Key Words
Fetal echocardiography, Doppler, color Doppler
Introduction
Malformation of the heart and arterial trunks is the most common form of congenital anomaly found in humans. They occur in approximately 6 of every 1000 live births and in 8 to 10 of every 1000 pregnancies. Fetal echocardiography, or the use of ultrasonic technologies to evaluate the fetal cardiovascular system, enables the diagnosis of structural heart defects and offers a way to observe complex physiologic processes prior to birth. The primary benefits of fetal echocardiography include the ability to counsel parents prior to birth as to the expectations for a child born with a congenitally malformed heart and the ability to implement appropriate postnatal management strategies in an anticipatory fashion so as to maximize outcome and to identify and treat cardiovascular diseases prior to birth.
Screening With Fetal Echocardiography
As imaging technologies and the skills of operators continue to improve, a higher percentage of congenital cardiac malformations can be detected accurately before birth. Analysis of large registries has shown that rates of detection can vary from 15% to 25%. Many studies have documented the utility of the four-chamber view of the heart in identifying the malformations during fetal life. In these studies, rates of detection using this view alone ranged from 4.5% to 81%. Malformations of the outflow tracts, however, and discordant ventriculoarterial connections, may frequently be missed when this view is used in isolation. When views showing the right and left ventricular outflow tracts are included in the obstetric screen, the reported detection rate increases to between 43.8% and 85.5%. In contrast, if fetal echocardiography—defined as a detailed, focused assessment of the fetal cardiovascular system—is performed, rates of detection are significantly higher and diagnostic accuracy rates may exceed 85% to 90%. Given the disparity in rates of detection between those specializing in fetal echocardiography and routine obstetric screening, the question of who should be referred for fetal echocardiography remains an important consideration.
Currently the American Society of Echocardiography recommends fetal echocardiography for fetal, maternal, and familial indications ( Box 8.1 ). In the past, many women were referred for fetal echocardiography due to a family history of congenital cardiac disease, an umbilical cord containing two vessels, maternal diabetes, or maternal exposure to teratogens. Patterns of referral, however, have changed because of improved techniques for imaging have become available. Referrals for fetal echocardiography producing a high yield now include an abnormal obstetric ultrasound evaluation, in which up to two-thirds of referrals have congenital cardiac malformations, and a chromosomal anomaly, with half of such referrals proving to have congenital cardiac lesions. Referrals with a low yield include the presence of a single umbilical artery or exposure to teratogens, with congenital cardiac lesions detected infrequently in mothers referred with these indications. A family history of congenital cardiac disease accounts for between one-quarter and one-third of all referrals for fetal echocardiography but less than one-twentieth of cases with detected malformations. Increased nuchal translucency noted on screening during the first trimester, from 10 to 13 weeks of gestation, has now emerged as an important indication for fetal echocardiography. Indeed, increased nuchal translucency of greater than 3 mm as seen during scanning when the fetus is from 10 to 14 weeks of age is a published marker for aneuploidy. Even in the absence of a chromosomal anomaly, fetuses found to have increased nuchal translucency on early scanning have been shown to be at increased risk for congenital cardiac disease. Fetuses conceived via artificial fertilization, particularly when intracytoplasmic injection of sperm is used, have a twofold increased risk of major birth defects, including congenital cardiac disease, compared with infants conceived naturally. Therefore fetal echocardiography has emerged as a benefit to this group of patients.
