To gain an understanding of pediatric third-degree acquired atrioventricular (AV) block (AVB), one must first understand the concepts behind AV blocks. In general, AV block refers to a conduction delay or interruption of the impulses generated in the atrium before they reach the ventricles. AV block may be transient or permanent, and the anatomic level at which it occurs varies. These features determine the clinical significance of this condition, which may range from none to severely symptomatic, including Stokes-Adams syncope, congestive heart failure, or sudden death.
In children, the most common cause of permanent acquired complete AV block is surgery for congenital heart disease. The second most common cause is congenital heart disease associated with complete AV block. Other etiologies of acquired AV block are often reversible and include digitalis and other drug intoxications, viral myocarditis, acute rheumatic fever, Lyme disease, and infectious mononucleosis. Rarely, complete AV block can occur as a complication of radiofrequency catheter ablation or during a diagnostic or interventional cardiac catheterization procedure (such as device closure of atrial or ventricular septal defects). Metabolic mitochondrial disorders and myotonic dystrophies can cause progressive AV block.
In adults, aside from the etiologies mentioned for the pediatric group, other causes of complete AV block include myocardial infarction (especially inferior), coronary spasm (usually of the right coronary artery), and neoplasms (eg, cardiac mesotheliomas). Almost exclusive to adult patients are 2 degenerative diseases of the specialized conducting system: Lev disease and Lenègre disease.
The following image depicts a normal intracardiac electrophysiologic study.
See also Atrioventricular Block, Pediatric Second-Degree Atrioventricular Block, Pediatric Third-Degree Congenital Atrioventricular Block, First-Degree Atrioventricular Block, Second-Degree Atrioventricular Block, and Third-Degree Atrioventricular Block.
Classification of Atrioventricular Blocks
Depending on the anatomic level at which the atrial impulse is interrupted before reaching the ventricles, the atrioventricular (AV) block (AVB) may be classified as first degree, second degree, or third degree. The term first-degree AV block is actually a misnomer, because nothing is blocked; instead, the conduction time of impulses from the atrium to the ventricles is prolonged (PR prolongation on ECG).
Second-degree AV block occurs either when successive impulses from the atrium to the ventricles are progressively delayed until conduction failure occurs (ie, Mobitz type I second-degree AV block, Wenckebach periodicity) or when an atrial impulse fails to conduct to the ventricles without any measurable lengthening of the preceding PR intervals (ie, Mobitz type II second-degree AV block) (see the first image below). Prolongation of the His-ventricle (H-V) interval may predispose individuals to complete or third-degree AV block (See the second image below).
In complete or third-degree AV block, the conduction of sinus or atrial impulses is entirely interrupted at the level of the AV node, in the bundle of His, or it is associated with an aberrantly located anatomic AV node that precludes normal anatomic communication to the distal conduction tissue. The atria and ventricles depolarize independently of each other, and the ventricular rate is typically slower than the atrial rate. The surface electrocardiograph (ECG) may reveal either narrow or wide QRS configurations, depending on the location of the interruption and the foci of the escape pacemaker tissue. For example, the QRS is narrow if the escape rhythm is supra-His bundle, or it is wide if the escape rhythm is intra-His or infra-His bundle. Escape ventricular rhythms with narrow QRS may be more stable than those with wide QRS morphologies. See the following images.
The atrioventricular (AV) node is composed of specialized cells arranged in 3 zones: (1) the atrial-transitional junction, (2) the transitional-compact node junction, and (3) the nodal-His junction or penetrating His bundle.  These regions display different electrical properties, such as slowing of AV conduction with progressively increasing atrial rates until conduction block occurs, which protects the ventricles from dangerously rapid rhythms during atrial tachycardias. Other properties of the AV nodal cells include their ability to pace the heart by producing "escape rhythms" in case of a block above their anatomic level.  These specialized cells, however, may begin to beat faster than the sinus node and may produce tachycardia.
Complete AV block (AVB) may occur immediately when the AV node is injured by direct trauma during surgery or catheterization, by ischemia during a coronary event, and by myocardial inflammation and infiltration during a viral illness. This condition may also occur years after mediastinal radiation secondary to fibrosis.
Certain congenital defects may also be inherently associated with vulnerability towards developing spontaneous AV block because of anatomic alteration in the arrangement of the AV node to the proximal His-Purkinje conduction tissue. These defects include L-transposition of the great arteries, some forms of double-outlet right ventricle or AV canal defects in the setting of heterotaxy syndrome, and other congenital heart lesions.
