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Complete Heart Block—Third-Degree Heart Block Mohamad Al-Ahdab Complete atrioventricular (AV) block can be defined as interruption in the transmission of the cardiac impulse from the atria to the ventricles due to an anatomical or functional impairment in the AV conduction system. The conduction disturbance can be transient or permanent. Congenital complete heart block (CHB), the most common and important form in children, was first described in 1901 by Morquio, who also noted a familial occurrence and an association with Stokes-Adams attacks and death. The presence of fetal bradycardia (40– 80 bpm) as a manifestation of CHB was first noted in 1921. The incidence of congenital CHB in the general population varies between 1 in 15,000 to 1 in 22,000 live-born infants.

almost all cases presenting in utero or during the neonatal period. Rarely it may explain a few cases occurring later (5% in one report). Other causes include myocarditis and various structural cardiac defects, particularly congenitally corrected transposition of the great arteries, AV discordance, or polysplenia with AV canal defect. Several genetic disorders such as familial atrial septal defect and Kearns-Sayre syndrome (Chapter 18) have been identified (Table 1). In most cases, CHB is characterized pathologically by fibrous tissue that either replaces the AV node and its surrounding tissue or by an interruption between the atrial myocardium and the AV node; other lesions that can occur include congenital absence of the AV node. The net effect is that the block is usually at the level of the AV node. The heart is otherwise structurally normal in these children.

ETIOLOGY In the absence of congenital heart disease, neonatal lupus is responsible for 60% to 90% of cases of congenital CHB. Antibodies from a mother with an autoimmune connective tissue disorder, most frequently lupus erythematosus, cross the placenta to the fetus during the first trimester and account for

Neonatal Lupus Complete heart block, hepatobiliary disease, malar rash, thrombocytopenia and, less frequently, myocarditis comprise the neonatal lupus primarily presenting in utero or in the neonate. Frequently the only manifestation of neonatal lupus, and by extension an

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COMPLETE HEART BLOCK THIRD-DEGREE HEART BLOCK

TABLE 1. Congenital Complete Heart Block (1 in 20,000 to 25,000 Live Births) No associated structural heart disease Accounts for 67% to 75% of affected infants Immune-mediated complete heart block Accounts for 80% of congenital complete heart block in the absence of structural defect Associated with maternal autoantibodies (anti-SSA/Ro antibodies and anti-SSB/La antibodies) which are the putative etiologic agents Because the mentioned antibodies are of the IgG class, transplacental passage and complete heart block do not appear until after 16 to 20 weeks of gestation High fetal wastage At least 80% of mothers with affected infants will have autoantibodies; most mothers will either have or develop overt symptoms of a systemic connective tissue disorder Discontinuity within the cardiac conduction tissue at level of the atrial axis, within the nodal-ventricular conduction tissue, or within the intraventricular conduction tissue∗ Other associations: (applies to complete heart block at birth as well as letter) Tumors and neoplasia Myocarditis and infections Familial, genetic, and metabolic Long QT syndrome Congestive heart failure occurs but less commonly Fair to good postnatal prognosis Pacemaker in the newborn period indicated by presence of congestive heart failure or a very slow heart rate (