Maternal Indications
- ■
Family history of congenital cardiac disease
- ■
Metabolic disorders, such as phenylketonuria or diabetes
- ■
Exposure to teratogens
- ■
Exposure to inhibitors of prostaglandin synthetase, such as ibuprofen, salicylic acid, or indomethacin
- ■
Infection with rubella
- ■
Autoimmune disease, such as systemic lupus erythematosus, or Sjögren syndrome
- ■
Familial inherited disorders, such as Ellis van Creveld syndrome, Marfan syndrome, or Noonan syndrome
- ■
Artificial fertilization
Fetal Indications
- ■
Abnormal result following obstetric ultrasonic screening
- ■
Extracardiac abnormality
- ■
Chromosomal abnormality
- ■
Arrhythmia
- ■
Hydrops
- ■
Increased nuchal translucency in first trimester
- ■
Multiple gestation and twin-twin transfusion
Timing of Fetal Echocardiography
Fetal echocardiography is best performed between 18 and 22 weeks’ gestation. At this gestational age there is adequate amniotic fluid to allow good visualization of the cardiac structures and vasculature. After 30 weeks’ gestation, increase in the fetal body mass and the shadowing effects of the fetal ribs may make the acquisition of images more difficult. Maternal transabdominal fetal imaging may be performed between 15 and 18 weeks’ gestation, although visualization may be suboptimal. At some centers, first-trimester screening for congenital heart disease may occur as early as 10 to 14 weeks’ gestation, especially in populations known to be at high risk, such as those noted to have increased nuchal translucency during scanning at 10 to 14 weeks, those with suspected aneuploidy, or those with a family history of congenital cardiac disease. Feasibility studies have demonstrated adequate image acquisition at these gestational ages for both maternal transabdominal and transvaginal imaging. At some centers with experienced practitioners, the prenatal detection rate for major congenital heart disease exceeds 86% to 90% within the first trimester.
Modalities for Imaging
Two-Dimensional Imaging
Cross-sectional imaging remains the gold standard for the diagnosis of structural cardiac disease during fetal life. The Pediatric Council of the American Society of Echocardiography recommends obtaining multiple cross-sectional tomographic views of the heart in order to make an accurate diagnosis. Fetal echocardiography should include an apical four-chamber view of the heart, an apical five-chamber view, a long-axis view of the left ventricular outflow tract, a long-axis view of the right ventricular outflow tract, a short-axis view at the level of the arterial trunks, a short-axis view at the level of the ventricles, a long-axis view of the caval veins, a view of the ductal arch, and a view of the aortic arch ( Table 8.1 ; Figs. 8.1 and 8.2 ). The American Institute of Ultrasound in Medicine also recommends the three-vessel tracheal view, which has proven useful in the assessment of critical congenital heart disease as well as aortic arch anomalies. The fetal heart rate should be documented and any arrhythmia confirmed with M-mode imaging. The diameters of the orifices of all valves should be measured in systole at right angles to the plane of flow. Reference ranges for the diameters of all valves over the course of gestation have been published. Cardiac dysfunction may be assessed by cross-sectional interrogation by the presence of ascites, pleural or pericardial effusions, skin edema, and cardiomegaly as defined by a ratio of cardiothoracic areas greater than 0.36. A ratio of cardiothoracic areas greater than 0.6 is associated with an extremely poor outcome.
| Feature | Essential Component |
|---|---|
| Anatomic overview |
|
| Biometric examination |
|
| Cardiac imaging views/sweeps |
|
| Doppler examination |
|
| Measurements |
|
| Examination of rhythm and rate |
|
Three-Dimensional Imaging
Three-dimensional (3D) imaging of the fetal heart has been performed for over 20 years. Studies have demonstrated improved diagnostic accuracy of fetal congenital heart disease with the performance of 3D ultrasound compared with two-dimensional (2D) imaging techniques alone. In addition, 3D ultrasound allows for better visualization of complex spatial relationships, which may be particularly helpful when counseling parents regarding potential postnatal surgical strategies for their unborn child.
