Background: Right bundle branch block (RBBB) occurs when transmission of the electrical impulse is delayed or fails to conduct along the right bundle branch. Thus, the right ventricle depolarizes via cell-to-cell conduction spreading from the interventricular septum and left ventricle to the right ventricle. This results in the characteristic ECG pattern shown in Image 1. 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.
The cardiac conduction system develops from rings of specialized tissue found in the embryonic heart tube. One theory describes 4 different 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.
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 separate 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 via 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. The blood supply to the AV node is 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, which 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 on 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 (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 right bundle branch block.
Pathophysiology: 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).
Electrophysiology of cardiac conduction
The heart is an electrical pump. In order for the heart to perform effectively, depolarization must occur in a manner that allows the atrial myocardium to contract before the ventricular myocardium. This permits blood to pass from the atria to the ventricles and from the ventricles to the great vessels.
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. Conduction is slowed as the impulse passes through the AV node, allowing atrial activation to occur before ventricular activation. The impulse is conducted from the AV node through the crux of the heart to the ventricles by the bundle of His (penetrating bundle) to the branching bundle, the bundle branches, the Purkinje fibers, and, finally, to the myocardial cells. When depolarization is complete, the ventricle repolarizes in preparation for conducting another impulse.
Types of right bundle branch block
Three types of RBBB have been identified by electrophysiologic studies. Proximal or central RBBB occurs when a conduction block exists just distal to the bundle of His or in the superior aspect of the right bundle branch. This is generally seen when the proximal bundle is injured during surgery for lesions with an inlet or membranous ventricular septal defect. Another type of RBBB occurs when the impulse is interrupted between the proximal and distal aspects of the right bundle branch; this type of RBBB 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.
In general, surgically induced RBBB results in no significant acute hemodynamic consequences and has a benign course over the long term. In rare cases, a concern exists for progression to complete heart block and sudden death, particularly if the RBBB pattern is accompanied by additional evidence of significant injury to the His-Purkinje system (eg, left anterior hemiblock, first-degree AV block). Patients who have undergone tetralogy of Fallot repair and 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 Kearns-Sayre syndrome or Brugada syndrome.
Mortality/Morbidity: Surgically induced RBBB generally results in no significant acute hemodynamic consequences and has a benign course over the long term. Rarely, 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 greater than 180 ms may be at risk for the development of 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 often results in significant pulmonary valve insufficiency and progressive right ventricular dilation as the patient ages. In addition, some patients may have residual stenosis at various levels in the pulmonary circulation. By the time the patient is in his or her late teens or young adult years, the right ventricle has been subjected to years of abnormal hemodynamics; those patients with RBBB and a markedly prolonged QRS duration (>180 ms) may be at higher risk for the development of ventricular tachycardia and sudden death.
History: The history in children with RBBB should include the following:
Physical: On physical examination, patients with RBBB have a persistently split second heart sound. In addition, one should always evaluate for findings consistent with postoperative heart disease, such as murmurs or a thoracotomy scar, pectus.
Bundle Branch Block,
Medical Care: The only treatment required for isolated RBBB is periodic follow-up and evaluation.
Consultations: Children with RBBB should be referred to a pediatric cardiologist for a careful and complete evaluation. If the RBBB is associated with a syndrome, consultation with other appropriate specialists is indicated (eg, an ophthalmologist for Kearns-Sayre syndrome; a geneticist, orthopedic surgeon, and pulmonologist in muscular dystrophy).
Activity: Patients with RBBB are not specifically limited in their activities; however, associated conditions may influence activity restrictions.
Further Inpatient Care:
Further Outpatient Care: