Teresa M. Bieker
Congenital heart disease is a leading cause of infant mortality, with a reported incidence of approximately 1 in 100 live births.1 However, these numbers are based on live-born infants and, therefore, probably underestimate the true incidence in the fetus.2 Early fetal loss and stillbirths are often the result of complex cardiac defects or chromosomal defects, which have an associated heart defect. For this reason, the incidence of congenital heart disease in the fetus has been reported to be as much as five times that found in live-born children1 (Table 9–1).
TABLE 9–1 Frequency of Congenital Heart Lesions Among Affected Abortuses and Stillborn Infants
In utero diagnosis of congenital heart disease allows a variety of treatment options to be considered, including delivery at an appropriate facility, termination, and in some cases, in utero therapy.3 Conversely, a normal fetal echocardiogram in the setting of an increased risk factor provides reassurance for both the patient and the physician.
INSTRUMENTATION AND TECHNIQUE
The American Institute of Ultrasound in Medicine (AIUM) Technical Bulletin on the performance of a basic fetal cardiac ultrasound recommends that a four-chamber view and both the right and the left ventricular outflow tracts be obtained on all obstetrical ultrasound exams.4 Evaluation of a four-chamber view alone may substantially decrease the detection rate of some major cardiac malformations.
When risk factors increase the likelihood of congenital heart disease, a formal and more detailed fetal echocardiogram should be performed.
Various reports advocate evaluation of the fetal heart at different gestational ages5; however, the AIUM Technical Bulletin recommends that fetal echocardiographic exams be performed between 18 and 22 weeks of gestation.4During this period, optimum image quality and, therefore, diagnostic accuracy are achieved. It should be borne in mind that even at 18 weeks of gestation, the fetal heart is a very small structure. Before this age, many cardiac structures may be too small to evaluate accurately.6 Recent advances in ultrasound have led to first-trimester screening of the fetal heart. Cardiac position, as well as a four-chamber view and outflow tracts, can be evaluated by a transabdominal or endovaginal approach. Obtaining these views, however, is dependent on fetal size and position.7 In addition, cardiac lesions such as Coarctation of the Aorta and hypoplastic left heart syndrome may be progressive lesions.8, 9 Therefore, scanning the fetus too early in gestation may result in a false-negative diagnosis.
Later in gestation, the echocardiographic exam may be hindered by increased attenuation from the fetal skull, ribs, spine, and limbs, as well as decreased amniotic fluid as pregnancy progresses.6
Fetal echocardiography requires the use of high-resolution ultrasound equipment.10 Preferred transducer frequencies usually range from 5 to 7 MHz, depending on gestational age, maternal body habitus, and the amount of amniotic fluid present. Equipment utilized for fetal echocardiography should have M-mode and pulsed Doppler capabilities to provide physiologic assessment, as well as color Doppler capabilities to assess spatial and directional information. All of these modalities are vital to performing a complete and accurate examination.
A family history of congenital heart disease is the most common indication to perform a fetal echocardiogram. Recurrence risk for fetuses varies depending on the type of lesion and their relationship to the affected relative.
The risk of congenital heart disease for a fetus with an affected sibling is approximately 2–4%.11, 12 If two or more siblings are affected, this risk increases to approximately 10% (Table 9–2). When the mother of the fetus has a congenital heart abnormality, the recurrence risk is also approximately 10–12%.12 An affected father carries a lower risk (Table 9–3).11, 12
TABLE 9–2 Recurrence Risk in Siblings for any Congenital Heart Defect
Exposure to known cardiac teratogens also increases the risk of having a fetus with a cardiac defect.13 The list of substances considered teratogenic is extensive.14 Specific occurrence risk varies with length and type of exposure, as well as the specific substance involved.
TABLE 9–3 Suggested Offspring Recurrence Risk for Congenital Heart Defects Given One Affected Parent
Chromosomal abnormalities have been reported to occur in 13% of live-born infants with a congenital heart defect.15, 16 The incidence of abnormal karyotype in the fetus with a congenital heart abnormality is approximately 35%.2, 17 Fetuses with an increased nuchal translucency during a first-trimester ultrasound also have an increased risk for congenital heart defects.18
The specific type and occurrence risk of a congenital heart defect vary depending on the chromosomal abnormality. Trisomy 21 is associated with a 40–50% occurrence of congenital heart disease,15 whereas in trisomy 13 and trisomy 18, the association is almost 100%.19 As with teratogenic agents, the list of abnormal karyotypes and syndromes associated with cardiac defects is extensive.14
Several maternal conditions may also carry an inherent risk to the fetus. Congenital heart disease is increased fivefold among infants of diabetic mothers,17 whereas phenylketonuria has a reported risk of 12–16%.20
Complete heart block in the fetus is associated with maternal collagen vascular disease (systemic lupus erythematosus). In these patients, circulating antinuclear antibodies of the SSA or SSB types damage the developing conduction tissue.21
Maternal infections such as human parvovirus and cytomegalovirus also have a reported association with cardiac defects in the fetus.22
Another indication for performing a fetal echocardiogram is the presence of extracardiac anomalies in a fetus.23 The overall incidence of extracardiac malformations in children identified as having a congenital heart abnormality ranges from 25% to 45% (Table 9–4).23 Cardiac abnormalities such as atrioventricular septal defects are associated with extracardiac defects in more than 50% of cases, while atrial septal defects, ventricular septal defects, tetralogy of Fallot, and cardiac malpositions are associated with extracardiac malformations in about 30% of cases.23
TABLE 9–4 Incidence of Associated Congenital Heart Defects Occurring with Extracardiac Malformations in Infants
A suspected structural or rhythm abnormality seen in the fetal heart on a routine obstetrical examination should also warrant a formal fetal echocardiogram to rule out an underlying structural abnormality or, in some cases, to implement in utero therapy.
Non-Immune hydrops fetalis is also an indication for fetal echocardiography. In some cases, it may reflect structural heart disease, while in others, it is the result of a dysrhythmia.21 Finally, massive polyhydramnios is a recognized indication for fetal echocardiography.21 An increase in amniotic fluid may be the result of congestive heart failure, but it is more likely related to associated defects in the fetus, such as those that cause difficulty in swallowing or compression of the esophagus. Although there are several predisposing indications to perform a fetal echocardiogram, up to 90% of congenital heart disease occurs in unselected “normal” obstetric patients.1 Therefore, routine obstetric scanning should identify the majority of fetuses with cardiac lesions that will need a formal fetal echocardiogram.
A fetal echocardiographic exam should always begin by determining fetal position. Unlike a pediatric or adult patient, the fetus cannot be placed in a standard position, nor can the heart be evaluated consistently from routine angles. Although the fetus may move throughout the exam, establishing basic position will allow the examiner to identify various cardiac structures more quickly.6 Once fetal position is determined, the location and orientation of the heart should be established. In a cross-sectional transverse view of the fetal chest, the correct orientation for the fetal heart is with the apex pointing to the left and the bulk of the heart occupying the left chest. The normal angle of the fetal heart, relative to midline is 45 ± 20°.24 The left atrium should be located closest to the fetal spine and the right ventricle nearest to the anterior chest wall. This normal orientation is termed levocardia.
The normal fetal heart should occupy approximately one-third of the fetal thorax.25 Fetal cardiac size can be calculated by measuring the diameter or the circumference of the heart and comparing it to the diameter or circumference of the fetal chest, respectively. When calculating this ratio, both measurements must be obtained from the same image.
The first view to obtain when beginning a fetal echocardiographic examination is the four-chamber view.14 There are two different four-chamber views. The apical four-chamber view and the subcostal four-chamber view. The apical four-chamber view is obtained in a transverse view of the fetal chest, with the transducer imaging the fetal heart from either the anterior or the posterior aspect.
In the apical four-chamber view, all four cardiac chambers can be visualized (Fig. 9–1). In addition, color or pulsed Doppler interrogation of the mitral and tricuspid valves can be performed. Doppler should be performed on the atrial side of the valves to assess for valvular insufficiency, whereas Doppler distal to the valves should be done to evaluate for stenosis or atresia. The two superior pulmonary veins should also be identified entering the left atrium from this projection. The two inferior pulmonary veins are usually not visualized on a fetal echocardiogram.
FIGURE 9–1. Apical four-chamber view showing the interventricular and interatrial septae parallel to the ultrasound beam.
The apical four-chamber view is not optimal for evaluating the interventricular septum (IVS). In this view, the angle of incidence of the sound beam is parallel to the interventricular septum and may result in an artifactual dropout of echoes at the level of the membranous portion, simulating a “pseudo” septal defect (Fig. 9–2).26
FIGURE 9–2. Apical four-chamber view showing a “pseudo” interventricular septal defect, caused by the septum being parallel to the sound beam.
By sliding the transducer cephalad from an apical four-chamber view, the aorta and pulmonary artery should be visualized side by side (Fig. 9–3). This confirms that both are present and normally of equal size.
FIGURE 9–3. Three vessel view. The aorta (a) and pulmonary artery (p) can be seen as two parallel structures by angling the transducer cephalad from the apical four-chamber view. The superior vena cava is seen in cross section. This view allows you to confirm that both great vessels are present and are of similar size.
The subcostal four-chamber view is obtained by imaging the fetal chest in a transverse projection from the anterior chest wall and angling the transducer slightly cephalad (Fig. 9–4). This view also allows identification and comparison of both atria and ventricles. It is ideal for obtaining M-mode measurements of the ventricles and interventricular septum by placing an M-mode cursor perpendicular to the septum at the level of the atrioventricular valves (Fig. 9–5).
FIGURE 9–4. Subcostal four-chamber view with the interventricular and interatrial septae perpendicular to the sound beam.
FIGURE 9–5. M-mode tracing of the right (r) and left (I) ventricles at the level of the atrioventricular valves. Arrow = interventricular septum (IVS).
Atrial measurements can be obtained by moving the M-mode cursor through both atria. The subcostal four-chamber view is also preferable for evaluating a fetal dysrhythmia by placing the M-mode cursor through an atrial wall and a ventricular wall simultaneously (Fig. 9–6). This allows visualization of the timing of dysrhythmic events and may aid in making a definitive diagnosis.
FIGURE 9–6. M-mode tracing through the right atrial (a, arrowhead) and left ventricular (v, open arrow) wall simultaneously to assess the response of each in the setting of a dysrhythmia.
Either pulsed Doppler or color Doppler can be used to evaluate the foramen ovale in the subcostal projection. Documentation of flow from the right atrium to the left atrium by either modality rules out restriction of the foraminal flap (Fig. 9–7). It is also valuable in assessing altered flow direction secondary to a structural defect. A spectral Doppler tracing will display normal foraminal flow as being twice the fetal heart rate.
FIGURE 9–7. Pulsed Doppler tracing of the foramen ovale documenting flow from the right atrium into the left atrium.
The interventricular septum (IVS) is best evaluated in the subcostal four-chamber view since the ultrasound beam is perpendicular to the intraventricular septum. Color Doppler is the best means of achieving this because it allows a large area to be evaluated simultaneously (Fig. 9–8). Pulsed Doppler may not detect flow across a septal defect if the sample volume is not precisely located.
FIGURE 9–8. Color Doppler image showing no flow crossing the intact interventricular septum.
Larger ventricular septal defects may be detected with gray scale imaging alone; however, many remain undetected by any means. Even when a ventricular septal defect is present, the pressures in the fetal heart are such that no flow may be appreciated across the interventricular septum.
Obtaining a subcostal four-chamber view is critical in performing a complete fetal echocardiogram. By angling the transducer systematically toward the fetal right shoulder from this view, most of the remaining fetal heart views are obtained.
A slight angulation from the subcostal four-chamber view toward the fetal right shoulder will result in visualization of a long-axis view of the proximal aorta. In this view, continuity of the anterior wall of the aorta with the interventricular septum and the posterior wall with the anterior leaflet of the mitral valve can by determined (Fig. 9–9). The aortic valve can be interrogated with pulsed Doppler in this view, both proximally looking for aortic insufficiency, and distally to detect stenosis or atresia.
FIGURE 9–9. Long-axis view of the aorta arising from the left ventricle. Continuity can be appreciated between the anterior wall of the aorta and the interventricular septum, and the posterior wall of the aorta with the anterior leaflet of the mitral valve.
The long-axis view of the aorta also provides another means of evaluating a dysrhythmia. By utilizing a wide sample gate and placing the pulsed Doppler cursor between the mitral and aortic valves, both left ventricular inflow and outflow can be evaluated simultaneously (Fig. 9–10). The inflow through the mitral valve will reflect rhythm disturbances occurring in the atria; whereas, the left ventricular outflow through the aorta reflects the ventricular response. Being able to visualize both events simultaneously may help in differentiating the type of dysrhythmia present.
FIGURE 9–10. Pulsed Doppler tracing showing the mitral valve inflow (arrow) simultaneously with the aortic valve (arrowhead) outflow.
The right ventricular outflow tract is visualized next by rotating the transducer further in the direction of the fetal right shoulder. In the normal fetus, this view demonstrates the pulmonary artery coursing cephalad, leftward, and posteriorly from the right ventricle (Fig. 9–11). The course of the pulmonary artery should cross the aorta in the normal fetus. In other words, by angling the transducer from the long-axis view of the aorta to the long-axis view of the pulmonary, the great vessels should “criss-cross” directions if they are correctly oriented. Color or pulsed Doppler are again used in this projection to evaluate the valve proximally for pulmonic insufficiency and distally for pulmonic stenosis or atresia.
FIGURE 9–11. Continuous angulation of the transducer toward the fetal right shoulder from the long-axis view of the aorta, results in a long-axis view of the pulmonary artery (arrow) arising from the right ventricle. This is the three-vessel view.
A further rightward rotation of the transducer will result in a sagittal view of the fetal thorax and thus a short-axis view through the ventricles (Fig. 9–12). The echogenic moderator band should be apparent near the apex to help in identifying the right ventricle.
FIGURE 9–12. Sagittal view of the fetus, showing a short-axis view of the right (r) and left (l) ventricles.
The short-axis view of the ventricles is useful for obtaining measurements of the ventricular free walls and interventricular septum, as well as chamber size. Color Doppler should be used in this view to again evaluate the interventricular septum for defects. With the color Doppler activated, the ventricles should be scanned from the apex to the level of the atrioventricular valves. If color is seen crossing the septum, pulsed Doppler can be used to confirm a septal defect.
From the short-axis view of the ventricles, a short-axis view of the great vessels can be obtained by angling the transducer slightly toward the fetal left shoulder (Fig. 9–13). In this view, the aorta appears as a circular structure with the pulmonary artery draping over it. The aortic, pulmonic, and tricuspid valves are usually well visualized in this projection. The main pulmonary artery can often be seen bifurcating into the ductus arteriosus and the right pulmonary artery. This view provides a reasonable angle from which the pulmonary and tricuspid valves can be interrogated with pulsed Doppler for insufficiency or stenosis or atresia. The great vessels can also be evaluated for size discrepancy in the short-axis view.
FIGURE 9–13. Angulation toward the fetus’s left shoulder from the short-axis view of the ventricles (Fig. 9–12), results in a short-axis view of the great vessels. The pulmonary artery (arrowhead) can be seen normally draping over the aorta (arrow).
Simultaneous M-mode through the aorta, and left atrium is another useful method for evaluating fetal dysrhythmias (Fig. 9–14). The atrial contraction will be depicted in atrial wall movement, while the ventricular response is reflected in the motion of the aortic valve.
FIGURE 9–14. Simultaneous M-mode through the aorta (a) and the left atrium (I) is a useful means of assessing a dysrhythmia.
In the normal heart, the short-axis view of the great vessels confirms the perpendicular relationship of the aorta to the pulmonary artery, thereby excluding such defects as complete or d-transposition of the great arteries or truncus arteriosus.
The aortic arch view is obtained from a sagittal plane of the fetal torso, with the transducer angled from the left shoulder to the right hemithorax. The aortic arch can be differentiated from the flatter, broader, more caudally located ductal arch by identifying the three brachiocephalic vessels arising from its superior aspect (Fig. 9–15). The aortic arch has been described as having a rounded “candy cane” appearance.25
FIGURE 9–15. Sagittal view of the aortic arch (arrow) with the three brachiocephalic vessels (arrowheads) arising from it.
Pulsed Doppler should be used to evaluate the arch from the aortic valve to the descending aorta, looking for areas of increased or decreased velocities. Of particular importance is the section of the arch between the origin of the left subclavian artery and the insertion of the ductus arteriosus, as this is where most in utero coarctations occur. It should be borne in mind, however, that diagnosis of Coarctation of the Aorta is extremely difficult, and a coarctation may be present even in the setting of a normal appearing aortic arch, with normal velocities. When evaluating the aortic arch, it is also important to remember to confirm a left-sided location of the descending aorta.
The ductal arch view is obtained by returning to a more anteroposterior axis of the thorax. It is often helpful to image the short-axis view of the great vessels and then angle the transducer slightly until the pulmonary artery/ductus arteriosus confluence connects with the descending aorta (Fig. 9–16). The ductal arch has a flatter appearance than the aortic arch. It is often referred to as having a “hockey stick” appearance.25 The ductal arch is composed the pulmonary artery, ductus arteriosus, and the descending aorta.
FIGURE 9–16. Sagittal view of the ductal arch with a flatter appearance than the aortic arch.
The final view that should be obtained is the right atrial inflow view, allowing visualization of the inferior and superior vena cavae. This is achieved by sliding the transducer rightward from the aortic arch while remaining in a sagittal plane of the fetus (Fig. 9–17).
FIGURE 9–17. Right atrial inflow view showing the inferior vena cava and the superior vena cava entering the right atrium.
Pulsed Doppler substantially enhances the ability to detect cardiac malformations in utero. It is an effective means of quantitating flow velocity in the cardiac vessels and across the heart valves, as well as determining flow direction. It is also a useful adjunct in differentiating dysrhythmias.21 In a standard fetal echocardiogram, pulsed Doppler should be used to evaluate all four cardiac valves, both proximal and distal to the valve. Pulsed Doppler interrogation of the foramen ovale should be done to document the presence of flow into the left atrium. The ductus arteriosus and aortic arch should also be interrogated to document the presence and normality of flow. In addition, pulsed Doppler of the pulmonary veins can be used to confirm their presence and course into the left atrium.
Technical factors to consider include attempting to place the Doppler cursor in the area of interest at an angle as close to 0° as possible by using transducer angulation and the angle correction capabilities of the equipment used. The sample gate should be set small enough so that interference from wall noise and transmitted flow from adjoining vessels or valves can be minimized. Wall filter should be set to eliminate unnecessary noise without losing essential low flow information, and velocity scale should be set to record maximum velocities accurately.
Color Doppler also plays an essential role in fetal echocardiography by providing a more efficient and expedient means of assessing normal and abnormal flow patterns in the fetal heart. Color Doppler supplies information on the presence or absence of flow, flow direction, and flow patterns. By superimposing color over the gray-scale image, morphologic and hemodynamic information can be assessed simultaneously.
Color Doppler allows visualization of flow in entire structures, such as the aortic arch, thus making it much more time efficient than pulsed Doppler. This efficiency is also prudent when imaging a fetus because color Doppler imaging produces lower peak intensities than pulsed Doppler.
Color Doppler can simplify the investigation of valvular stenosis or insufficiency, again, by sampling a large area and identifying areas of turbulence or flow reversal. In some cases, color may aid in visualizing such cardiac structures as the outflow tracts, which may be difficult to see with gray-scale imaging alone. Also, it may occasionally lead to detection of an abnormality not obvious on the gray-scale image, such as valvular stenosis, small ventricular septal defects, and flow reversal within the aortic and ductal arches.
Equipment used for fetal echocardiography should have specific fetal cardiac capabilities utilizing higher pulse repetition frequencies, which allow color imaging at a frame rate fast enough to evaluate the rapid fetal heart rate. Using a narrow color field or reducing the image depth, when possible, may be necessary to maintain an adequate frame rate.
It is important to remember that color Doppler will only provide mean velocity information; therefore, pulsed Doppler is a necessary adjunct to color Doppler to provide quantitative information regarding peak velocities.
Power Doppler, in general, may hold several advantages over color Doppler such as increased sensitivity, lack of aliasing, and direction independence. In the fetal heart, however, flow direction and maximum velocities are essential in making an accurate diagnosis. Therefore, power Doppler is typically not useful except for establishing the presence of blood flow.
Although M-mode echocardiography is not routinely necessary in fetal echocardiographic examinations, it is essential to differentiate some dysrhythmias.27 By placing the M-mode cursor through both an atrial and ventricular wall or structure simultaneously, the response of both structures can be visualized, aiding in identifying the type of dysrhythmia. M-mode can also be used to acquire measurements of chamber size and wall thickness; however, it is not absolutely necessary because these measurements can also be obtained from two-dimensional (2D) images.
M-mode is also helpful in evaluating contractility in heart abnormalities, which may affect wall motion, such as cardiomyopathies, and is a quick and accurate method of measuring fetal heart rate.
3D and 4D Ultrasound
The advances in three-dimensional (3D) and four-dimensional (4D) ultrasound have recently been applied to evaluate the fetal heart.
Two-dimensional imaging of the fetal heart remains the gold standard; however, there are advantages to 3D volume imaging. By obtaining a volume set of the fetal heart, a third scan plane is obtained. This may be advantageous when a fetus is in a difficult position.28, 29
Several 3D/4D techniques have been applied to scan the fetal heart. The most common include spatiotemporal image correlation and multiplanar reconstruction.
Spatiotemporal image correlation (STIC): The transducer performs a sweep and a volume set is acquired. The images are then correlated with the fetal heart rate and a complete cardiac cycle is produced.28, 29
Multiplanar reconstruction: A volume set is obtained and all three image planes are displayed on the screen. The sonographer or physician can then manipulate each plane as needed to obtain a surface rendering in a fourth image.28, 29
ANATOMY AND PHYSIOLOGY
Several important structural and physiologic differences exist between the fetal and adult cardiovascular systems.14 Unlike the adult, fetal oxygen and carbon dioxide exchange takes place in the placenta. For oxygenated blood to reach the systemic circulation and deoxygenated blood to return to the placenta for oxygenation, the fetal cardiovascular system contains several shunts not present in the adult.
In utero, oxygenated blood travels from the placenta to the fetus via the umbilical vein. After entering the fetus, the majority of this blood travels through the ductus venosus, bypassing the liver, and entering the inferior vena cava. The remainder of this oxygenated blood enters the liver and mixes with the portal circulation.
After entering the inferior vena cava, this oxygenated blood mixes with deoxygenated blood returning from the lower extremities of the fetus. It then enters the right atrium. As it enters the right atrium, the majority of blood is shunted across the foramen ovale into the left atrium. A smaller amount of blood mixes with the desaturated blood returning from the fetal head and upper extremities. This blood travels into the right atrium and into the pulmonary artery. Resistance to blood flow is high in utero; therefore, the majority of the blood that enters the pulmonary artery passes directly into the descending aorta via the ductus arteriosus.
The blood that was shunted through the foramen ovale into the left atrium mixes with a small amount of desaturated blood returned from the lungs by way of the pulmonary veins. This blood then enters the left ventricle and then the aorta. As this blood travels through the aortic arch, a majority passes through the head and neck vessels to supply the fetal head and upper extremities. The remainder continues down the descending aorta, mixes with blood from the ductus arteriosus, and flows out of the fetus by way of the umbilical arteries to the placenta.
The three shunts present in utero, the ductus venosus, the foramen ovale, and the ductus arteriosus, all normally close after birth. The ductus arteriosus closes almost immediately after birth. This results in increased pressure within the left atrium which combined with decreased pressure in the right atrium causes the foramen ovale to close. Complete fusion of the foramen ovale is usually complete by 1 year of age. The umbilical arteries also close immediately after birth. This leads to closure of the ductus venosus.
