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KEY POINTS
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Key Points

  • The most common fetal bradyarrhythmia diagnosed prenatally is congenital complete heart block (CHB), usually occurring in association with circulating maternal anti-Ro or anti-La antibodies, but also occurring together with structural fetal cardiac malformation.

  • Sonographic diagnosis of complete heart block (CHB) is made using M-mode echocardiography, demonstrating complete dissociation between atrial and ventricular rates; varying degrees of cardiac failure and hydrops may also be present.

  • Immune-mediated CHB usually leads to permanent damage to the fetal cardiac conduction system, and controversy exists as to whether subsequent prenatal therapy by maternal administration of corticosteroids will have any meaningful benefit.

  • Other options for prenatal therapy include maternal betamimetic administration, although its role is generally limited by maternal side effects; additionally experimental approaches to fetal cardiac pacing have also been described in cases with very poor prognosis.

  • While vaginal delivery is possible with appropriate intensive fetal surveillance, for practical reasons, most such fetuses are delivered elective by cesarean; delivery should occur in a tertiary care facility with appropriate pediatric cardiology backup available.

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CONDITION
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The diagnosis of fetal arrhythmias has become increasingly common as echocardiographic evaluation of the fetal heart has improved and been pursued earlier in gestation. Two-dimensional echocardiography can distinguish normal from disordered cardiac anatomy as early as 16 weeks of gestation (Kleinman et al., 1980; Allan et al., 1983, 1984a). Similarly, fetal arrhythmias can be accurately characterized with the addition of M-mode echocardiography (Allan et al., 1983; Devore et al., 1983). Although the vast majority of fetal arrhythmias reported are either extrasystoles (75%) or tachyarrhythmias (15%), fetuses with bradyarrhythmia due to complete heart block (CHB) account for 9% of all cases (Kleinman and Evans, unpublished data, 1988). CHB is seen in association with severe congenital heart disease in up to 53% of cases (Schmidt et al., 1991). In this setting the prognosis is poor, with a survival rate of less than 15% (Shenker, 1979; Teteris et al., 1979; Allan et al., 1983; Crawford et al., 1985; Cameron et al., 1989). However, CHB complicates structural congenital heart disease in only 0.4% to 0.9% of the cases (Camm and Bexton, 1984; Olah and Gee, 1993). CHB is observed with normal cardiac anatomy in up to 50% of cases (Kleinman and Evans, unpublished data, 1988). CHB with normal cardiac anatomy is usually associated with transplacental passage of maternal antibodies, anti-SSA or anti-SSB (anti-Ro or anti-La), in mothers with connective-tissue diseases (McCue et al., 1977; Scott et al., 1983; Litsey et al., 1985; Taylor et al., 1988).

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The most common form of congenital heart block seen in the fetus is third-degree, or complete, atrioventricular (AV) block. First-degree AV block is the prolongation of the P–R interval and is difficult to detect prenatally. Second-degree AV block occurs either as a progressive lengthening of the P–R interval, ...

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