The five axial views for optimal fetal heart screening. The color image shows the trachea (Tr), heart and great vessels, liver, and stomach, with the five planes of insonation indicated by polygons corresponding to the grayscale images, as indicated. (I) Most caudal plane, showing the fetal stomach (St), cross-section of the descending aorta (dAo), spine (Sp), and liver (Li). (II) Four-chamber view of the fetal heart, showing the right and left ventricles (RV, LV) and atria (RA, LA), foramen ovale (FO) and pulmonary veins (PV) to the right and left of the dAo. (III) Left ventricular outflow tract view, showing the aortic root (Ao), LV, RV, LA and RA and a cross-section of the descending aorta (dAo). (IV) Slightly more cephalad view (right ventricular outflow tract view), showing the main pulmonary artery (MPA), bifurcation into the right (RPA) and left (LPA) pulmonary arteries, and cross-sections of the ascending aorta (Ao) and descending aorta (dAo). (V) Three vessels and trachea view showing the superior vena cava (SVC), pulmonary artery (PA), ductus arteriosus (DA), transverse aortic arch (from proximal Ao to dAo) and trachea (Tr). L = left; R = right.
Fetal cardiology in a tertiary center
Fetal cardiology units are best situated as an integral part of a fetal medicine unit. This allows joint fetal cardiology and obstetric appointments so that patients can receive a full assessment and diagnosis (including invasive testing, if required) on the same day. It also facilitates joint counseling by fetal cardiologists and fetal medicine specialists for the mothers of fetuses with a combination of cardiac and extracardiac abnormalities.
Indication for referral to fetal cardiology
The majority of fetuses with CHD do not have any known predisposing risk factors. However, some factors are recognized to increase the risk of CHD in the fetus and, depending on local resources, warrant referral to fetal cardiology.[13]
Risk factors are generally considered to be either maternal or fetal.
Maternal indications
There are a number of conditions that, when present in the mother, increase the risk of heart disease in the infant.
Maternal autoantibodies
The most common of these referral indications is the presence of maternal autoantibodies (anti-Ro, anti-La) that have a well-established link with the development of heart block in the fetus. In cases with no previous affected infant, the reported incidence is about 4%. If a previous child has developed heart block in utero, then the risk is higher, in the region of 19%.[14] The autoantibodies may also cause inflammation of the heart with consequent fibrosis and impairment of function; rare cases of rupture of the atrioventricular valve tensor apparatus have also been reported.[15]
More recent evidence suggests that the level of maternal autoantibodies may be important in determining the individual risk of development of heart block, and that serial echocardiography should be limited to women with high anti-Ro titres.[16]
Maternal/paternal or family history of CHD
A maternal/paternal or family history of CHD increases the risk of the baby being affected, but the degree of increased risk varies from 2–3% in most polygenic syndromes, to as high as 50% in cases of autosomal dominance.[17–19]
Previous affected child
The presence of nonsyndromic CHD in a previous child increases the risk to around 2–6% for subsequent pregnancies.[17] In some conditions (in particular, a history of left heart obstructive lesion), this risk may be up to 10%.[18] Concordance between recurrence of the same type of lesion varies widely, depending on the lesion.[19]
Exposure to teratogenic drugs
There are many drugs with known teratogenic effects, and sometimes accidental or unavoidable exposure to these medications occurs in pregnancy. Examples include anticonvulsant medication, ethanol, lithium, angiotensin-converting enzyme (ACE) inhibitors, and retinoids.[13]
Metabolic risk factors
There is an increased risk of congenital abnormalities including CHD in maternal diabetes, particularly when this is pregestational.[20] Specific lesions that occur more commonly are ventricular septal defects, TGA, and heterotaxy. The elevation of risk is five times that of the general population.[21] Those at highest risk are patients with poor glycemic control in the first trimester of pregnancy. Maternal phenylketonuria is also associated with an elevated risk of CHD, particularly in those with poor metabolic control in the first trimester of pregnancy.[22,23]
There is some evidence that preconception folic acid supplementation is protective against the development of CHD.[24,25]
Fetal indications
Suspected abnormality at screening
The most common fetal indication and the indication that has the highest yield of confirmed CHD is a suspected abnormality of the heart at the 20-week anomaly screening examination. The guidelines published by the British Congenital Cardiac Association state that all patients should be seen at a tertiary center within 5 working days from a suspected diagnosis of CHD at screening (www.bcs.com/documents/Fetal_Cardiology_Standards_2012_final_version.pdf). In practice, most units would aim to see these patients within 48 h in consequence of the considerable anxiety this causes to the parents.