CONGENITAL CARDIAC ABNORMALITIES
Ventricular Septal Defect
The pooled reported frequency of congenital heart lesions among affected abortuses and stillborn infants shows that ventricular septal defect (VSD) is the most common type of heart defect found (Table 9–1).14
In children, VSDs account for 20–57% of cases of congenital heart defects.2 Unfortunately, it is also one the most commonly missed defects in utero. The sonographic diagnosis of a VSD is based on identifying an interruption in the ventricular septum. This area of dropout may be bordered by a hyperechoic specular reflector, representing the blunted edge of the intact portion of the septum (Fig. 9–18A). The subcostal four-chamber view is often the most useful view in detecting a VSD.
FIGURE 9–18A. Subcostal, four-chamber view showing an anechoic area in the membranous portion of the interventricular septum representing a ventricular septal defect.
VSDs are classified into membranous and muscular defects. Membranous VSDs occur at the base of heart, near the valves. Membranous VSDs are the most common type and usually occur in isolation. Muscular VSDs are located in the muscular septum, at the apex of the heart. Muscular VSDs are divided into four types: inlet, outlet, trabecular, and apical defects. They are typically multiple and are characterized by their location.14
VSDs vary in size and may be singular or multiple. Obviously, smaller defects are more difficult to recognize in utero. In addition, spontaneous closure of a VSD may occur during later gestation. Therefore, a defect that was present earlier in pregnancy may not be present when reevaluated.
Pulsed and color Doppler are useful in making the diagnosis of a VSD. In fact, some small defects that are virtually unseen by 2D alone may be visualized with the utilization of color (Fig. 9–18B). However, it should be borne in mind that because of the near equal pressures of the right and left ventricles in utero, flow across a small VSD may not be appreciated by either color or pulsed Doppler.
FIGURE 9–18B. Subcostal, four-chamber view of a small membranous ventricular septal defect shown by color Doppler.
Atrial Septal Defect
Atrial septal defects (ASDs) account for approximately 6.7% of congenital heart disease in live-born infants.30 Overall, ASDs are twice as common in females as in males. It is difficult to make the diagnosis of ASD in utero because of the normal atrial shunt, the foramen ovale, which allows blood flow from the right atrium to the left atrium in the fetus. Most in utero ASDs are best visualized in the subcostal four-chamber view (Fig. 9–19).
FIGURE 9–19. Subcostal, four-chamber view showing an anechoic area (arrowhead) in the interatrial septum representing an atrial septal defect.
An ostium secundum ASD would appear as a larger than expected area of dropout in the vicinity of the foramen ovale. An ostium primum ASD would result in the absence of the lower portion of the atrial septum, just above the atrioventricular valves. As with VSDs, color Doppler may be a useful adjunct in making the diagnosis.
Atrioventricular Septal Defect
Atrioventricular septal defect (AVSD) refers to a constellation of cardiac malformations that include abnormal development of the interatrial septum, the interventricular septum, and the atrioventricular (mitral and tricuspid) valves. It is also referred to as an atrioventricular canal defect or endocardial cushion defect. Approximately 30% of AVSDs in the fetus are associated with polysplenia.3 Of these, most are accompanied by complete heart block. Chromosomal abnormalities, especially Down syndrome, are associated in up to 78% of cases.3 When an AVSD is present without complete heart block, it is more likely to be associated with abnormal chromosomes.
A complete AVSD can usually be appreciated from either a subcostal or an apical four-chamber view (Fig. 9–20A). The endocardial cushion is absent, creating a wide opening within the center of the heart (Fig. 9–20B). The continuity between the interatrial and interventricular septa and the atrioventricular valves is lost. Instead of identifying separate mitral and tricuspid valves, one single multileaflet valve is seen.
FIGURE 9–20A. Apical four-chamber view in a fetus with an atrioventricular septal defect. A singular atrioventricular valve can be seen closing within a ventricular and atrial septal defect.
FIGURE 9–20B. Apical four-chamber view in the same fetus. The valve is open showing the large endocardial cushion defect.
A partial form of AVSD occurs less frequently. With this, two atrioventricular valves are present; however, their leaflet formation is always abnormal. This may be difficult to appreciate by ultrasound, with the presence of an atrial and a ventricular septal defect being the only clue that an abnormality is present. In a partial AVSD, the apical four-chamber view is useful in demonstrating the abnormal insertion level of the atrioventricular valves. In the normal heart, the tricuspid valve has a more apical insertion than the mitral valve. When a partial AVSD is present, the two atrioventricular valves appear to insert at the same level.5
Additional echocardiographic views of the heart such as a short-axis view and long-axis views of the aorta and pulmonary artery, may be useful in defining the extent of the AVSD, as well as identifying associated cardiac malformations.
Hypoplastic Left Heart Syndrome
Hypoplastic left heart syndrome (HLHS) refers to a group of structural abnormalities affecting the left side of the heart. Its hallmark is a small left ventricle, which can be accompanied by aortic atresia, a hypoplastic ascending aorta, an atretic or hypoplastic mitral valve, and a small left atrium (Fig. 9–21).4 HLHS results from decreased blood flow into or out of the left ventricle. This lack of blood flow results in the underdevelopment of the left ventricle.6Sonographically, a very small left ventricle is usually seen. This is apparent in either a four-chamber view or a short-axis view of the ventricles.
