Fetal Echocardiography: Congenital Heart Disease

Chromosomal Abnormalities

The frequency of chromosomal abnormalities in infants with congenital heart disease is estimated as 5% to 10% from postnatal data. In a control study of 2100 live-born infants with cardiovascular malformations, chromosomal abnormalities were found in 13%. In this study, Down syndrome occurred in more than 10% of the infants, with the other trisomies each constituting the remaining 3%.

The frequency of abnormal karyotypes in fetuses with cardiac defects has been commonly found at 30% to 40%. Of these fetuses, most have trisomy 21, followed by trisomy 13, trisomy 18, and Turner syndrome. The association of congenital heart defects and chromosomal abnormalities is lower in live-born infants than in fetuses because of the high in utero mortality of the fetus with trisomy 18, trisomy 13, and Turner syndrome (trisomy 45,X).

The occurrence of associated extracardiac abnormalities in fetuses with cardiac defects and chromosomal abnormalities is on the order of 50% to 70%. In the fetus with a single cardiac abnormality, the incidence of chromosomal abnormalities is still increased (15% to 30%). The most common single cardiac abnormality is the ventricular septal defect.

Certain cardiac abnormalities are more likely associated with chromosomal defects. In general, malformations of the right side of the heart are rarely associated with karyotypic abnormalities (e.g., pulmonic stenosis and tricuspid atresia). On the other hand, abnormalities such as atrioventricular septal defect (AVSD), perimembranous ventricular septal defect, tetralogy of Fallot, double-outlet right ventricle, coarctation of the aorta, and hypoplastic left heart are often associated with chromosomal abnormalities.

The incidence of cardiac defects in the fetus with trisomy is increased, with trisomy 21 showing the highest rate at 40% to 50%, Turner syndrome (45,X) showing 25% to 40%, and more than 90% having cardiac defects with trisomies 13 and 18 (see Table 36.1 ).

Familial Risks of Congenital Heart Disease

Most congenital heart defects have more than one cause, with genetic and environmental factors both playing a role. Only about 10% to 15% of all congenital heart defects have been attributed to known chromosomal abnormalities, genetic syndromes, and teratogenic embryopathies.

The recurrence risk for an isolated congenital cardiovascular malformation is modified for each family based on the number of affected relatives and the severity of the abnormality. In general, a recurrence risk of 1% to 5% is estimated for the majority of congenital cardiac abnormalities.

Studies have shown that the contribution of genetic factors increases the risk of congenital heart disease significantly. For example, a mother who has had a child with an abnormality of the left side of the heart (mitral atresia or aortic atresia) has a significantly higher risk (13%) of delivering another child with a form of left-sided heart disease. This risk increases significantly with each pregnancy.

The Four-Chamber View

All sonographers should be familiar with the routine four-chamber view that is part of the normal obstetric examination. This view is easily obtainable after 16 weeks of gestation, although the anatomy becomes more distinctly imaged with ultrasound between 18 and 22 weeks of gestation. The four-chamber view is normal when the following conditions are seen:

• 1.

  The fetal situs is normal. (The heart is in the left side of the chest and its apex points to the left, the stomach is to the left, the aorta is anterior and to the left of the spine, and the inferior vena cava is anterior and to the right of the spine; Fig. 36.1A ).

  

• 2.

  The size of the heart in relation to the chest is normal (ratio of heart to thorax = 1:3) (see Fig. 36.1B ).

• 3.

  The two atria are equal in size, and the flap of the foramen ovale is seen to move toward the left atrium. (The atria should constitute about one-third of the size of the heart; Fig. 36.1C ).

• 4.

  The two ventricles are equal in size and contractility. (The ventricles should constitute about two-thirds of the size of the heart; Fig. 36.1D ).

• 5.

  The interatrial and interventricular septa are completely formed and normal in thickness (see Fig. 36.1E ).

• 6.

  The atrioventricular valves are normal in thickness, position, and opening (see Fig. 36.1F ).

Several cardiac abnormalities may be recognized with the four-chamber view alone, such as a large ventricular septal defect, AVSD, hypoplastic left or right heart, mitral or tricuspid atresia, Ebstein anomaly, and total anomalous pulmonary venous return (TAPVR; discussed later in this chapter). However, many cardiac abnormalities may be missed with only the four-chamber view. Abnormalities of the cardiac structure (especially the great vessels) that are not located in the four-chamber plane may show a normal four-chamber view, but the specific abnormality will be missed if a complete fetal echocardiogram with multiple sweep cine loop images is not conducted (e.g., transposition of the great arteries, truncus arteriosus, coarctation of the aorta, small outlet ventricular septal defect, and tetralogy of Fallot). Chapter 35 covers the normal fetal echocardiographic examination.

