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Estimated Time for Completion: approximately 1 hour
Date of Release and Review: January 2, 2014, January 12, 2017
Expiration Date: January 15, 2020
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Congenital heart disease (CHD) occurs in approximately 8/1,000 neonates1. The incidence of CHD in stillbirths is approximately 10 times greater2,3. If fetuses with aneuploidy and extra-cardiac malformations are excluded, survival rates of isolated CHD detected prenatally are equivalent to neonatal statistics4.
The sonographic evaluation of the fetus for anomalies is a major component of the 2nd trimester ultrasound examination. The role of the anatomic survey specifically seeks to detect the more common significant fetal congenital anomalies. The time of onset and sonographic manifestations of specific congenital anomalies varies markedly5. Some congenital anomalies, i.e. coarctation of the aorta, are detectable at variable gestational ages.
22.1% of infant deaths are due to congenital anomalies; one-third of which are incompatible with life6. 35% of infant deaths from congenital malformations are related to cardiovascular anomalies7.
The fetal heart is the most difficult anatomic structure to appropriately image during a 2nd trimester ultrasound examination. In the original guidelines for an obstetrical ultrasound examination published in 19868 only four anatomic structures were included (Table I). Five years later, a 4-chamber cardiac view of the fetal heart was added to the anatomic survey9. By 2003 the anatomic survey had expanded to include the outflow tracts "if technically feasible"10. Recently11, the outflow tracts became part of the anatomic survey without a qualifying statement.
A 4-chamber cardiac view has the potential to detect up to 60% of major congenital heart defects12. However, the detection rate varies markedly between institutions and is primarily determined by the experience of the sonographer/sonologist performing the examination13. When experienced sonographers (> 2,000 routine 2nd trimester ultrasound examinations) are compared to recent hires, the detection rate of a 4-chamber cardiac view (Fig. 1) and outflow tracts (Figs. 2,3) was 75% and 36%, respectively14.
Other variables that affect detection rates of congenital anomalies on a 4-chamber cardiac view of the heart include gestational age at the time of the examination, amniotic fluid volume, and associated congenital anomalies15.
An appropriate 2-dimensional evaluation of the fetal heart requires the steps outlined in Table II.
The left and right sides of the fetus must be established, in order to correctly determine cardiac and abdominal situs. The normal cardiac axis is 22-75 degrees (Figs. 4,5)16. The cardiac circumference is approximately 50% of the chest (Fig. 6). The moderator band in the apex of the right ventricle and a slightly offset tricuspid valve towards the apex of the heart permit delineation of the right and left ventricles (Fig 1). Atrial situs requires the identification of the inferior and superior vena cava entering the right atrium.
Although the 4-chamber view is quite effective in detecting ventricular or atrial disproportion, it does not evaluate the left and right ventricular outflow tracts. The cardiac defects that are not detectable in the 4-chamber cardiac view are outlined in Table III.
The 90% confidence interval for the right/left ventricle and pulmonary artery/aorta ratios are 0.79-1.24 and 0.84-1.41, respectively. Cardiac asymmetry (Fig. 7) in one or both of these ratios is associated with an increased risk of a structural malformation17,18. The right ventricle to left ventricular ratio may be abnormally high with intrauterine growth restriction19.
Abnormalities of the right ventricle include tricuspid atresia. This is the third most common form of cyanotic heart disease with a prevalence of 0.3-0.7% in patients with congenital heart disease20. The most common type of tricuspid atresia is muscular (90%). It is characterized by a local fibrous thickening at the expected site of the tricuspid valve21. Flow is not detected across the tricuspid valve and the tissue at the tricuspid valve is echogenic.
With an associated ventricular septal defect (VSD), there is a hypoplastic right ventricle. The ventricular septum is rarely intact with tricuspid atresia. However, the VSD is usually small22. In the absence of a VSD, blood does not enter the right ventricle and its normal development is prevented, resulting in a single (left) ventricle (Fig. 8).
With tricuspid atresia the right atrium is enlarged. After delivery, an interatrial communication, i.e. a stretched patent foramen ovale, is necessary for survival. Other sonographic findings with tricuspid atresia include left ventricular dilatation/hypertrophy (Fig. 9), a dilated mitral valve and mitral regurgitation23.
