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The heart of the matter Antenatal diagnosis of fetal heart malformation at 11 to 14 weeks gestation The incidence of fetal cardiac malformation amongst live births (eight per 1000) is six times greater than chromosomal abnormalities and four times that of neural tube defects. Furthermore, over half of these babies have a significant defect which contributes to 20 per cent of all neonatal deaths and up to 50 per cent of all infant deaths.1 Importantly, there is good evidence supporting the view that prenatal detection of these major defects improves p ostnatal Dr Simon Meagher outcome, particularly for lesions such as transposition of the great FRANZCOG arteries (TGA) and other arterial duct dependent lesions. For this reason amongst o thers, there is increasing international interest in the prenatal detection of congenital heart defects (CHD). Detection rates of fetal cardiac malformation in the mid-trimester is variable and dependent on many factors, including o perator expertise, patient size, fetal position and the complexity of the s pecific cardiac lesion. For example, a recent population study in Victoria revealed a detection rate for the more obvious abnormalities such as hypoplastic left heart and double outlet/ inlet ventricle of 84 per cent and 74 per cent respectively, whereas detection rates in more subtle lesions such as coarctation and transposition were 26 per cent and 17 per cent respectively.2 Identification of all these abnormalities in the first trimester is unquestionably more difficult. The fetus at 11 to 14 weeks is approximately 6 to 8 cm in length. The fetal heart size at 12 weeks measures 11 mm long and 6 mm across, therefore, approximately the size of the fifth fingernail. This said, the axial and lateral resolution of current ultrasound systems using high frequency endovaginal ultrasound is 0.1 mm. Also, recently published detection rates in specialist centres now approximate the detection rates at the mid-trimester and thus it is a matter of time before the majority of CHD in Australia will become detectable at 11 to 14 weeks. Nuchal translucency and heart defects In the first trimester, an increased nuchal translucency (NT) is a strong indicator of underlying congenital heart disease, far stronger than a past history of CHD or diabetes. Of all births with CHD, 25 to 30 per cent would be expected to demonstrate an increased NT at 11 to 14 weeks. Conversely, the incidence of CHD in fetuses with an increased NT is five per cent. Also, the greater the NT, the greater the chance of underlying CHD ( Table 1). Therefore, patients with an NT of greater than 8.5 mm and normal chromosomes over 60 per cent will have underlying CHD.3 Thus, in patients with a normal karyotype following chorionic villus sampling or amniocentesis, the increased NT should be regarded as a pointer to cardiac malformation. Indeed, once the NT reaches 3 mm, a targeted fetal echocardiogram should be considered. Of note in the presence of an increased NT, there is no obvious bias towards any particular lesion or sidedness of heart defect. Table 1. Bands of risk for increased nuchal translucency and congenital heart malformation.3 NT Range Incidence of major CHD NT in the range 2.5-2.9 mm 2.4% (10/416) NT in the range 3.0-3.4 mm 2.6% (8/306) NT in the range 3.5-4.4 mm 3.1% (12/384) NT in the range 4.5-6.4 mm 8.3% (13/157) NT in the range 6.5-8.4 mm 19% (8/42) NT in the range > 8.5 mm 64.3% (9/14) Tricuspid regurgitation Tricuspid regurgitation (TR) at 11 to 14 weeks is seen in up to 70 per cent of fetuses with Trisomy 21 and has been introduced in the United Kingdom to the Fetal Medicine Foundation first trimester nuchal translucency program. Like nuchal translucency, TR is also an indicator of CHD and recent evidence suggests its association with CHD may be independent of nuchal thickening (Figures 3a and 3b). Its assessment in the first trimester may help subselect the group of fetuses with an increased NT and congenital heart malformation. While TR assessment is not yet routine practice in Australia, it seems likely that assessment of this marker in the future may help identify fetuses with major CHD. Ductus venosus Absence or reversal of flow in the ductus venosus (DV) at 11 to 14 weeks is an indicator of Down syndrome. Recent data, however, also show that Doppler velocimetry of the DV can help improve the predictive capacity of increased NT in the detection of major congenital heart defects in chromosomally normal fetuses. In patients with a thickened NT and normal chromosomes following invasive testing, the finding of an absent or reversed A-wave in the ductus venosus is associated with a three-fold increase in the likelihood of a major cardiac defect, whereas the finding of normal ductal flow is associated with a halving in risk for such defects. Therefore, in patients with normal karyotype following invasive testing where absent ductous venosus (ADV) or reversed ductous venosus (RDV) has been observed, a fetal echocardiogram should be considered. There is also data emerging to suggest that an abnormal first trimester DV blood flow is a risk factor not only for Continued on page ? The heart of the matter Figure 1. A normal four-chamber view of the heart at 12 weeks on both B-mode (left) and colour Doppler e xamination (right). Normal atrial and ventricular symmetry is observed. Figures 3a and 3b. Figure 2. Normal outflow tracts at 12 weeks are demonstrated on this duplex image in which B-mode is shown on the left and directional power Doppler on the right. The calibre and direction of the main pulmonary artery (MPA), right pulmonary artery (RPA), aorta (AO), ductus arteriosis (AD) and descending aorta (DA) are all seen more easily with colour Doppler examination. Doppler assessment of the right heart reveals tricuspid regurgitation (TR) which indicates the possibility of underlying CHD. The heart of the matter CHD, but an indicator of other structural anomalies and perinatal death in fetuses with normal NT.4 Assessment of fetal heart anatomy The fetal heart is best assessed at both transabdominal and transvaginal examination. There should be few reasons why a transvaginal examination is not offered routinely to all patients at the 11 to 14-week scan. Accepting the limitations of probe movement and accepting the unpredictability of fetal position, greater cardiac details are obtained in most patients with this approach (Figure 1). The ease of recognition of normal fetal heart anatomy at 12 versus 11 weeks is striking. A one-week increase in fetal size makes a significant difference in cardiac anatomy resolution. For this reason, patients are best booked after 12 weeks of gestation. The identification of normal cardiac structures at 12 weeks has a strong negative predictive value which is of clinical relevance in the setting where first trimester risk factors have been identified (see below). The four chambers of heart and outflow tracts are identifiable in skilled hands in up to 90 per cent of patients. ‘Improved high frequency vaginal probes, increasing sensitivity of colour Doppler technology and ever increasing maternal body h abitus, should steer clinicians towards routine endovaginal u ltrasound examination of the fetal heart during the first trimester.’ Colour Doppler examination in the first trimester is a powerful tool in the routine assessment of fetal cardiac structure and will provide detail of the outflow tracts in patients where views on gray scale are limited (Figure 2). Also, colour Doppler examination has the potential to unveil TR or muscular septal defects, which for the greater part are not seen on B-mode examination at this stage in gestation (Figure 3). 4D ultrasound using spatio-temporal image correlation (STIC) technology is emerging as a useful screening and diagnostic tool for CHD at the mid-trimester and beyond. At this point in time however, its application in the first trimester is limited. However, 3D static volume acquisition of the heart using colour (‘glass body’ render) has potential in its assessment of cardiac function including tricuspid regurgitation and septal defects. With respect to fetal cardiac malformation, the image sequences upon which mid-trimester diagnoses rely are similar in the first trimester. In transposition of the great vessels, the major arteries arise from the heart in parallel (Figure 6 ); in Tetralogy of Fallot the aorta overrides the inter-ventricular septum (Figure 7); and in hypoplastic left heart, the left ventricle is small and the right heart chambers and right ventricular outflow tract are dilated (Figures 5a and 5b). The diagnoses of more subtle abnormalities such as right-sided aortic arch and atrioventricular (AV) canal defects can be more difficult (Figures 8a and 8b). Conclusion Improved high frequency vaginal probes, increasing sensitivity of colour Doppler technology and ever increasing maternal body habitus, should steer clinicians towards routine endovaginal ultrasound examination of the fetal heart during the first trimester. Patient acceptance of a vaginal scan is predictably high once the benefits of the examination are explained. While the learning curve in first trimester fetal heart diagnosis may be steep, current evidence suggests that a diagnosis may be reached in over 90 per cent of cases. The first trimester provides a unique opportunity in that there are several markers at this time which lead to the diagnosis of CHD. One in every three cases of major CHD present with an increased NT and Doppler velocimetry of the ductus venosus. The tricuspid valve can help improve the predictive capacity of increased NT in the detection of major congenital heart defects in both euploid and aneuploid fetuses. While first trimester detection rates of CHD in specialist centres now approximate second trimester detection rates, it may take time before this translates into routine practice in low-risk populations. While climbing this learning curve, caution needs to be exercised in the first instance, with a low threshold to seeking a second opinion, or at least considering a repeat examination of the fetal heart later in pregnancy. While resolution of ultrasound increases and the understanding of cardiac diagnosis improves, it seems unlikely however, that the 11 to 14-week scan will completely replace the mid-trimester cardiac assessment in the near future. This is not to say we should not strive to reach a diagnosis at earlier stages in pregnancy, particularly since there is a captive population who are having a routine scan performed for other reasons, namely NT assessment. The heart of the matter Figures 4a and 4b. These figures demonstrate normal and reversed ductus venosus (RDV) waveform at 12 weeks gestation. The presence of a reverse A-wave raises the possibility of CHD. Figure 5a. Hypoplastic left heart at 12 weeks. On the four-chamber view, a dilated right atrium (RA) and dilated right ventricle (RV) are seen. The left atrium is small and the left ventricle hypoplastic. Figure 5b. Hypoplastic left heart at 12 weeks. The right ventricular outflow tract view demonstrates a dilated main pulmonary artery (MPA) and dilated right pulmonary artery (RPA). Ductus Arteriosis (DA). The heart of the matter Figure 6. Figure 7. Figure 8a. Figure 8b. Transposition of the great vessels at 12 weeks. Both the aorta (AO) and pulmonary artery (PA) arise from the heart in parallel. Right-sided aortic arch at 12 weeks. The three-vessel tracheal view reveals the transverse aortic arch (TAo) diverging from the main pulmonary artery (PA) and to the right of the trachea. Tetralogy of Fallot at 12 weeks. The dilated aorta is seen sitting astride the interventricular septum. Cardiac exstrophy at eight weeks. The heart of the matter Figure 9. Atrioventricular (AV) canal defect at 12 weeks. There is a common AV valve, absent septum primum and ventricular septal defect. Figure 10. Hypoplastic right heart at 12 weeks. RV=right ventricle, LV =left ventricle, PE=pericardial effusion. References 1. Yagel S, Cohen SM and Achiron R. Examination of the fetal heart by five short-axis views: a proposed screening method for comprehensive cardiac evaluation. Ultrasound Obstet Gynecol 2001; 17:367-369. 2. Chew C, Halliday JL, Riley MM and Penny DJ. Population-based study of antenatal detection of congenital heart disease by ultrasound examination. Ultrasound Obstet Gynecol 2007; 29:619-624. 3. Ghi T, Huggon IC, Zosmer N and Nicolaides KH. Incidence of major structural cardiac defects associated with increased nuchal translucency but normal karyotype. Ultrasound Obstet Gynecol 2002; 18:610-614. 4. Oh C, Harman C, Baschat AA. Abnormal first-trimester ductus venosus blood flow: a risk factor for adverse outcome in fetuses with normal nuchal translucency. Ultrasound Obstet Gynecol 2007; 30:192-196.