Raised nuchal translucency at first-trimester screening
In fetuses with a normal karyotype, elevated NT is independently associated with an increased risk of CHD; it can therefore be used as a screening tool to aid prediction of pregnancies at a higher risk of CHD.[26] The risk of CHD increases in proportion to the nuchal measurement (Table 9.1).[27] It is recommended that all women with fetal NT measurements above the 99th centile (3.5 mm) should be referred for detailed fetal echocardiography. Depending on local resources, fetuses with a NT measurement of >95th centile (2.5 mm) are also screened in some obstetric units.
NT (mm) | Risk of major CHD | ||
|---|---|---|---|
| Pregnancies (n/1000) | Percentage | Probability | |
| >95th centile to 3.4 | 17a | 1.7 | ~1:50 |
| 3.5–4.4 | 32b | 3 | ~1:35 |
| 4.5–6.4 | 83b | ~10 | ~1:10 |
| 6.5–8.4 | 190b | ~20 | ~1:5 |
| >8.5 | 643b | ~65 | >1:2 |
b Ghi et al. (2001) [Ghi T, Huggon IC, Zosmer N, Nicolaides KH. Incidence of major structural cardiac defects associated with increased nuchal translucency but normal karyotype. Ultrasound Obstet Gynecol. 2001 Dec;18(6):610–14.]
CHD = coronary heart disease; NT = nuchal translucency
The underlying pathophysiology that explains the subcutaneous edema resulting in a raised NT measurement in some fetuses with CHD is incompletely understood, but may include cardiac failure, abnormal lymphatic development, and venous congestion in the head and neck.[28]
Extracardiac abnormality
The finding of an extracardiac abnormality often prompts referral to a fetal cardiologist. The abnormalities associated with a particularly increased risk of CHD are exomphalos (one study reported that 30% of such cases have CHD) and duodenal atresia (approximately 20% of such cases have CHD[29]). Other abnormalities such as diaphragmatic hernia, in addition to being associated with an increased risk of CHD,[30] may have functional consequences for the fetal heart because of mediastinal shift and external compression. Depending on the local provision of fetal echocardiography, other extracardiac abnormalities with a lower association with CHD are not routinely referred to a fetal cardiologist but may instead be screened by a fetal medicine specialist.
Equipment required
Imaging of the fetal heart requires high-resolution fetal ultrasound machines that currently are widely available in obstetric ultrasound units. Continuing challenges to obtaining good image resolution are unfavorable fetal position, fetal movement, and maternal factors. Thus, probes of an appropriate ultrasound frequency, as well as repeat scanning at a later gestational age, are needed to account for these factors. High frequency ultrasound probes are required to optimize definition of small fetal heart structures, especially in the first trimester. Standard detailed assessment of the fetal heart involves the use of color Doppler, pulsed wave Doppler, and M-mode, in addition to two-dimensional imaging. Other modalities such as power Doppler and continuous wave Doppler are desirable and will help to refine a complex diagnosis.
The examination is usually performed transabdominally, most commonly with curvilinear probes, although in some situations, particularly in first-trimester imaging, a transvaginal probe may improve image resolution.
Equipment should have facilities for recording both still and moving images. It is increasingly a requirement that the images should be stored for medicolegal purposes; this also allows review of the evolution of a defect throughout pregnancy.
Ultrasound is considered a safe imaging modality for the fetus and has been widely used in pregnancy for many years without reports of harm. However, in consequence of the theoretical risk of using a technique that transfers energy to the developing fetus, guidelines from the British Medical Ultrasound Society advise use of the ALARA (“As Low As Reasonably Achievable”) principle, which requires the operator to limit the length of scan and energy used to that necessary for diagnosis. This is particularly the case with earlier gestation scans and the use of techniques that use higher energy, such as Doppler and harmonic imaging (www.bmus.org/policies-guides/pg-safetystatements.asp).
Performing the fetal echocardiogram
The initial step in examination of the fetal heart is to establish the fetal lie to ensure correct identification of the left and right sides of the fetus on the projected image. Following this, the horizontal position of the heart in the fetal thorax means that five standard planes that sweep from a caudal to cranial direction in the fetus can identify the majority of cardiac abnormalities (Figure 9.1).[3] In this way, the situs, atrioventricular, and ventriculoarterial connections are established. Abnormalities affecting the size or function of the chambers of the heart can be detected. Additional sagittal and coronal views are then used to refine details of the diagnosis, along with color and pulsed wave Doppler. In this way, a full segmental sequential diagnosis of the congenital heart defect can be achieved.
