Chapter 4 Isolated Congenital Complete Heart Block
Congenital complete atrioventricular block was recognized in 1901 when Morquio1 described familial recurrence with Stokes-Adams attacks and death in childhood. In 1908, van den Heuvel2 published the electrocardiogram of a patient with complete heart block and syncopal episodes that dated from infancy. Thirteen years later, White and Eustis3 described slow fetal heart rate; at birth, the electrocardiogram disclosed complete heart block.3 Davis and Stecher4 distinguished congenital from acquired complete heart block; shortly thereafter, Yater5–7 established criteria for the clinical diagnosis of the congenital form. Currently, complete atrioventricular block is regarded as congenital when it is diagnosed in utero, at birth, or within the neonatal period.8
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.9 The electrocardiogram is a simple but secure means of identifying complete heart block (Figures 4-1 and 4-2). Atrioventricular dissociation is a disorder of both conduction and impulse formation9 and is not considered in this chapter.
Figure 4-1 Electrocardiogram from a 7-year-old boy with isolated congenital complete heart block. P waves are independent of QRS complexes. The QRS complex is normal in configuration and duration, and its axis is normal. Deep Q waves and tall R waves are found in leads V5 and V6. The R wave in lead V1 is relatively tall, with an R/S ratio of 1:1. T waves are deeply inverted in right precordial leads and are tall and peaked in left precordial leads. See Figure 4-5 for rhythm strip.
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) and left isomerism (see Chapter 3). The incidence rate 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.9–12 Block can be within the atrioventricular (AV) node or within the His bundle (or infra-Hisian), and the discontinuity in conduction can be anatomic or functional.13,14 Narrow QRS complexes indicate that the subsidiary pacemaker is above the bifurcation of the His bundle,9 but morphologic abnormalities have been identified at multiple levels.7,13,15,16 The connection between atrial muscle and atrioventricular node is deficient or absent.13,16 The node can be congenitally absent or defective13,14,16,17 and separated from the His bundle,18 which supports the view that the AV node and bundle of His originate as separate structures that normally are destined to join during early fetal development.15 Disruption can be within the His bundle,13,19 at the origins of the bundle branches, or in the right or left bundle branch.16 Anatomic defects occasionally exist in the AV node itself or at the junction of AV node and atrial muscle.15 The fetal sinoatrial and atrioventricular nodes can calcify.20 Nevertheless, spontaneous changes from complete to incomplete heart block21,22 or even to sinus rhythm23,24 indicate that interruption of the conduction pathways is not necessarily anatomic, complete, or permanent (Figure 4-3).
Myocardial contractile force is augmented by the slow heart rate, the long diastolic filling period, and the increased end-diastolic volume and fiber length,9,25 so stroke volume increases and basal cardiac output is maintained.26,27 An increase in cardiac output with exercise is chiefly rate dependent.26 Submaximal isotonic exercise generally provokes an appropriate increase in ventricular rate and cardiac output, but higher workloads are accompanied by blunted hemodynamic and rate responses.26 An increase in stroke volume and, to a lesser degree, an increase in arteriovenous oxygen difference partially compensate for the blunted rate response.25 Despite compensatory mechanisms, oxygen consumption during submaximal exercise is significantly lower than in healthy age-matched and sex-matched control subjects.25 A subset of patients with congenital complete heart block has development of dilated cardiomyopathy for which no definite cause has been found.28
An association between maternal lupus erythematosus and congenital complete heart block was reported in 1966 and confirmed a decade later.9,29–32 Congenital heart block is a passively transferred autoimmune disease that affects the offspring of mothers with Ro/SSA autoantibodies.33 Sinus node disease may occur in children with prenatal exposure to anti-Ro or anti-La antibodies.34 Complete heart block in utero or at birth is strongly associated with the neonatal lupus syndrome and with maternal antibodies to 48-kD SSB/La, 52-kD SSA/Ro, and 60-kD SSA/Ro ribonucleoproteins.35 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.35 Anti-SSA/Ro associated with third-degree heart block is irreversible.36 Dilation of the ascending aorta is present in a large proportion of pediatric patients with isolated congenital complete heart block.37
History
Congenital complete heart block necessarily begins in utero10,38 and must be distinguished from the bradycardia of fetal distress,39,40 a distinction that is more certain when the slow heart rate is detected before the onset of labor.40 However, congenital complete heart block usually comes to attention because an inappropriately slow heart rate is detected in an otherwise healthy neonate or infant (see subsequent section, Arterial Pulse).