Cardiomyopathy

Cardiomyopathy is a condition of the myocardial tissue in the heart. This disease process may be caused by exposure to a virus (Coxsackie or mumps) or to bacteria, which leads to an infection that causes cardiomyopathy. Errors of metabolism may also cause cardiomyopathy. Endocardial fibroelastosis has also been associated with cardiomyopathies and hypoplastic left heart syndrome. Asymmetric septal hypertrophy (as seen in patients with hereditary idiopathic subaortic stenosis) and concentric hypertrophy (as seen in some uncontrolled diabetic mothers) have been reported.

Myocarditis is characterized by necrosis and destruction of myocardial cells and an inflammatory infiltrate. In a viral cardiomyopathy, all four chambers are dilated, with thinning of the myocardial walls. Gross valvular regurgitation may be present, resulting from the stretched mitral and tricuspid annulus ( Fig. 36.5 ). Cardiac function is decreased severely, leading to congestive heart failure with pericardial effusion, bradycardia, and death. The general prognosis for a fetus with evidence for a cardiomyopathy is poor. Serial fetal echoes are performed to monitor chamber size, regurgitation, and contractility.

Pericardial Effusion

Pericardial effusion is an abnormal collection of fluid surrounding the epicardial layer of the heart. In the four-chamber view, a hypoechoic area in the peripheral part of the epicardial/pericardial interface of 2 mm or less is considered within normal limits and does not represent a pericardial effusion. The separation must be seen on the M-mode to separate both in systole and in diastole and be greater than 2 mm. A separation that surrounds the heart (from the atrioventricular junction around the apex of the heart) may be associated with hydrops fetalis ( Fig. 36.6 ).

With a small pericardial effusion, the separation of the pericardium from the epicardium may localize toward the posterolateral and apical walls of the heart. The larger effusion will extend to the atrioventricular groove posteriorly and around the anterior right ventricular wall.

Pericardial effusion may be seen secondary to indomethacin therapy with premature closure of the patent ductus arteriosus. Pericardial effusion has also been associated with coxsackievirus, cytomegalovirus, parvovirus, human immunodeficiency virus, intrauterine growth restriction, and aneuploidy.

Atrial Septal Defect

An atrial septal defect allows communication between the left atrium and right atrium. The locations of three common atrial septal defects are shown in Fig. 36.7 . There are three common forms of atrial septal defects: ostium secundum, ostium primum, and sinus venosus. The ostium secundum defect is the defect in the central atrial septum near the foramen ovale and is the most difficult to see in utero, as the flap of the foramen ovale is mobile at this period of development. The ostium primum defect is usually associated with the chromosomal abnormality of trisomy 21 and often will have a cleft mitral valve and abnormalities of the atrioventricular septum. The least common septal defect is the sinus venosus defect seen near the superior vena cava entrance into the right atrium. This may be associated with a partial anomalous pulmonary venous return .

The atrial septal defect is not always recognized during fetal life unless part of the intra-atrial septum is missing. The foramen ovale remains open in the fetal heart until after birth, and the pressures change between the right and left sides of the heart to force the foramen to close completely. Failure of the foramen to close may result in atrial septal defect, secundum type. An atrial septal defect provides communication between the right and left atrium. The defect must be quite large in the fetus to be identified by ultrasound.

The area of the foramen ovale is thinner in the fetus than the surrounding atrial tissue; therefore, with echocardiography, the area is prone to signal dropout, particularly in the apical four-chamber view when the transducer is parallel to the septum. Any break in the atrial septum in this view must be confirmed by the short-axis, or subcostal, view (the transducer is inferior to the heart and angled cephalad in a transverse or short-axis plane), in which the septum is more perpendicular to the transducer. Because of beam-width artifacts, the edges of the defect may be slightly blunted and appear brighter than the remaining septum.

In utero, the natural flow in the atrium is right to left across the foramen (as the pressures are slightly higher on the right). A small reversal flow may be present. The foramen should flap into the left atrial cavity. The flap should not be so large as to touch the lateral wall of the atrium; when this redundancy of the foramen occurs, the sinoatrial node may become agitated in the right atrium and cause fetal arrhythmias. The sonographer should be sure to sweep inferior to superior along the atrial septum to identify the three parts of the septum: the primum septum, fossa ovalis, and septum secundum.

Ostium Secundum Atrial Septal Defect

The most common atrial defect is the secundum atrial septal defect, which occurs in the area of the fossa ovalis ( Fig. 36.8 ). Usually, an absence of the foramen ovale flap is noted, with the fossa ovalis opening larger than normal.