Color Doppler
Color Doppler interrogation adds tremendous value to the assessment of fetal cardiovascular well-being by establishing the degree of valvar stenosis and regurgitation, if present. Mild tricuspid regurgitation may be seen throughout gestation and is frequently a benign finding, but tricuspid regurgitation detected during early scanning, from 11 to 14 weeks, may be a marker for aneuploidy even in the absence of structural heart disease. In contrast, regurgitation across the mitral, pulmonic, or aortic valves is usually not a normal finding and suggests pathology secondary to underlying structural cardiac disease or fetal cardiac failure. Cardiovascular physiology can also be assessed by color Doppler echocardiography by determining the direction of blood flow. In the normal fetal circulation, the direction of shunting is right to left at both the patent foramen ovale and the arterial duct. Abnormal directions of flow at these sites may suggest cardiac disease. For example, left-to-right flow at the patent foramen ovale or bidirectional shunting through the arterial duct with reversal of flow in the transverse aortic arch may indicate inadequacy of the left ventricle.
Pulse-Wave Doppler
Doppler echocardiography is a powerful tool with which to assess cardiovascular physiology and function and is an important part of the comprehensive evaluation of the fetal cardiovascular system. Indeed, Doppler techniques may enable the practitioner to sort out complex physiologies and to recognize cardiac compromise, which might not be clear by 2D and color Doppler techniques alone. The Doppler signal can be analyzed by the pulsatility index, defined as the (peak systolic velocity – end-diastolic velocity)/time-averaged mean velocity, the resistance index, defined as the peak systolic velocity – end-diastolic velocity/end-diastolic velocity, or the systolic/diastolic (S/D) ratio.
Umbilical Artery
Doppler evaluation of the umbilical artery provides vital information concerning the health and state of the placental circulation. Doppler interrogation of the umbilical cord should be performed in a free loop of the cord during fetal apnea ( Fig. 8.3 ). Reference values for the umbilical arterial Doppler flow pattern over the course of gestation have been published. The healthy placenta should have very low vascular resistance, and hence the Doppler spectral display will demonstrate substantial antegrade flow during both systole and diastole ( Fig. 8.4 ). However, gestational age factors heavily into the interpretation of the umbilical arterial flow pattern, since diminished or even absent flow in diastole is a normal finding in the first trimester. Abnormally elevated placental vascular resistance, as seen in cases of intrauterine growth retardation, can be identified by the presence of diminished, or even reversed, flow in diastole (see Fig. 8.4 ). Multiple studies have demonstrated poor neonatal outcomes or intrauterine fetal demise in cases of absent or reversed end-diastolic flow in the umbilical artery. In various forms of congenital heart disease, the umbilical artery resistance may be elevated, yet it usually remains within the 95% confidence interval for gestational age.
Middle Cerebral Artery
Doppler interrogation of the middle cerebral artery provides an assessment of cerebrovascular resistance in the fetus and may also offer important information regarding fetal well-being. Fig. 8.5 demonstrates the normal Doppler flow pattern for the middle cerebral artery as well as the Doppler flow patterns for a fetus with low and high cerebrovascular resistance. In the normal middle cerebral Doppler flow pattern, most of the flow occurs in systole, with only a small portion occurring in diastole. Reference values for the middle cerebral artery over the course of gestation have been published. An elevated peak systolic velocity in the middle cerebral artery is a sensitive marker for fetal anemia and may also be predictive of perinatal mortality in fetuses with intrauterine growth retardation.
Changes in cerebrovascular resistance may be seen in conditions of altered cardiac output and with different forms of congenital cardiac disease. These changes are a manifestation of autoregulatory mechanisms of the fetal cardiovascular system, in which there is a natural tendency to preserve flow of blood to vital organs, such as the brain. When blood flow to the brain is diminished due to an overall decrease in cardiac output because of myocardial dysfunction, or because of anatomic impediment, vascular resistance will be lower than normal in the middle cerebral artery as the brain “attempts” to augment volume and flow. Hence, in the presence of left-sided obstructive lesions, the resistance measured in the middle cerebral artery decreases. In the setting of right-sided obstructive lesions, in contrast, the resistance increases (see also Chapter 76 ). In fetuses with cardiovascular compromise, such as intrauterine growth retardation, there may be a redistribution of the cardiac output away from the placenta and toward the brain, the so-called brain-sparing effect. Typically the ratio of resistance in the middle cerebral artery compared with the umbilical artery is greater than 1. With hypoxia or inadequate cardiac output, there may be “cephalization” of flow, characterized by a ratio of resistance in the middle cerebral artery compared with the umbilical artery of less than 1.