FIGURE 9–21. Apical four-chamber view in a fetus with hypoplastic left heart syndrome. The left heart (arrow) is nearly obliterated, whereas the right ventricle and right atrium are enlarged.
When a small left ventricle is identified, accompanying abnormalities of the mitral and aortic valves must be determined. With valve atresia or hypoplasia, the valve orifice will appear smaller than normal for gestational age. Color and pulsed Doppler will demonstrate a lack of blood flow through the valve. The aorta itself will also appear small or atretic. In some cases, the walls of the aorta will appear more hyperechoic than expected. Blood flow through the ascending aorta may be absent or reversed. Reversal of flow represents blood flowing through the ductus arteriosus and then retrograde through the ascending aorta.
HLHS has a very poor prognosis, carrying a 25% mortality rate within the first week of life. All untreated infants die within the first 6 weeks. Treatment of this lesion usually involves surgical repair via a two-stage Norwood procedure or heart transplantation.10
Hypoplastic Right Heart
Hypoplastic right heart is the result of either pulmonary atresia with intact ventricular septum or tricuspid atresia.14 As with HLHS, it occurs when normal blood flow into or out of the ventricle is compromised. Sonographic findings include a small right ventricle accompanied by a small or atretic and pulmonary artery and valve. Either an apical or a subcostal four-chamber view is most useful in assessing this abnormality (Fig. 9–22). Color and pulsed Doppler will confirm the absence of blood flow across the pulmonary valve.
FIGURE 9–22. Subcostal, four-chamber view in a fetus with hypoplastic right heart syndrome. The right ventricle (arrow) is nearly obliterated, whereas the right atrium (ra), left atrium (la) and left ventricle (lv) are enlarged.
In tricuspid atresia, flow will be absent or substantially decreased across the tricuspid valve (Fig. 9–23). The pulmonic valve is usually stenotic, so an increased velocity may be appreciated distal to the valve. If severe stenosis is present, no flow may be detectable, making differentiation from pulmonary atresia difficult. Color Doppler is essential for evaluating the interventricular septum for defects. As stated previously, this should be done in a subcostal four-chamber view.
FIGURE 9–23. Apical, four-chamber view showing tricuspid atresia. There is no flow across the echogenic, nonmobile tricuspid valve.
Both tricuspid atresia and pulmonary atresia with intact ventricular septum are associated with a large left atrium and a dilated, often hypertrophied, left ventricle. The aortic root may be dilated with either entity. This left-sided enlargement is a result of the vast quantity of blood being forced across the foramen ovale because it is unable to enter the right ventricle.10 Retrograde blood flow within the ductus arteriosus is also possible because of the increased flow through the aorta and the accompanying decreased or absent flow through the pulmonary artery.
Univentricular heart is defined as the presence of two atrioventricular valves or a common atrioventricular valve emptying into a single ventricle. From either four-chamber view, only three chambers are present, two atria and one large ventricle (Fig. 9–24).
FIGURE 9–24. Subcostal, four-chamber view showing the single ventricle (arrow) present in a univentricular heart.
If two atrioventricular valves are identified, but one appears atretic, the most likely diagnosis is tricuspid or mitral atresia, which is usually considered a defect separate from a univentricular heart. The aorta and pulmonary artery are almost always transposed in the setting of a univentricular heart. Pulmonary atresia or stenosis is also common. Univentricular heart has been associated with asplenia or polysplenia in 13% of cases.11
Coarctation of the Aorta
When the diagnosis of Coarctation of the Aorta is made in utero or in early infancy, it is easily correctable, but if left undetected, the effects can be devastating. Coarctation is a narrowing of the aortic lumen, which results in an obstruction to blood flow. In 98% of cases, this narrowing occurs between the origin of the left subclavian artery and the ductus arteriosus.12 The severity of a coarctation can range from a slight narrowing of the distal end of the arch to severe hypoplasia of the entire arch.
Intuitively, the in utero diagnosis of a coarctation seems straightforward. It is, however, extremely difficult. Subtle changes associated with coarctation, such as a narrowing of the aortic arch, may not be appreciated, even when the arch is well visualized (Fig. 9–25). This may be caused by the physiological shunts present in the fetal heart, allowing for the severity of the narrowing not to present until after birth.
FIGURE 9–25. Sagittal view of the aortic arch showing a normal appearing arch in the setting of a Coarctation of the Aorta.
These shunts may also explain why Doppler velocities may not be affected in the presence of a coarctation. Interestingly, one of the most reliable signs of a coarctation in utero to be reported is a right ventricular dimension greater than that expected for a gestational age (Fig. 9–26). Pulmonary artery size may also be increased.13
FIGURE 9–26. Apical four-chamber view in a fetus with Coarctation of the Aorta. The only clue in this case was a right ventricle (arrow) that was slightly larger than expected for gestational age.
