history in children with RBBB should include the following:

• History of congenital heart disease

• History of cardiac surgery

• History of palpitations, general energy and activity level, exercise tolerance, dizziness, and/or syncope

• Family history of known arrhythmias, including bundle branch block, complete heart block, and placement of a pacemaker or defibrillator

• Family history of premature or sudden unexplained death, myocardial infarction in individuals younger than 45 years, syncope, seizures, or fetal loss

Physical: On physical examination, patients with RBBB have a persistently split second heart sound with normal respiratory variation in the splitting interval. In addition, one should always evaluate for findings consistent with postoperative heart disease, such as murmurs or a thoracotomy scar, pectus.

Causes:

• Hereditary factors

• Hereditary RBBB was observed in 4 Lebanese families and has been mapped to chromosome 19.

• A subset of patients with Brugada syndrome has mutations in SCN5A, the gene encoding for the voltage-gated cardiac sodium channel.

• Risk factors

• In children, most cases of RBBB occur after intracardiac surgery, such as congenital heart surgery associated with repair of a VSD and cardiac transplantation. LBBB has also been described in patients undergoing transcatheter closure of perimembranous VSDs.

• LBBB has been associated with cardiomyopathy, myocarditis, congestive heart failure, atrial septal defect (ASD), and Ebstein anomaly.

• A transient form of RBBB may be observed in patients with premature atrial contractions or supraventricular tachycardia. This occurs when an early impulse is conducted from the AV node to the His bundle while the right bundle branch is still refractory but the left bundle is not. Conduction down the right bundle branch is therefore delayed or blocked, resulting in a transient RBBB pattern on the ECG.

• Right anterior hemiblock is described in children with perinatal exposure to HIV type 1.

• Associated syndromes

• Duchenne muscular dystrophy is an X-linked myopathy characterized by early onset and rapid progression with muscular weakness and pseudohypertrophy seen in the second year of life. Cardiac findings include mitral valve prolapse, pulmonary flow murmur, and an S3 or S4 gallop. (See also Muscular Dystrophy.)

• Myotonic dystrophy is characterized by muscular dystrophy, myotonias, hypogonadism, frontal balding, and cataracts. Congenital Muscular Dystrophy manifests with neonatal hypotonia, paresis, and myotonia that occurs after 5 or longer. The adult form of myotonic dystrophy is the most common muscular dystrophy seen in adults. ECG findings may include first-degree AV block, left anterior fascicular block, and intraventricular conduction delay. Patients may have arrhythmias and/or Stokes-Adams attacks. (See also Muscular Dystrophy.)

• Kearns-Sayre Syndrome is a mitochondrial myopathy with the physical findings of ptosis, chronic progressive external ophthalmoplegia, and abnormal retinal pigmentation. Patients are at risk for heart block and sudden death. Rare patients present with dilated cardiomyopathy and heart failure.

• Brugada syndrome is a channelopathy mediated by the SCN5A gene. The RBBB pattern seen in patients with this syndrome is not actually RBBB but is a function of their unusual repolarization abnormality. The ECG shows ST-segment elevation in leads V1-V3, and patients are at risk for sudden cardiac death. Use of cocaine consumption or the antiarrhythmic propafenone may unmask ECG findings consistent with Brugada syndrome.

• Patients may have isolated RBBB or RBBB with a left anterior fascicular block.

• RBBB has been associated with blunt chest trauma and polymyositis.

	DIFFERENTIALS	Section 4 of 10
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Bundle Branch Block, Left

Other Problems to be Considered:

Interventricular conduction delay
Right bundle branch aberrancy (premature atrial contractions, supraventricular tachycardia)
Premature ventricular contractions, other ventricular arrhythmia
Paced ventricular beat
Brugada syndrome