Following surgical procedures
Postsurgical complete atrioventricular (AV) block (AVB) is the most common cause for acquired AV block in children, resulting from trauma to the AV node at the time of surgery (ie, hemorrhage, ischemia, necrosis, inflammation, traumatic disruption). This condition may occur following repairs, including those for ventricular septal defect (VSD), L-transposition of the great arteries (especially), tetralogy of Fallot (TOF), aortic stenosis, and other forms of congenital heart disease.  These patients not only have very slow heart rates, but they are also prone to asystole, with a high risk for sudden death if left unpaced.
Transient AV block has also been associated with junctional ectopic tachycardia (JET) following surgery for congenital heart disease.  Risk factors for JET in this patient population included younger age at time of surgery, longer aortic cross clamping time, and surgical procedures in proximity to the AV node. 
Transient and permanent complete AV block has been reported during complicated catheter manipulation in a diagnostic cardiac catheterization or following transcatheter closure of ASD and VSD. 
Complete AV block may occur with systemic infections that cause myocardial inflammation and infiltration, such as diphtheria, Borrelia burgdorferi infection (Lyme disease), Chagas disease, Rocky Mountain spotted fever, Yersinia enterocolitica infection, infectious mononucleosis, bacterial endocarditis, and viral myocarditis.
Diphtheria is an infection of mucous membranes or skin caused by Corynebacterium diphtheriae, of which certain strains produce the diphtheria toxin. This protein can cause myocarditis, polyneuritis, and other systemic effects. Myocarditis occurs in 10-25% of patients with diphtheria and may develop during the acute phase or after several weeks. This consists of varying degrees of heart block, including complete AV block and arrhythmias, such as atrial fibrillation, premature ventricular beats, ventricular tachycardia, and ventricular fibrillation.
Lyme disease is caused by the tick-transmitted spirochete B burgdorferi. It begins with an expanding skin lesion known as erythema migrans. Within several days or weeks, the spirochete spreads hematogenously, and approximately 8% of patients develop cardiac involvement. The most common abnormality is fluctuating degrees of AV block (ie, first-degree, second-degree, and third-degree AV block). Other more diffuse forms of cardiac involvement include left ventricular dysfunction, cardiomegaly, or pancarditis. Cardiac involvement usually lasts for a few weeks, although it may recur.
Chagas disease is an endemic zoonosis in certain regions of Central and South America. It is caused by the protozoan Trypanosoma cruzi and is transmitted by hematophagous triatomae insects (ie, reduviids). These insects become infected after sucking blood that contains parasites from vertebrate hosts (eg, birds, mammals). Parasites reproduce in the guts of reduviids, and infective forms are discharged with feces at the time of subsequent blood meals. Therefore, the second host becomes infected with parasites because of skin abrasions that have been contaminated with infected feces.
Symptomatic chronic Chagas disease becomes apparent years or even decades after infection. Right bundle branch block is the most common electrocardiographic (ECG) abnormality, but other types of AV block, including complete AV block, are frequently observed. Other forms of rhythm disturbances include premature ventricular contractions, tachyarrhythmias, and bradyarrhythmias. Cardiomyopathy results in right-sided or biventricular heart failure.
Inflammatory and coronary disease
Rheumatic fever is an inflammatory disease that occurs from pharyngeal infection with group A streptococci, with multisystemic involvement. In acute rheumatic carditis, all the layers of the heart may be compromised. Although most patients with carditis are asymptomatic, some have valvular damage of the mitral and/or aortic valves that may produce regurgitation and even congestive heart failure. In terms of rhythm disturbance in patients with carditis, tachycardia disproportionate to the degree of fever and varying degrees of heart block may occur. The most common type of AV block is first degree.
Reiter syndrome is a seronegative arthropathy that may be accompanied by pancarditis with involvement of the AVN (ie, varying degrees of AVB) and proximal aortitis with valve regurgitation. Both of these features are more common in patients with long-standing disease and peripheral joint involvement.
Sarcoidosis is an inflammatory systemic disease characterized by formation of granulomas that most frequently affect the lungs, lymph nodes, skin, eyes, and liver. However, it may affect any organ. When it affects the heart, it may cause different types of conduction disturbance, from bundle branch block to complete AV block. [6, 7] Interestingly, the AV block may be reversible. [6, 8] Sudden death has been described.