Fetal echocardiography is used to establish the diagnosis and determine whether the heart block is isolated or associated with congenital heart disease, which is usually left isomerism or ventricular inversion (see previous discussion). Only 14% of fetuses with coexisting congenital heart disease survive as neonates.10 Conversely, 85% of fetuses with isolated intrauterine complete heart block live beyond the neonatal period.10,26,38 This survival rate is similar to that of isolated congenital complete heart block diagnosed after birth, in which approximately 90% of infants and children are alive at long-term follow-up.10 The fetal cardiac rhythm may change from sinus to second-degree atrioventricular block to complete heart block within hours or weeks after birth, which suggests that immunologic damage occurs early, develops slowly, or appears late.10
A tendency for female preponderance in congenital complete heart block has been found,41 and approximately 76% of mothers of affected children are white.35 Familial heart block has been well established.9,22,39,42–45 In Morquio’s1 original description, atrioventricular block recurred in five of eight siblings (see previous mention). Osler46 reported Stokes-Adams attacks in a patient who had relatives with slow pulses. Familial congenital heart block can become overt years after birth43,47 and can be characterized by right or left bundle branch block.48 Almost all degrees and forms of heart block have occurred in different members of the same family. Four generations of a single family had right bundle branch block, left anterior fascicular block, bifascicular block, and complete heart block.49 Mothers with systemic lupus erythematosus and one child with neonatal heart block are at greater risk of having subsequent offspring with heart block.45 Children of mothers with lupus not only can have congenital heart block but subsequently can have development of the overt connective tissue disease.50 Maternal lupus may not become manifest for years after birth of an infant with congenital complete heart block.51 Long-term outlook for the mothers is more reassuring, however.52,53
The key determinants of clinical stability in isolated congenital complete heart block are the ventricular rate, the hemodynamic adjustments at rest and with exercise (previously discussed), and the presence of inherently normal myocardium.9,54 Nevertheless, patients can have development of dilated cardiomyopathy attributed to bradycardia, and a strong relationship exists between SSA/Ro and SSB/La antibodies and cardiomyopathy.28,52 Although subnormal exercise performance has been reported in children and adolescents with congenital complete atrioventricular block,23,55,56 exercise tolerance is generally normal or nearly so, and endurance performance is occasionally normal.25,57 One patient was an ardent ice hockey player, and other patients have included Air Force pilots,58,59 a cricket player,55 a 56-year-old woman who for 20 years walked 2 miles to her daily job on a farm,24 a 38-year-old man who had won boxing matches in his youth,24 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.54 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 healthy women with congenital heart block,60,61 but Stokes-Adams attacks occasionally occur during gestation or the puerperium.61
A substantial majority of young patients with isolated congenital complete heart block are asymptomatic, but the mortality rate even in infancy and childhood is estimated at 8%,62 and longevity in adolescents and adults is less than normal.23,41,62 The heart may not respond adequately to increased circulatory demands, especially in the vulnerable neonatal period.9,41,54,62 Infants with slow ventricular rates may succumb to congestive heart failure before physiologic adaptation is achieved.62 Metabolic acidosis slows the heart rate,63 and the stress of febrile illnesses in infancy are poorly tolerated. Congenital complete heart block may come to light in toddlers because of night terrors or irritability.9 Stokes-Adams episodes are uncommon in the young but pose tangible hazards, with symptoms that range from mild dizziness to syncope and convulsions.24,55 Neurologic sequelae follow cardiac arrest. A Stokes-Adams episode with sudden death can occur in previously asymptomatic patients (Figure 4-4). Fatal Stokes-Adams seizures occasionally occur in childhood, but death rarely accompanies the initial episode.23,41 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.23,55 Syncope and sudden death are usually caused by bradycardia, but ventricular tachycardia and fibrillation also play a role.25 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.57 Serious symptoms and complications are more likely in patients with daytime heart rates below 50 beats per minute, wide QRS complexes, a blunted rate response to graded exercise, a disproportionate increase in left ventricular internal dimensions, or subnormal left ventricular function.64
Arterial pulse
A pulse rate inappropriately slow for age often leads to the diagnosis of congenital complete heart block. At birth, the heart rate in affected patients is seldom more than 90 beats per minute, and in infants, the rate is seldom more than 65 to 70 beats per minute.9 After infancy, the basal rate generally exceeds 50 beats per minute (Figure 4-5) and not uncommonly reaches 60, 70, or even 80 beats per minute.23 The pulse rate in congenital complete heart block is slow but is relatively rapid compared with acquired complete heart block.23,24 In healthy young adults, rates of 40 to 60 beats per minute may be mistaken for sinus bradycardia.23 A Royal Canadian Air Force veteran described by Mathewson and Harvie58 was a case in point. His slow heart rate was initially attributed to the sinus bradycardia of physical conditioning. Strenuous exercise was tolerated without difficulty. Complete heart block thought to be congenital was subsequently recognized.
Figure 4-5 Long strip of lead 2 of the electrocardiogram shown in Figure 4-1. Independent P waves are identified with arrows. The PP intervals are shorter when separated by a QRS complex (positive chronotropic effect). The ventricular rate is 48 to 50 beats per minute. The QRS complexes are narrow.
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