The Fetal Heart

Atrial Septal Defect

An atrial septal defect (ASD) results from an error in the amount of tissue resorbed or deposited in the interatrial septum. It is the fifth most common form of CHD and is the most common form in adult patients. ASDs occur in 1 per 1500 live births and comprise 6.7% of CHD in live-born infants. ASDs occur twice as often in females as males. ASDs are associated with a variety of cardiac, extracardiac, and chromosomal abnormalities. ASDs can be classified by embryogenesis, size, or relationship to the fossa ovalis.

Embryologically, between the fourth and sixth weeks of gestation the primitive atrium is divided into right and left halves. The septum primum, a crescent-shaped membrane, develops along the cephalad portion of the atrium and grows caudally toward the endocardial cushions. The space between these two structures, termed the ostium primum, disappears when the septum primum fuses with the endocardial cushion. Before complete fusion, however, multiple small fenestrations develop in the septum primum, coalescing to form the ostium secundum.

A second crescent-shaped membrane subsequently develops just to the right of the septum primum. As this membrane grows toward the endocardial cushion, it partially covers the ostium secundum. Its crescent-shaped lower border never entirely fuses with the endocardial cushion, leaving an opening, the foramen ovale ( Fig. 37.19 ).

Ostium secundum ASDs make up more than 80% of all ASDs and generally occur in isolation. This ASD is caused by excessive resorption of the septum primum (foraminal flap) or by inadequate growth of the septum secundum ( Fig. 37.20A ). The ostium primum ASD is the second most common type and is located low in the atrial septum, near the atrioventricular (A-V) valves. Although the ostium primum ASD may occur alone, it is more frequently associated with a more complex congenital cardiac anomaly, the atrioventricular septal defect (AVSD) ( Fig. 37.20B ).

The sinus venosus ASD is a rare defect that can be divided into two types: (1) sinus venosus ASD of the SVC, with the defect adjacent to the SVC, and (2) sinus venosus ASD of the IVC, with the defect adjacent to the IVC. The first type is often associated with anomalous pulmonary venous return (APVR) ( Fig. 37.20C ). Coronary sinus ASDs, located at the ostium of the coronary sinus in the right atrium, may also occur but are exceedingly rare.

The prenatal sonographic diagnosis of ASD is difficult because the normal patent foramen ovale, which allows blood to flow from the right to the left atrium in utero, itself represents an ASD. It can be difficult to distinguish a small, pathologic ASD from the normal patent foramen ovale. The foraminal flap, or septum primum, is clearly visualized on the four-chamber view. It has a “loose pocket” configuration, appearing either circular or linear in shape as it opens into the left atrium ( Fig. 37.21 ).

The septum secundum, which is thick and relatively stationary, makes up the majority of the atrial septum. The foramen ovale is an opening in the septum secundum. The septum secundum and foramen ovale are well visualized in the four-chamber views. The maximal size of the normal foramen ovale differs by 1 mm or less from the aortic root diameter at all gestational ages. An ostium secundum ASD appears as a larger than expected defect in the central portion of the atrial septum near the foramen ovale. Alternatively, it can appear as a deficient foraminal flap.

If the lowest portion of the atrial septum (just adjacent to the A-V valves) is deficient, an ostium primum defect should be suspected ( Fig. 37.22 ). Color Doppler ultrasound may be helpful in the diagnosis of larger ASDs. However, small ASDs are commonly obscured by the normal flow through the patent foramen ovale.

A large right-to-left shunt is physiologic in utero, and thus an ASD usually does not compromise the fetus hemodynamically. After birth, the shunt may cause right ventricular overload and pulmonary hypertension. Spontaneous closure of an ASD will occur in approximately two-thirds of patients. Patients with small ASDs may remain asymptomatic into their 50s.

Ventricular Septal Defect

Isolated VSD is the most common cardiac anomaly, accounting for 30% of heart defects diagnosed in live-born infants and 9.7% diagnosed in utero. VSDs are associated with other cardiac anomalies in 50% of cases. Of the structural cardiac defects, VSDs have the highest recurrence rate and the highest association with teratogen exposure. They are classified according to their position in the interventricular septum ( Fig. 37.23 ) as membranous or muscular VSD (inlet, trabecular, outlet).

About 80% of VSDs occur in the membranous portion of the septum. However, because most membranous defects also involve a portion of the muscular septum, they are usually referred to as perimembranous defects. The subcostal four-chamber view provides optimal evaluation of the interventricular septum. At sonography, a VSD appears as an area of discontinuity in the interventricular septum. When the defect is small, this diagnosis is problematic, and at least one-third of VSDs are missed on the four-chamber view. Color Doppler imaging may improve the diagnostic accuracy for VSD. However, most are missed on fetal echocardiography. Small VSDs not detectable on gray-scale echocardiography may be documented with color Doppler ultrasound in some cases ( Fig. 37.24 ). In the setting of an isolated VSD, color Doppler ultrasound imaging typically shows bidirectional interventricular shunting, with a systolic right-to-left shunt and a late diastolic left-to-right shunt.