In approximately 20-50% of cases with tricuspid atresia, the great vessels will be transposed24. Table IV outlines some of the cardiac defects that may be associated with tricuspid atresia25.
The classification of tricuspid atresia is based on the morphology of the valve and associated cardiac defects26.
Tricuspid atresia may lead to heart failure. As a result, pregnancies with this diagnosis should be scanned at regular intervals. Tricuspid atresia is rarely associated with other extra-cardiac anomalies.
The differential diagnosis of a hypoplastic right ventricle also includes pulmonary atresia with and without an intact ventricular septum and severe pulmonary stenosis.
Pulmonary atresia occurs in approximately 1% of stillbirths and 7% of live born infants with congenital heart disease3. Because of retrograde ductal arteriosus flow, pulmonary artery hypoplasia is present in only one-third of fetuses with pulmonary stenosis. Hence, all newborns with pulmonary atresia and most newborns with pulmonary stenosis, will have a ductal dependent circulation27.
Right atrial dilatation, with or without tricuspid regurgitation (Fig. 10) and right ventricular hypertrophy, is frequently associated with pulmonary stenosis/atresia.
Secondary dysplasia of the tricuspid valve has been associated with pulmonary atresia. If a VSD is present, a small right ventricle is rare. When a ventricular septal defect is not present with tricuspid or pulmonary atresia, the right ventricle is not only hypoplastic, but its walls are hypertrophic and the right ventricle is hypokinetic. Endocardial fibroelastosis may also occur.
Milder forms of pulmonary stenosis, in contrast to atresia, are not usually detectable in utero. The fetal presentation of pulmonary stenosis is based on the severity of the lesion27. The pulmonary valve may appear thickened and motion of the pulmonary leaflets is restricted. With narrowing of the pulmonary valve aperture, an increased velocity will be detected across the valve (Fig. 11).
The right atrial dilatation in pulmonary stenosis or atresia is due to tricuspid insufficiency and/or elevated right ventricular filling pressure27.
The absence of detectable forward flow in the pulmonary artery does not exclude a patent pulmonary valve27. In "functional" pulmonary artery obstruction, ventricular pressure does not exceed pulmonary artery pressure due to severe tricuspid incompetence28. The in utero diagnosis of functional pulmonary atresia is suggested by a pulmonary artery that is not hypoplastic and Doppler flow proximal to the pulmonary valve indicating regurgitation29.
Pulmonary stenosis may progress in utero to complete atresia.
When pulmonary atresia is associated with a ventricular septal defect, the pulmonary trunk is quite small. However, in the presence of an intact ventricular septum, the diameter of the pulmonary trunk is either normal or enlarged. The difference in pulmonary diameter is likely due to the pathophysiology of the lesion. In the presence of a VSD, the small pulmonary trunk is due to a congenital cardiac anomaly. When pulmonary atresia is associated with an intact interventricular septum, the normal appearance of the pulmonary valve and trunk suggests that the pulmonary atresia developed at a later gestational age and may be an acquired abnormality30.
Obstructive cardiac lesions can change in severity as gestation advances. A reduced rate of chamber or vessel size is a result of reduced blood flow31.
Assessment of the ductus venosus is important in evaluating the heart with diastolic dysfunction and incipient cardiac failure (Fig. 12).
With either tricuspid or pulmonary atresia, the pulmonary artery is smaller than the aorta. The three-vessel view (3vv) (Fig. 13) is obtained by moving the scanning plane cephalad from the 4-chamber view toward the upper mediastinum. The main pulmonary artery, ascending aorta, and superior vena cava are imaged in cross-section. The 3vv can be used to evaluate: 1) alignment; 2) vessel size; 3) aggressive arrangement; 4) number of vessels; 5) color flow mapping; and 6) aorta arch sidedness32. In severe pulmonary stenosis and tricuspid atresia, the disproportion in the size of the pulmonary artery and aorta is evident33
The in utero sonographic detection of pulmonary atresia/stenosis and tricuspid atresia is 50% to 100%34,35. The mortality with untreated tricuspid atresia is 90% by 1 year of age. However, with surgery there is a 50% survival rate at age 1536.