Isolated congenital complete heart block

Definition

Currently, complete atrioventricular (AV) block is regarded as congenital when it is diagnosed in utero, at birth, or within the neonatal period. Complete heart block is characterized by a random relationship between atrial and ventricular activation. Atrial impulses are not conducted to the ventricles which are depolarized in response to a subsidiary pacemaker. The electrocardiogram is a simple but secure means of identifying complete heart block ( Figs. 4.1 and 4.2 ). Atrioventricular dissociation is a disorder of both conduction and impulse formation and is not considered in this chapter.

Fetal echocardiography permits intrauterine diagnosis, and isolated complete heart block can be distinguished from cases with coexisting congenital heart disease, most commonly ventricular inversion (see Chapter 6 ) or left isomerism (see Chapter 3 ). The incidence of congenital high-degree heart block, either complete or with more than 50% of blocked atrial impulses, has been estimated at 1 in 2500 to 1 in 20,000 live births. Block can be within the AV node, within the His bundle, or infra-Hisian, and the discontinuity in conduction can be anatomic or functional. ^, Narrow QRS complexes indicate that the subsidiary pacemaker is above the bifurcation of the His bundle, but morphologic abnormalities have been identified at multiple levels. ^{, , ,} The connection between atrial muscle and AV node is deficient or absent. ^, The node can be congenitally absent or defective ^{, , ,} and separated from the His bundle, supporting the view that the AV node and bundle of His originate as separate structures that normally are destined to join during early fetal development. Disruption can be within the His bundle, ^, at the origins of the bundle branches, or in the right or left bundle branch. Anatomic defects occasionally exist in the AV node itself or at the junction of AV node and atrial muscle. The fetal sinoatrial and AV nodes can calcify. Nevertheless, spontaneous changes from complete to incomplete heart block, ^, or even to sinus rhythm, ^, indicate that interruption of the conduction pathways is not necessarily anatomic, complete, or permanent ( Fig. 4.3 ).

An association between maternal lupus erythematosus and congenital complete heart block was reported in 1966 and confirmed a decade later. ^, Congenital heart block is a passively transferred autoimmune disease that affects the offspring of mothers with Ro/SSA autoantibodies. Sinus node disease may occur in children with prenatal exposure to anti-Ro or anti-La antibodies. Complete heart block in utero or at birth is strongly associated with neonatal lupus syndrome and with maternal antibodies to 48-kD SSB/La, 52-kD SSA/Ro, and 60-kD SSA/Ro ribonucleoproteins. Congenital heart block is an important model of passive autoimmunity, with cardiac injury believed to be in response to active transport of maternal immunoglobulin G (IgG) autoantibodies into the fetal circulation. Anti-SSA/Ro associated with third-degree heart block is irreversible. Dilation of the ascending aorta is present in a large proportion of pediatric patients with isolated congenital complete heart block. Neurodevelopmental sequelae are described. Mothers with systemic lupus erythematosus and one child with neonatal heart block are at greater risk of having subsequent offspring with heart block. Children of mothers with lupus not only can have congenital heart block, but can subsequently develop the overt connective tissue disease. Maternal lupus may not manifest for years after birth of an infant with congenital complete heart block. Long-term outlook for the mothers is more reassuring, however.

The history

Congenital complete heart block necessarily begins in utero ^, and must be distinguished from the bradycardia of fetal distress, ^, a distinction that is more certain when the slow heart rate is detected before the onset of labor. However, congenital complete heart block usually comes to attention because an inappropriately slow heart rate is detected in an otherwise normal neonate or infant (see section The Arterial Pulse, later).

There is a tendency for female preponderance in congenital complete heart block, and approximately 76% of mothers of affected children are Caucasian. Familial heart block has been well established. In Morquio’s original description, AV block recurred in five of eight siblings. Osler reported Stokes-Adams attacks in a patient who had relatives with slow pulses. Familial congenital heart block can become overt years after birth ^, and can be characterized by right or left bundle branch block. Almost all degrees and forms of heart block have occurred in different members of the same family. Genetic determinants have been established. Four generations of a single family had right bundle branch block, left anterior fascicular block, bifascicular block, and complete heart block.

The key determinants of clinical stability in isolated congenital complete heart block are the ventricular rate, the hemodynamic adjustments at rest and with exercise, and the presence of inherently normal myocardium. Nevertheless, patients can develop dilated cardiomyopathy attributed to bradycardia, and a strong relationship exists between SSA/Ro and SSB/La antibodies and cardiomyopathy. ^, Although subnormal exercise performance has been reported in children and adolescents with congenital complete AV block, ^{, ,} exercise tolerance is generally normal or nearly so, and endurance performance is occasionally normal. ^, One patient was an ardent ice hockey player, and other patients have included Air Force pilots, ^, a cricket player, a 56-year-old woman who for 20 years walked 2 miles to her daily job on a farm, a 38-year-old man who had won boxing matches in his youth, and a 48-year-old man who experienced a normal response to a Royal Canadian Air Force decompression chamber at age 23 years, and subsequently flew jet aircraft. High levels of physical activity are not desirable but are at least possible despite the exercise limitations described previously. Pregnancy is generally uneventful in otherwise normal women with congenital heart block, ^, but Stokes-Adams attacks occasionally occur during gestation or the puerperium.

A substantial majority of young patients with isolated congenital complete heart block are asymptomatic, but mortality even in infancy and childhood is estimated at 8%, and longevity in adolescents and adults is less than normal. ^{, ,} Congenital complete heart block may come to light in toddlers because of night terrors or irritability. Stokes-Adams episodes are uncommon in the young but pose tangible hazards, with symptoms ranging from mild dizziness to syncope and convulsions. ^, A Stokes-Adams episode with sudden death can occur in previously asymptomatic patients ( Fig. 4.4 ). One patient experienced recurrent Stokes-Adams episodes between 2 and 4 years of age, but the attacks gradually diminished and finally vanished, leaving him able to participate in cricket, football, and swimming. ^, Syncope and sudden death are usually caused by bradycardia, but ventricular tachycardia and fibrillation also play a role. Frequent ventricular ectopic beats have been recorded during nocturnal monitoring, and young patients sometimes experience unifocal, multifocal, or repetitive ventricular ectopic beats during treadmill exercise. Serious symptoms and complications are more likely in patients with daytime heart rates below 50 beats/min, wide QRS complexes, a blunted rate response to graded exercise, a disproportionate increase in left ventricular internal dimensions, or subnormal left ventricular function.

Physical appearance

Growth, development, and general appearance are normal. Cyanosis can accompany congestive heart failure in bradycardic infants.