Sonographic Findings

Doppler tracings of the septal defect with the sample volume placed at the site of the defect show a right-to-left flow with a velocity of 20 to 30 cm/sec. Color flow Doppler is performed in the apical four-chamber and subcostal views and may be useful to outline the size and direction of flow as it crosses the foramen ovale ( Fig. 36.9 ). The flow patterns of the mitral and tricuspid valves are slightly increased with the elevated shunt flow.

Ostium Primum Septal Defect

The primum septal defect is deficient in the lower (inferior) portion of the septum, near the crux of the heart ( Fig. 36.10 ). It may be seen in AVSD malformation in which there is malalignment of the atrioventricular valves secondary to the defect. In addition, a cleft of the mitral valve is present, causing mitral regurgitation into the left atrial cavity.

Sonographic Findings

The primum septal defect is best imaged in the four-chamber plane that is parallel to the transducer beam. The gain should be reduced to clearly identify the atrial septum. Look for the flap of the foramen ovale. This defect may be part of an AVSD or a primary defect with or without a cleft mitral valve.

Sinus Venosus Septal Defect

The sinus venosus atrial septal defect is technically more difficult to visualize with echocardiography. This defect lies in the superior portion of the atrial septum, close to the inflow pattern of the superior vena cava ( Fig. 36.11 ).

Sonographic Findings

Sinus venosus septal defects are best visualized with the subxiphoid four-chamber view. If signs of right ventricular volume overload are present, with no atrial septal defect obvious, care should be taken to study the septum in search of a sinus venosus type of defect. Partial anomalous pulmonary venous drainage of the right pulmonary vein is usually associated with this type of defect; thus it is important to identify the entry site of the pulmonary veins into the left atrial cavity. Color flow mapping is useful in this type of problem because it allows the sonographer to visualize the venous return to the left atrium and a flow pattern crossing into the right atrial cavity.

Ventricular Septal Defect

Ventricular septal defect is the most common congenital lesion of the heart, accounting for 30% of all structural heart defects. The septum is divided into two basic segments: the membranous and muscular areas ( Fig. 36.12 ). The septum lies in a curvilinear plane and has different areas of thickness. There are several sites where ventricular septal defects may occur within the septum. Muscular defects occur more inferior in the septum, usually are very small, and may be multiple ( Fig. 36.13 ). Often, smaller defects will close spontaneously shortly after birth. This type of muscular defect is more difficult to image with echocardiography.

Membranous Septal Defect

The (perimembranous) ventricular septal defect may be classified as membranous, aneurysmal, or supracristal ( Fig. 36.14 ). The significant anatomic landmark is the crista supraventricularis ridge. The defect lies either above or below this ridge. Defects that lie above are called supracristal . Supracristal defects are located just beneath the pulmonary orifice so that the pulmonary valve forms part of the superior margin of the interventricular communication. Defects that lie below the crista are called infracristal and may be found in the membranous or muscular part of the septum. Infracristal defects are the most common.

Sonographic Findings

The lesion may be partially covered by the tricuspid septal leaflet, and the sonographer must carefully evaluate this area with Doppler and color flow tracings. The membranous defect is found just below the aortic leaflets; sometimes, the aortic leaflet is sucked into this defect ( Fig. 36.15 ).

The presence of an isolated ventricular septal defect in utero usually does not change the hemodynamics of the fetus. Defects smaller than 2 mm are not detected by fetal echocardiography as these are beyond the limits of resolution. Care must be taken in the four-chamber view to carefully sweep the probe posterior to record the inlet part of the septum to anterior to record the outlet part of the septum.

Ventricular septal defects may close with the formation of aneurysm tissue, which is commonly found along the right side of the septal defect ( Fig. 36.16 ). These aneurysms generally protrude into the right heart in one of the following three directions: (1) above the tricuspid valve and into the right atrium, (2) directly into the septal leaflet of the tricuspid valve, or (3) below the tricuspid leaflets and into the right ventricular cavity. Usually, these aneurysms are small, but obstruction may occur in the right ventricular outflow tract when they become large.

Atrioventricular Septal Defect

The endocardial cushion defect is also called ostium primum atrial septal defect, atrioventricular canal malformation, endocardial cushion defect , and atrioventricular septal defect (AVSD) ( Fig. 36.17 ). These defects are subdivided into complete, incomplete, and partial forms.