Fetal medicine assessment
Following a prenatal diagnosis of CHD, because of the high frequency of extracardiac abnormalities found in fetuses with CHD,[31] it is advisable for a detailed anomaly scan to be performed by a fetal medicine specialist. There is also a strong association of CHD with karyotype abnormalities. The risk of karyotype abnormality varies with the cardiac diagnosis. For example, there is a very high risk of aneuploidy for a complete atrioventricular septal defect and a very low risk for simple TGA.[32]
As part of the fetal medicine consultation, if the risk of underlying genetic abnormality is increased, then the option of invasive testing is discussed. Amniocentesis can be performed from 16 weeks onward using ultrasound guidance to take a sample of amniotic fluid through the abdominal wall. It is generally a well-tolerated procedure with a low risk of complications. The risk of miscarriage or premature labor as a result of the procedure is widely quoted to be in the region of 0.5%, although some suggest that the risk is lower after correction for spontaneous miscarriages that would have occurred anyway. An initial polymerase chain reaction test result for trisomy 21, 13, and 18 is available within 3 working days and a full karyotype analysis after 2 weeks. Fluorescent in situ hybridization (“FISH”) testing for 22q11 microdeletion can also be requested and is particularly indicated for conotruncal abnormalities.
Increasingly, microarray comparative genomic hybridization is becoming the genetic test of choice. This molecular cytogenetic technique enables detection of copy number changes across the whole genome by comparison of the patient’s genome with a reference. This is a very sensitive method: its main limitations are that it cannot detect chromosomal abnormalities for which there is no copy number change (such as mosaicism, balanced translocations, or inversions). It can also result in findings that are not clinically significant.
Counseling
Following fetal echocardiography, the diagnosis of a congenital heart defect has to be explained to the parents in a manner they can understand. This is best done away from the scanning room. Most units also have fetal clinical nurse specialists who play an important role in both counseling and longer-term support of the families.
The diagnosis is explained with the aid of diagrams. The likely course in pregnancy and the likely short- and long-term outcomes are explained. Evidence-based outcome data are used to provide parents with an estimation of risk for the procedures that their child will undergo, along with the potential short- and long-term complications, long-term survival, and quality of life.
Progression of disease in pregnancy
One important aspect that needs to be considered when counseling the parents is evolution of the heart defect during the course of pregnancy. Perhaps the best illustration of this is evolution of left or right ventricular outflow tract obstruction before birth.[33,34] Early studies on lamb fetuses demonstrated that reduced filling of a cardiac chamber can negatively impact growth of the chamber and that artificially narrowing of the left ventricular outflow tract resulted in myocyte hyperplasia, leading to hypertrophy of the myocardium.[35] The consequence is that mild narrowing of a semilunar valve in midtrimester can evolve to atresia through the course of pregnancy, and that an apex-forming left ventricle can evolve to become a hypoplastic left heart. For that reason, serial assessment is required for continuing pregnancies.
The recognition of this dynamic process has led to an exploration of possible targets of fetal intervention that could modify this process.
Termination of pregnancy
In some cases, the severity of the heart defect is such that the parents do not wish to continue the pregnancy. Termination of pregnancy usually involves the induction of labor. An additional factor that needs to be considered is that if a termination is performed after 21 weeks and 6 days, then the Royal College of Obstetricians and Gynaecologists in the UK recommends that, to avoid the rare situation of a live birth, feticide should be performed. The legal limit for termination in the UK for maternal indications (i.e. “continuance of the pregnancy would involve risk, greater than if the pregnancy were terminated, of injury to the physical or mental health of the pregnant woman or any existing children of her family”) is 24 completed weeks. There is no gestational age limit if “there is substantial risk that if the child were born it would suffer from such physical or mental abnormalities as to be seriously handicapped” (clause E of the 1967 Abortion Act). The definition of “seriously handicapped” is not defined in law, which gives clinicians and parents some scope for personal interpretation. Clinicians working in other jurisdictions may have less room for maneuver, and termination of pregnancy is illegal under all circumstances in many parts of the world. The ramifications of this are beyond the scope of this text.
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