The prognosis for an infant with an isolated VSD is excellent, and many such defects go undetected. The rate of spontaneous closure of isolated muscular VSDs by 5 years of life is much higher (65%) than that for isolated perimembranous VSDs (20%). Overall about 40% of VSDs spontaneously close in the first year of life, and 60% resolve by 5 years of age. However, large defects detected in the fetus are associated with an 84% mortality. Concurrent cardiac, extracardiac, and chromosomal anomalies (trisomy 13, 18, 21, and 22) are associated with a worse prognosis.

VSDs may be extremely difficult to diagnose in utero, particularly when small in size. In addition, many small VSDs will close in utero or shortly after birth. A “pseudo” VSD in the membranous portion of the septum may be appreciated when evaluating the interventricular septum from an apical four-chamber view. This occurs when the angle of insonation is parallel to the septum, causing an artifactual dropout of the thin, membranous septum.

Atrioventricular Septal Defect

AVSD refers to a spectrum of cardiac abnormalities involving various degrees of deficiency of the interatrial and interventricular septa and of the mitral and tricuspid valves. These defects arise when the endocardial cushions fail to fuse properly and were previously called endocardial cushion defects or A-V canal defects. Almost two-thirds of fetuses with AVSD have additional cardiac anomalies. About one-third are associated with left atrial isomerism (both atria anatomically resemble the left), and of these the majority of affected fetuses have complete heart block. Chromosomal (especially trisomy 21 ) or extracardiac anomalies are associated in 78% of AVSDs.

Embryologically, in the primitive heart, the common atrium and ventricle communicate through the A-V canal. Development of the endocardial cushion results in division of the single, large A-V canal into two separate orifices, separating the atria from the ventricles ( Fig. 37.25 ). The interatrial and interventricular septa develop concurrently, eventually dividing the single atrium and ventricle into right and left portions. When the endocardial cushions fail to fuse properly, normal development of the mitral and tricuspid valves cannot occur, and an AVSD results ( Fig. 37.26 ).

AVSDs are divided into complete and partial or incomplete forms. In both, the A-V valves are abnormal. In complete AVSD a single, multileaflet valve is present, whereas in incomplete AVSD two of the leaflets (bridging leaflets) are connected by a narrow strip of tissue, resulting in the appearance of two valve orifices. Complete AVSD has variable amounts of deficient tissue in the atrial and ventricular septa. The incomplete form is associated with an ostium primum ASD. At fetal echocardiography, 97% of AVSDs are complete, although after birth only 69% are complete. The fetal incidence of AVSD is four times greater than that in the live-born population, indicating a high incidence of in utero demise.

AVSDs are considered balanced when the A-V junction is connected to both the right and the left ventricle, such that blood flow is relatively evenly distributed. If this connection exists with primarily one ventricle, such as in the setting of a hypoplastic left ventricle, it is termed an unbalanced AVSD.

Sonographically, a defect in the atrial or ventricular septum with an associated single abnormal A-V valve is visible in a four-chamber view ( Fig. 37.27 , Video 37.9 ). The abnormal valve should be suspected when the normal offset of the A-V valves is not visualized or when only a single A-V valve is appreciated in a short-axis view. Demonstration of two A-V valve orifices allows for differentiation between complete and incomplete forms of AVSD.

Color Doppler ultrasound demonstrates an open area of flow across the AVSD and the abnormal A-V valve. Color Doppler ultrasound imaging is particularly useful in the detection of valvular insufficiency. Holosystolic valvular insufficiency is closely associated with fetal hydrops and a worsening prognosis. Frequently, a left ventricular–to–right atrial jet can be identified across the ostium primum defect before the onset of holosystolic valvular insufficiency. Cardiac malformations associated with AVSD include septum secundum ASD, hypoplastic left heart syndrome (HLHS), valvular pulmonary stenosis, coarctation of the aorta, and tetralogy of Fallot (TOF). A meta-analysis of published cases of AVSD diagnosed prenatally confirmed that chromosomal anomalies are common, occurring in 25% to 58% of affected fetuses. Therefore karyotyping is indicated. Associated extracardiac anomalies are common, including omphalocele, duodenal atresia, tracheoesophageal atresia, facial clefts, cystic hygroma, neural tube defects, and multicystic kidneys.

The fetus with an AVSD and associated defects has a poor prognosis. When hydrops is present, few survive the neonatal period. Despite advances in pediatric cardiothoracic surgery, the overall outcome for antenatally diagnosed AVSD remains variable, with many studies reporting 5-year to 15-year survival rates below 50%. Others have reported excellent long-term results with newer surgical techniques such as the two-patch technique with early complete cleft closure for complete AVSD repair.

