Malformations of the heart and arterial trunks are the most common form of congenital anomalies 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 diagnosis of structural heart defects, and offers a way to observe complex physiological 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, the ability to implement appropriate postnatal management strategies in an anticipatory fashion, so as to maximise outcome, and the ability 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%. 1–5 Many studies have documented the utility of the four-chamber view of the heart in identifying the malformations during fetal life. 5–11 In these studies, rates of detection using this view alone ranged from 4.5% to 81%. 5–11 Malformations of the outflow tracts, however, and discordant ventriculo-arterial 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 obstetrical screen, the reported detection rate increases to between 43.8% and 85.5%. 7,12,13 In contrast, if fetal echocardiography, defined as a detailed, focused assessment of the fetal cardiovascular system, is performed, then rates of detection are significantly higher, and diagnostic accuracy rates may exceed 85% to 90%. 12,14 Given the disparity in rates of detection between those specialising in fetal echocardiography and routine obstetrical 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 ( Table 10-1 ). 15 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 on account of improved techniques becoming available for imaging. Referrals for fetal echocardiography producing a high yield now include an abnormal obstetrical 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. 16,17 Referrals with a low yield include presence of a single umbilical artery, or exposure to teratogens, with congenital cardiac lesions detected infrequently in mothers referred with these indications. 16,17 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. 16,17 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 aged from 10 to 14 weeks is a published marker for aneuploidy. 18–23 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. 24,25 Fetuses conceived via artificial fertilisation, particularly when using intracytoplasmic injection of sperm, have a twofold increased risk of major birth defects, including congenital cardiac disease, compared to infants conceived naturally. 26 While somewhat controversial, it is increasingly recognised that the frequency of congenital anomalies is increased in this population, and fetal echocardiography is therefore of benefit to these families.
| Maternal Indications |
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| Fetal Indications |
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TIMING OF FETAL ECHOCARDIOGRAPHY
Fetal echocardiography is best performed between 18 and 22 weeks of gestation. At this gestational age, there is adequate amniotic fluid to allow good visualisation of the cardiac structures and vasculature. After 30 weeks gestation, the increase in the fetal body mass, and the shadowing effects of the fetal ribs, may make acquisition of images more difficult. Maternal transabdominal fetal imaging may be performed between 15 and 18 weeks gestation, although visualisation may be suboptimal. New interest has emerged in early maternal transabdominal or trans-vaginal scanning at 11 to 14 weeks of 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 acquisition of images at these gestational ages for both maternal transabdominal and trans-vaginal imaging. Diagnostic transabdominal images have been reported in 98.7% of cases, with abnormalities detected in 13 of 226 fetuses studied. 27 In this study, only 4 of 213 minor structural abnormalities were missed on the initial scan. 27 Others have shown that the four-chamber view can be obtained using trans-vaginal imaging in all cases, with extended views of the heart obtained in almost all of these instances. 28 In this study, three cases of major congenital cardiac disease were diagnosed on the initial trans-vaginal scan, while three cases were diagnosed later in gestation. 28
MODALITIES FOR 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. 15 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 15 ( Table 10-2 and Figs. 10-1 and 10-2 ). 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. 15 Reference ranges for the diameters of all valves over the course of gestation have been published. 29 Cardiac dysfunction may be assessed by cross sectional interrogation by the presence of ascites, pleural or pericardial effusions, skin oedema, and cardiomegaly, as defined by a ratio of cardiothoracic areas of greater than 0.36. 30 A ratio of cardiothoracic areas greater than 0.6 is associated with an extremely poor outcome. 31
| Feature | Essential Component |
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| Anatomical overview |
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| Biometric examination |
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| Cardiac imaging views/sweeps |
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| Doppler examination |
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| Measurements |
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| Examination of rhythm and rate |
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Colour Doppler interrogation adds to the assessment of fetal cardiovascular wellbeing 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, 32,33 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. 34 In contrast, regurgitation across the mitral, pulmonary, 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 colour 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 oval foramen and the arterial duct. Abnormal directions of flow at these sites may suggest cardiac disease. For example, left-to-right flow at the patent oval foramen, or bidirectional shunting through the arterial duct with reversal of flow in the transverse arch, may indicate inadequacy of the left ventricle. 35,36
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. Pathological conditions associated with elevated central venous pressures may manifest as changes in the Doppler flow patterns in the inferior caval vein, the venous duct, and the umbilical vein. After 18 weeks gestation, flow in the venous duct should be all antegrade with atrial contraction. As central venous pressure increases, there is first decreased flow, and ultimately reversal of flow seen with atrial contraction in the venous duct ( Fig. 10-3 ). In the umbilical vein, there is normally continuous forward flow at low velocity. As central venous pressure increases, notching is seen at the end of diastole in the umbilical venous flow. In severe cardiovascular compromise, there is absence of flow at end-diastole, with venous pulsations seen in the umbilical vein.
Doppler evaluation of the arterial system provides important information regarding fetal cardiovascular wellbeing. Doppler evaluation of the umbilical artery provides information concerning the health and state of the placental circulation ( Fig. 10-4 ). The healthy placenta has a very low vascular resistance, and hence Doppler spectral display will demonstrate substantial antegrade flow during the diastolic phase of the cardiac cycle. Abnormally elevated placental vascular resistance, as seen in cases of intra-uterine retardation of growth, or in the donor fetus of a twin-twin transfusion syndrome, can be identified by the presence of diminished, or even reversed, flow in diastole ( Fig. 10-5 ).
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 auto-regulatory 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 flow to the brain is diminished due to an overall decrease in cardiac output as a consequence of myocardial dysfunction, or due to an anatomical impediment, vascular resistance will be lower than normal in the middle cerebral artery as the brain attempts to augment volume and flow ( Fig. 10-6 ). 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. 37,38 In addition, in fetuses with cardiovascular compromise, there may be a redistribution of the cardiac output away from the placenta and toward the brain, the so called brain sparing effect. 39–41 Typically, the ratio of resistance in the middle cerebral artery compared to the umbilical artery is greater than 1. With hypoxia or inadequate cardiac output, there may be cephalisation of flow, characterised by a ratio of resistance in the middle cerebral artery compared to the umbilical artery of less than 1. Assessment of patterns of antegrade flow across the atrioventricular valves also provides important information regarding diastolic function and ventricular compliance ( Fig. 10-7 ). Compared to mature myocardium, the fetal myocardium is comprised of greater non-contractile elements. 42 As a result, the fetal heart exhibits impaired myocardial relaxation, reflected by a lower velocity of the Doppler E wave compared to that of the A wave. Over the course of gestation, the ratio of the velocities of the E to A waves increases progressively. 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. 10-8 ).
