Although the obstetrical definition of fetal bradycardia is a fetal heart rate (FHR) < 110 beats per minute at any time during gestation,¹ recent studies suggest that an FHR < third percentile for gestational age² may have superior sensitivity and specificity. Bradycardia may be either sinus bradycardia with 1:1 atrioventricular (AV) conduction, AV block, or blocked atrial bigeminy (see Part 3, Chapter 1). Bradycardia may also occur with congenital (structural) heart abnormalities (ie, congenital heart disease [CHD]). In these cases, the sinoatrial (SA) node or AV node is usually displaced, duplicated, or even absent. For the fetal cardiologist, the distinction between the anti-Ro/SSA-mediated AV block (discussed in Part 3, Chapter 5) and AV block associated with CHD is important due to the differences in pre- and postnatal outcome (FIG. 3.4.1).

Approximately 30% of newborns with 3° (or complete) AV block have associated CHD.³,⁴,⁵,⁶ The fetal presentation is much more common than 30%, suggesting many die in utero. Thus, the prognosis for a fetus with 3° AV block and CHD is extremely poor. Review of several larger series identified that fewer than 15% were alive at the end of the neonatal period (TABLE 3.4.1). Fetuses at highest risk for a poor outcome are those with hydrops and a ventricular rate of <50 to 55 beats per minute (bpm). If the fetus is liveborn, the infant many succumb during the neonatal period even if they receive a pacemaker.

In most published studies, a majority of affected fetuses with sinus bradycardia or 3° AV block had either heterotaxy (an abnormality where the internal thoracoabdominal organs demonstrate abnormal arrangement across the left-right axis of the body)⁹ (FIG. 3.4.2A and B) or discordant AV connection (also known as congenitally corrected transposition of the great arteries [cc-TGA]) (FIG. 3.4.2C). In heterotaxy, abnormal cardiac development often leads to atrial appendage isomerism, resulting in either bilateral paired right atria (right atrial isomerism, RAI) or paired left atria (left atrial isomerism, LAI). Fetuses with RAI often have dual SA nodes (because they are right-sided structures) resulting in an ectopic atrial rhythm with a slower beat rate than normal (FIG. 3.4.3). On the other hand, fetuses with LAI have bilateral left atrial morphology and frequently lack a normal SA node. Bradycardia in this setting may result in a nonsinus atrial pacemaker from a left atrial focus, or they may have AV block (FIG. 3.4.4). The exact pathophysiology of AV block in LAI lies in the discontinuity between the AV node and the conduction axis.¹⁰ This discontinuity is found to be distal to the AV node, in contrast to the pattern of atrial-axis discontinuity found in anti-Ro/SSA AV block caused mainly by replacement of the SA node and AV node by a loose fibrous tissue.

Other features of LAI include ipsilateral pulmonary venous return, an interrupted inferior vena cava with azygous continuation to the right or left superior vena cava, bilateral vena cava, and a spongy myocardium (ventricular noncompaction)¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸ (FIGS. 3.4.5 and 3.4.6). Conduction system disease in LAI can present before 13 weeks and be progressive (FIGS. 3.4.7 and 3.4.8). In subjects with cc-TGA, the inversion of the ventricles often leads to disruption of the AV conduction axis and AV block, which usually appears in the third trimester. Familial recurrence of LAI has been documented in some case series,¹⁴,¹⁵ raising the possibility of an underlying heritable pathogenic variant, while recurrence of cc-TGA has not been reported.