Arterial Duct
The arterial duct plays a key role in the fetal circulation, allowing most of the deoxygenated blood to bypass the lungs, which are under high resistance in utero. At 20 weeks’ gestation, about 13% of the combined cardiac output reaches the lungs, although this percentage increases to nearly 25% by 30 weeks’ gestation. The normal Doppler flow pattern for the arterial duct demonstrates the majority of flow in systole, with only a small proportion occurring in diastole. The arterial duct should be large and unrestrictive in utero. Indeed, constriction of the arterial duct leads to increased afterload on the right ventricle, which may be characterized by tricuspid regurgitation, pulmonary insufficiency, and right ventricular dysfunction. Multiple medications—including corticosteroids, high-dose aspirin, and prostaglandin synthetase inhibitors—have been shown to cause constriction of the arterial duct. In cases of severe constriction of the arterial duct, prompt recognition of this finding, discontinuation of causative agents, and early delivery may be critical to avoid intrauterine fetal demise. Fig. 8.6 demonstrates both the normal and a constricted arterial duct Doppler flow pattern.
Umbilical Vein
In the umbilical vein there is normally continuous forward flow at low velocity. Reference values for the umbilical venous Doppler have been published previously. As in the case of the umbilical artery, tracings should be obtained during fetal apnea, since respiratory effort in the second and third trimesters may influence the umbilical venous Doppler flow pattern. As central venous pressure increases, notching is seen at the end of diastole in the umbilical venous flow. In severe cardiovascular compromise, absence of flow at end-diastole and venous pulsations may be noted. Fig. 8.7 illustrates both the normal umbilical venous Doppler flow pattern and an abnormal pattern with venous pulsations in a fetus with heart failure.
Venous Duct
The venous duct is a key site of shunting within the fetal cardiovascular system, enabling highly oxygenated blood within the umbilical vein to bypass most of the hepatic circulation and return to the heart via the inferior caval vein. Absence of the venous duct may be associated with aneuploidy and congenital anomalies and can also lead to the development of hydrops fetalis if the umbilical vein bypasses the liver and inserts directly into the inferior caval vein. Resistance within the venous duct gradually declines over the course of gestation. The normal Doppler flow pattern—comprising s, d, and a waves—is shown in Fig. 8.8 . The s wave corresponds to ventricular systole, the d wave to ventricular diastole, and the a wave to the nadir during atrial contraction. Reference ranges for the venous duct over the course of gestation have been published. After 18 weeks’ gestation, flow in the venous duct should be all antegrade with atrial contraction. However, as central venous pressure increases, first there is decreased flow, next there is diminished flow, and finally there is reversal of flow with atrial contraction in the venous duct (see Fig. 8.8 ). Reversal of flow with atrial contraction may be seen in a variety of pathologic conditions, including the Ebstein anomaly, pulmonary atresia with intact atrial septum, complete heart block, and intrauterine growth restriction. In these cases, central venous pressure is elevated.
Atrioventricular Valves
The Doppler flow pattern across the mitral and tricuspid valves reflects important information regarding diastolic function and ventricular compliance. In contrast to mature myocardium, fetal myocardium comprises a greater amount of noncontractile elements, causing impaired relaxation. Consequently the normal Doppler flow consists of a two-peak pattern with a smaller E wave, representing passive filling during early diastole, and a larger a wave, representing active atrial contraction. In severe cases of diastolic dysfunction—such as in the recipient twin with twin-twin transfusion syndrome, or in ventricles compromised by endocardial fibroelastosis—the E and a waves may merge into a single peak. Fig. 8.9 illustrates the two-peak Doppler flow pattern for the tricuspid valve in a normal fetus and a single-peak Doppler flow pattern in a recipient twin with twin-twin transfusion syndrome.