Incomplete Atrioventricular Septal Defect

The failure of the endocardial cushion to fuse is termed an incomplete atrioventricular septal defect . This condition results in a membranous ventricular septal defect, abnormal tricuspid valve, primum atrial septal defect, and cleft mitral valve ( Fig. 36.18 ). A cleft mitral valve means that the anterior part of the leaflet is divided into two parts (medial and lateral). When the leaflet closes, blood leaks through this hole into the left atrial cavity. The leaflet is usually somewhat malformed, further causing regurgitation into the atrium. In addition, there is a communication between the left ventricle and right atrium (left ventricular to right atrial shunt) because of the absent primum atrial septum and membranous interventricular septum. The ventricular septal defect occurs just below the mitral ring and is continuous with the primum atrial septal defect.

Complete Atrioventricular Septal Defect

The endocardial defect is characterized by the insertion of the chordae from the cleft mitral and tricuspid valve into the crest of the ventricular septum or a right ventricular papillary muscle ( Fig. 36.19 ). The most primitive form is called a complete atrioventricular septal defect . This defect has a single, undivided, free-floating leaflet stretching across both ventricles. A two-dimensional (2D) sweep from the mitral to the aortic valves would show the anterior mitral leaflet swinging through the ventricular septal defect in continuity with the tricuspid valve. The tricuspid valve is said to cap the mitral valve. The anterior and posterior leaflets are on both sides of the interventricular septum, causing the valve to override or straddle the septum. This is a more complex abnormality to repair because the defect is larger, and the single atrioventricular valve is more difficult to manage clinically, depending on the amount of regurgitation present. The regurgitant jet may extend from the right ventricle to the left atrium, secondary to the valve deformity and increased right heart pressures. Complete AVSDs are frequently associated with malpositions of the heart (mesocardia and dextrocardia) and atrioventricular block (abnormal rhythm secondary to distortion of the conduction tissues).

AVSDs are frequently associated with other cardiac defects, including truncoconal abnormalities, coarctation of the aorta, and pulmonary stenosis or atresia. There is an increased incidence of Down syndrome (50% of trisomy 21 babies have congenital heart disease) and asplenia and polysplenia syndromes.

Occasionally, complete absence of the interatrial septum is noted in the fetal four-chamber view. With color flow, the entire atria are completely filled throughout systole and diastole. This is termed common atria .

With a partial AVSD, the fetus has only some of the previously described findings, usually an absent primum atrial septum and a cleft mitral valve.

Sonographic Findings

Echocardiographically, the ideal views are the long-axis, short-axis (to search for abnormalities in the atrioventricular valves, such as presence of cleft), and four-chamber views (to search for chordal attachment, overriding, or straddling of the valves). The atrioventricular valves share a superior and inferior bridging leaflet that results in a functional single large valve. This may be well demonstrated in the short-axis view of the ventricle as the single large valve appears as a wide circle figure-of-8 sign . The crux of the heart is carefully analyzed by slowly sweeping the transducer anterior (toward the aorta outlet) to posterior (toward the atrioventricular valve inlet) to record the outlet and inlet portions of the membranous septum.

Doppler and color flow techniques are extremely useful in determining the direction and degree of regurgitation present in the atrioventricular valves and the direction of shunt flow (increased right-sided heart pressure causes a right ventricular to left atrial shunt in the fetus).

Tricuspid Atresia/Stenosis

Tricuspid atresia is the interruption of the growth of the tricuspid leaflet that begins early in cardiac embryology. This interruption involves the growth of the tricuspid apparatus, causing the valve to be hypoplastic or atretic.

In tricuspid atresia, the inflow portion of the right ventricle has failed to form, and a membrane or dimple in the floor of the right atrium represents the position where the tricuspid valve should have originated ( Fig. 36.20 ). A ventricular septal defect may be present to help shunt blood into the hypertrophied right ventricle. The right ventricular outflow tract and pulmonary artery are generally diminished in size.

Sonographic Findings

Echocardiographically, the tricuspid valve is best visualized on the four-chamber view ( Fig. 36.21 ). The findings in tricuspid atresia are a large dilated left ventricular cavity with a small, underdeveloped right ventricular cavity. The echogenic tricuspid annulus is seen with no valvular movement. The mitral valve is clearly the dominant atrioventricular valve. On the long- and short-axis views, the right ventricle is seen as a slit-like cavity just anterior to the interventricular septum.

Color flow imaging shows the incoming blood entering the right atrium and crossing the patent foramen ovale to enter the left side of the heart. If no blood flow passes the tricuspid orifice, pulmonary stenosis is present. However, if a ventricular septal defect is present, the blood flows from the high-pressure left ventricle across the defect into the hypertrophied right ventricle and out the pulmonary outflow tract.