Bundle Branch Block, Right

 

INTRODUCTION

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.

Embryology

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.

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 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.

 

Natural history

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.

Frequency:

  • In the US: The most common cause of RBBB in children is surgery associated with repair of an isolated ventricular septal defect or another form of congenital heart disease that includes a ventricular septal defect, eg, double-chambered right ventricle, AV canal, or tetralogy of Fallot. Incidence of RBBB ranges from 25-81% for repair of a ventricular septal defect alone to 60-100% for repair of tetralogy of Fallot. The variation of RBBB after surgery is likely due to proximity of the ventricular septal defect to the His-Purkinje system and surgical technique; for example, RBBB is less common in a transatrial repair of a ventricular septal defect or exclusion ventriculotomy.

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.

CLINICAL

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. In addition, one should always evaluate for findings consistent with postoperative heart disease, such as murmurs or a thoracotomy scar, pectus.

Causes:

  • Inheritance: Most cases of RBBB in children occur after intracardiac surgery. Hereditary RBBB has been observed in 4 Lebanese families and has been mapped to chromosome 19. A subset of patients with Brugada syndrome have mutations in SCN5A, the gene encoding for the voltage-gated cardiac sodium channel.
  • Risk factors:
    • RBBB in children is most commonly seen after congenital heart surgery associated with repair of a ventricular septal defect. It has been associated with cardiomyopathy, myocarditis, congestive heart failure, atrial septal defect, Ebstein anomaly, and postoperative cardiac transplantation.
    • A transient form of RBBB may also 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.
  • RBBB may be associated with certain syndromes such as the following:
    • 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.
    • Myotonic dystrophy is characterized by muscular dystrophy, myotonias, hypogonadism, frontal balding, and cataracts. The congenital form presents with neonatal hypotonia, paresis, and myotonia that occurs 5 or more years later. 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 experience arrhythmias and/or Stokes-Adams attacks.
    • 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 the development of heart block and sudden death. Rarely, patients present with dilated cardiomyopathy and heart failure.
    • Brugada syndrome is a channelopathy mediated by the SCN5A gene. The RBBB pattern seen in these patients is not actually RBBB but is a function of the unusual repolarization abnormality seen in these patients. The ECG also manifests ST segment elevation in leads V1 through V3 and in patients are at risk for sudden cardiac death.
    • Patients may also have isolated RBBB or RBBB with a left anterior fascicular block.
    • RBBB has been associated with blunt chest trauma and polymyositis.

DIFFERENTIALS

Bundle Branch Block, Left


Other Problems to be Considered:

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


WORKUP

Lab Studies:

  • Serum: If myocarditis or cardiomyopathy seem to be reflected in the RBBB pattern, consider troponins, creatine kinase, erythrocyte sedimentation rate, and other cardiomyopathy laboratory tests (eg, carnitine).
  • Standard ECG: Ambulatory ECG (ie, Holter monitoring) may be indicated in postoperative patients who are being monitored periodically or in patients with a new RBBB that has a poorly understood etiology and clinical impact.

Imaging Studies:

  • An echocardiogram or myocardial perfusion studies may be indicated if the RBBB is new or if concern exists about coexisting cardiac problems that cannot be assessed adequately by means of a routine ECG. For example, the standard criteria for ventricular hypertrophy or ischemia are less helpful in a patient with a preexisting intraventricular conduction defect.

Other Tests:

  • The ECG findings in RBBB reflect the underlying pathophysiology. Transmission of the electrical impulse via the left bundle is normal, resulting in normal depolarization of the septum and left ventricle. This creates the initial R wave in lead I and V1 and the Q wave in V6. The electrical impulse in the right bundle branch is either delayed or fails to conduct. 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.
  • The ECG findings necessary to confirm the diagnosis of RBBB include the following:
    • For complete RBBB, the duration of the QRS complex is prolonged for the patient's age. The maximum QRS duration for newborns younger than 6 days is 0.07 seconds, for patients aged 1 week to 7 years is 0.08 seconds, and for patients aged 7-15 years is 0.09 seconds. In incomplete RBBB, an rSR' pattern is seen in lead V1, but the QRS duration is within the normal limits.
    • An rSR' or rR' pattern, with the initial r wave less than the R' or r', may be seen in leads V1 through V3R (see Image 3). The initial R wave represents septal activation, the S wave represents left ventricular activation, and the R' represents activation of the right ventricle from the septum and left ventricle.
    • The S wave, which represents left ventricular activation, is wide in leads I and V6 (see Image 6).
    • The QRS axis may be normal, or right or left axis deviation may be present.
    • The T wave is almost always inverted in lead V1 and may be inverted in V2. In the other precordial leads and in the limb leads, the T wave is directed opposite to the terminal portion of the QRS complex.

Procedures:

  • Invasive cardiac catheterization and electrophysiology testing may be necessary in patients at risk for heart block or ventricular tachycardia and for documenting cardiomyopathy, myocarditis, or coexisting hemodynamic abnormalities.

TREATMENT

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.

FOLLOW-UP

Further Inpatient Care:

  • Inpatient ECG telemetry may be required in cases of RBBB in which the risk for significant arrhythmia is suspected (see Natural history).

Further Outpatient Care:

  • Individuals with unoperated or postsurgical heart disease and RBBB should have an ECG performed annually to evaluate for interval changes that may reveal progression of the underlying conduction defect or other potential rhythm abnormalities (eg, sinus bradycardia, supraventricular or ventricular ectopy).

Complications:

  • If RBBB progresses to heart block, the patient may experience fatigue, exercise intolerance, dizziness, syncope, or sudden death.
  • Some patients who have undergone tetralogy of Fallot repair and have a RBBB pattern and a markedly prolonged QRS duration may be at higher risk for ventricular tachycardia or sudden death.

Prognosis:

  • The prognosis for patients with isolated RBBB is excellent because the course of RBBB generally is benign.
  • In other patients with RBB, such as those who have undergone a heart transplantation or repair of congenital heart disease and those with Brugada syndrome, Kearns-Sayre syndrome, myocarditis, or cardiomyopathy, the prognosis depends on the underlying condition causing the RBBB.

Patient Education:

  • Patients, parents, and primary medical providers should be informed that RBBB per se does not typically predict specific future problems or management concerns.
 

MISCELLANEOUS

Medical/Legal Pitfalls:

  • Although the risk of RBBB progressing to complete heart block and sudden death is low, patients require workup and consultation by a pediatric cardiologist. Failure to have the child properly evaluated could result in an unnecessary death and the legal ramifications resulting from such a tragedy.

PICTURES

 
Caption: Picture 1. ECG tracing demonstrating normal sinus rhythm and sinus rhythm with right bundle branch block.
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Picture Type: Rhythm Strip
Caption: Picture 2. Bundle branch block, right. Anatomy of the penetrating portion of the atrioventricular (AV) bundle.
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Caption: Picture 3. ECG depiction of electrophysiologic events of right bundle branch block.
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Caption: Picture 4. Pathophysiology of right bundle branch block.
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Caption: Picture 5. Bundle branch block, right. ECG depiction of electrophysiologic sequence of events that occur in normal cardiac conduction.
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Picture Type: Image
Caption: Picture 6. Bundle branch block, right. ECG demonstrating wide S wave.
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Picture Type: Image