Ebstein Anomaly

Ebstein anomaly is characterized by inferior displacement of the tricuspid valve, frequently with tethered attachments of the leaflets, tricuspid dysplasia, and right ventricular dysplasia ( Fig. 37.28 , Video 37.12 ). Ebstein anomaly makes up approximately 7% of cardiac anomalies in the fetal population and has an incidence of 0.5% to 1% in high-risk populations. It occurs in approximately 1 per 20,000 live births.

Early data from biased retrospective studies suggested that lithium use during pregnancy was associated with an estimated 500-fold increase in the incidence of Ebstein anomaly in exposed fetuses. It is now clear that the increased risk is less than 2%. Ebstein anomaly may be associated with a variety of structural cardiovascular defects, particularly pulmonary atresia or stenosis, arrhythmias, and chromosomal anomalies.

Ebstein anomaly is readily detected in utero. The sonographic diagnosis rests on recognition of apical displacement of the tricuspid valve into the right ventricle, an enlarged right atrium containing a portion of the “atrialized” right ventricle, and a reduction in size of the functional right ventricle. Differential diagnosis includes tricuspid valvular dysplasia, Uhl anomaly, and idiopathic right atrial enlargement, but none of these has an inferiorly displaced tricuspid valve, the most reliable sign of Ebstein anomaly. Ebstein anomaly is one of the few structural defects that may cause substantial cardiac dysfunction in utero, frequently with cardiomegaly, hydrops, and tachyarrhythmias. Examination with spectral and color Doppler ultrasound is helpful in demonstrating tricuspid valve regurgitation, which causes further enlargement of the right atrium and ventricle. Tethered distal attachments of the tricuspid valve, marked right atrial enlargement, and left ventricular compression with narrowing of the pulmonary outflow tract are all associated with a poor prognosis. Fetuses diagnosed with Ebstein anomaly and tricuspid dysplasia have a poor prognosis, with an overall perinatal mortality (fetal demise or death before neonatal discharge) of 45%.

Arrhythmias, particularly supraventricular tachycardias (SVTs), are common with Ebstein anomaly and can further compromise the fetus. Overall, the 3-month mortality rate for patients diagnosed in utero is 80%. Surgical correction of Ebstein anomaly in young children is associated with a low mortality rate and an excellent quality of life. Because clinical presentation, treatment options, and prognosis are inconsistent, case-by-case management is variable.

Hypoplastic Right Ventricle

In general, hypoplastic right ventricle occurs secondary to pulmonary atresia with intact interventricular septum. It has an incidence of 1.1% among stillbirths. Tricuspid atresia may be associated with a hypoplastic right ventricle, but this is not as common. Pathophysiologically, hypoplasia of the right ventricle develops because of a reduction in blood flow secondary to inflow impedance from tricuspid atresia or outflow impedance from pulmonary arterial atresia. Typical sonographic findings include a small, hypertrophic right ventricle and a small or absent pulmonary artery ( Fig. 37.29 , Video 37.11 ). Spectral Doppler ultrasound may be helpful in demonstrating decreased flow through the tricuspid valve or pulmonary artery. Congestive heart failure and hydrops may develop from tricuspid regurgitation. After birth, closure of the ductus arteriosus frequently results in neonatal death. Prognosis improves with preoperative prostaglandin infusion to maintain the patency of the ductus.

Hypoplastic Left Heart Syndrome

In hypoplastic left heart syndrome, the left ventricular cavity is pathologically reduced in size. HLHS constitutes approximately 7% to 9% of all congenital cardiac lesions. It has a 2 : 1 male predominance and a recurrence risk of 0.5%. The small left ventricle results from decreased blood flow into or out of the left ventricle. The primary abnormalities include aortic atresia, aortic stenosis, and mitral valve atresia. It is associated with coarctation of the aorta in 80% of cases. The primary sonographic feature of HLHS is a small left ventricle ( Fig. 37.30 , Video 37.10 ). The mitral valve is typically hypoplastic or atretic, as is the aorta. Color Doppler ultrasound is extremely helpful in the setting of HLHS, usually demonstrating the absence of flow through the mitral and aortic valves.

Three decades ago this syndrome had an extremely poor prognosis, with 25% mortality in the first week of life and most untreated infants dying within 6 weeks. Comfort care was provided, but little could be done to prolong survival. Currently it is expected that up to 70% of newborns with HLHS may reach adulthood owing to advancements in surgical techniques, perioperative management, and postoperative care. Prenatal diagnosis of HLHS is beneficial for preventing ductal shock and keeping affected infants stable in the preoperative stage. Monophasic blood flow across the mitral valve, restricted or absent flow through the foramen ovale, and retrograde flow through the aorta are all considered poor prognostic signs in utero. Despite significant advancements in medical and surgical management over the past 30 years, follow-up studies indicate that children with HLHS often experience major developmental delays and decreased exercise performance, even after heart transplantation. A recent meta-analysis found that although deficits remain, substantial improvement in neurodevelopment has occurred over the past 20 years in patients with surgically corrected HLHS.