Pediatric Left Bundle Branch Block
- Author: Bahram Kakavand, MD, FACC; Chief Editor: Stuart Berger, MD more...
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.[1] Thus, the left ventricle slowly depolarizes 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 the image below.
ECGs show a normal sinus rhythm and a sinus rhythm with a left bundle branch block. 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 left bundle branch block.
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 septa, 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 has traditionally been 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. In the past, the slow and fast pathways were believed to be parallel. However, data from radiofrequency ablations of the AV nodal and AV reentrant tachycardias have demonstrated that the slow pathway is more closely associated with anterior-superior aspect of the os of the coronary sinus. Furthermore, the data show the fast pathway is located slightly superior and posterior to the AV node.
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 the image below).
Anatomy of the penetrating portion of the atrioventricular (AV) bundle. 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 the image below).
ECG depicts electrophysiologic events in normal cardiac conduction. AV = atrioventricular. 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 left bundle branch block
Complete left bundle branch block 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 normally proceeds, and the right ventricle depolarizes in the normal fashion. In complete left bundle branch block, 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 left bundle branch block (see the images below).
Pathophysiology of left bundle branch block. AV = atrioventricular; LV = left ventricular; RV = right ventricular. 
ECG depicts electrophysiologic events of left bundle branch block. Incomplete left bundle branch block 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 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.
Epidemiology
Frequency
United States
Left bundle branch block in children is associated with cardiovascular disease or surgery and is not observed in the general population. Left bundle branch block may occur in as many as 20% of individuals after aortic valve replacement.
Mortality/Morbidity
Left bundle branch block 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 left bundle branch block itself. Patients with left bundle branch block, left axis deviation, and first-degree heart block or left bundle branch block associated with near-syncope or syncope require close follow-up and/or electrophysiologic study. In adults, left bundle branch block can disturb coronary perfusion of the left anterior descending coronary artery by shortening the duration of diastolic flow.
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