Coronary ischemia and degeneration of the conducting system
Ischemia from acquired coronary disease (eg, Kawasaki disease) or from inferior wall infarcts can transiently or permanently damage the conduction tissue, leading to variable degrees of AV block.
Hypertension and aortic and/or mitral stenosis are believed to accelerate the degeneration of the conducting system through calcification and fibrosis.
Neuromuscular and neurocutaneous disease
Myotonic dystrophy forms 1 and 2
Myotonic dystrophy forms 1 and 2 are neuromuscular diseases inherited in an autosomal dominant fashion, with genetic anticipation, in which cardiac involvement consists of disorders of impulse formation and, especially, AV conduction. They are typically progressive and are caused by an expanded CTG repeat in the 3-prime untranslated region of the DMPK gene and by an expanded CCTG repeat in intron 1 of the ZNF9 gene, respectively. These diseases begin as asymptomatic PR prolongation and lead to complete AV block. Syncope and sudden death may occur; therefore, symptomatic patients require insertion of a permanent pacemaker. Diagnosis is mainly clinical.
Kearns-Sayre syndrome (also known as oculocraniosomatic disease) is a neuromuscular disease caused by a mitochondrial DNA deletion. Its characteristic triad includes progressive external ophthalmoplegia, pigmentary degeneration of the retina, and progressive cardiac conduction defects, including complete heart block. Other features may include ataxia, hearing loss, dementia, short stature, delayed secondary sexual characteristics, hypoparathyroidism, hypothyroidism, and peripheral neuropathy. The diagnosis is usually made clinically and by demonstration of ragged red fibers on skeletal muscle biopsy. Any progression in conduction abnormalities (such as from first- to second- degree AV block) is a clear indication for permanent pacemaker implantation in Kearns-Sayre syndrome, given the unpredictability of disease progression.
Tuberous sclerosis is a neurocutaneous disorder caused by mutations in any of 4 separate genetic loci in which cutaneous lesions of multiple types are associated with tumors and malformations of the central nervous system (CNS). Mental retardation may be severe, and patients develop intractable seizures. Cardiac rhabdomyomas within the AV node may cause complete AV block and different types of rhythm disturbances, such as ventricular tachycardia when localized in the ventricles. Diagnosis is clinical. The earliest lesions are leaf-shaped hypopigmented spots scattered over the trunk and limbs; they are observed better using a Wood lamp than with the naked eye.
Almost exclusive to adult patients are 2 degenerative diseases of the specialized conducting system: Lev disease and Lenègre disease. In Lev disease, calcification and sclerosis of the cardiac skeleton frequently involve the mitral and aortic valves, the central fibrous body, and the summit of the ventricular septum. Lenègre disease is thought to represent a primary sclerodegenerative disease of the conducting system with no involvement of the myocardium or fibrous skeleton of the heart.
Intracardiac tumors constitute a very rare cause of acquired complete AV block that occurs through invasion of the conduction system.  The term cardiac mesothelioma refers to heterotopic epithelial replacement of the AV node; it is a rare entity and, thus, a rare cause for complete AV block. Cases of congenital complete AV block and complete AV block in adults secondary to cardiac mesotheliomas have been reported. [10, 11, 12, 13, 14, 15] Definitive diagnosis is made only at autopsy, upon which necropsy may reveal extensive infiltration of the AV node and proximal bundle by mesothelioma tissue. Metastatic melanoma has also been described to cause sudden complete AV block and sudden death. [16, 17]
Abnormal protein deposition
Amyloidosis is a systemic disease in which abnormal production of immunoglobulins leads to their deposit in various tissues such as tongue, intestines, skeletal and smooth muscles, nerves, skin, ligaments, heart, liver, spleen, and kidneys. When the heart is affected, cardiomyopathy with marked left ventricular wall thickening, severely decreased systolic and diastolic function, and complete AV block may ensue. [20, 21, 22]
Drugs, chest trauma, and irradiation
Drugs such as digoxin, beta-blockers, and calcium channel blockers may cause transient complete AV block, which can be fatal if the ventricular escape rhythm is inadequate to maintain brain blood flow or if asystole is prolonged.
Clonidine, tricyclic antidepressants, penetrating chest trauma, and radiation may cause complete AV block. Mediastinal radiation for lymphomas such as Hodgkin lymphoma has been described to cause complete AV block. [23, 24, 25, 26]
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