This finding may be subtle; therefore, measurements of the ventricles and great vessels should always be performed in the fetus at risk for coarctation such as those with Turner syndrome (XO) or a prior family history of left heart anomalies. It is also important to remember that Coarctation of the Aorta is often a progressive lesion, with the distal arch becoming more hypoplastic as pregnancy advances. Reversal of blood flow through the foramen ovale is often, but not always, present with a coarctation.
Aortic and Pulmonic Stenosis
Congenital aortic stenosis is an obstruction of the left ventricular outflow tract. Aortic stenosis is classified into three types: valvular, subvalvular, and supravalvular stenosis. Sixty to 70% of patients with aortic stenosis have valvular stenosis. On ultrasound, aortic stenosis may appear as a thickened or immobile valve. Flow distal to the aortic valve is increased in velocity. With severe stenosis, no flow or reversed flow may be seen.14
Congenital pulmonic stenosis is an obstruction or narrowing of the right ventricular outflow tract. Pulmonary stenosis is classified as obstructive or valvular. A thickened valve or muscular ring may be seen by ultrasound. By pulsed Doppler, there is increased velocity distal to the valve. Pulmonic stenosis can been found in the recipient twin of twin-to-twin transfusion syndrome.14, 31
Ebstein anomaly is defined as the inferior displacement of the tricuspid valve leaflets from their normal location. Ebstein anomaly is an uncommon cardiac lesion, with a reported incidence of 1 in 20,000 live births.15 It has often been associated with maternal lithium use; however, more recent data have shown this association to be substantially less than previously reported.16
The sonographic diagnosis of Ebstein anomaly is usually straightforward. Apical displacement of the tricuspid valve leaflets is readily apparent from either four-chamber view (Fig. 9–27). This results in “atrialization” of the right ventricle, which, along with the tricuspid insufficiency that is almost always present, causes an often massively enlarged right atrium. This is turn, causes the axis of the heart to be severely levocardic, giving the heart a very horizontal position within the fetal chest. Pulmonary atresia or stenosis, as well as dysrhythmias are not uncommon with Ebstein anomaly. Ebstein anomaly frequently causes in utero cardiac dysfunction, resulting in cardiomegaly and hydrops fetalis.
FIGURE 9–27. Apical four-chamber view in a fetus with Ebstein anomaly. The tricuspid valve (arrow) is displaced apically, causing atrialization of the right atrium (ra) and a small right ventricle (rv). la = left atrium, lv = left ventricle.
Tetralogy of Fallot
Tetralogy of Fallot consists of four classic structural defects: a ventricular septal defect, aortic override of the VSD, pulmonary stenosis, and right ventricular hypertrophy.14 Because of the normal shunts present in the fetus, the right ventricular hypertrophy may not occur in utero. To diagnose this malformation in utero, an aortic root overriding the interventricular septum must be identified (Fig. 9–28). It is often not possible to make this diagnosis solely from a four-chamber view, either apical or subcostal. The VSD may be seen on the four-chamber view; however, color Doppler should be used to confirm that the defect is real and not artifactual. A slight angulation of the transducer toward the fetus’ right shoulder from a subcostal four-chamber view, or cephalad from an apical four-chamber view should allow the overriding aorta to be appreciated. Dilatation of the aortic root is usually present in later gestation.17
FIGURE 9–28. Apical view of the heart in a fetus with tetralogy of Fallot showing the aorta overriding a ventricular septal defect.
Once an overriding aorta has been observed, the diagnosis of tetralogy of Fallot relies on the evaluation of the right ventricular outflow tract. This is usually best accomplished in either a long-axis view of the pulmonary artery or a short-axis view of the great vessels. The pulmonary artery will appear small, often so much so that it cannot be identified. Pulsed Doppler interrogation of the pulmonic valve may show a greatly increased velocity, indicative of stenosis, or absence of flow in the setting of severe stenosis or atresia. Retrograde flow through the ductus arteriosus may also be present. Making an accurate diagnosis of tetralogy of Fallot relies on identifying the pulmonary artery. If a pulmonary artery cannot be visualized, the differential diagnosis would include pulmonary atresia with a VSD. If there is no main pulmonary artery arising from the right ventricle but smaller pulmonary artery branches are seen arising from the overriding aorta, the diagnosis is truncus arteriosus.
When the diagnosis of tetralogy of Fallot is established, the laterality of the aortic arch should be determined because approximately 25% of cases are associated with a right-sided aortic arch.19
Truncus arteriosus is rare. It is an embryological failure that results in a single great vessel arising from the heart.14 The systemic, pulmonary, and coronary circulations are all supplied by this single great vessel. Sonographically, truncus arteriosus appears very similar to tetralogy of Fallot. A VSD is present, and the singular great vessel overrides the defect, similar to the aorta in tetralogy of Fallot (Fig. 9–29). The difference is that the pulmonary artery arises from this great vessel, not the right ventricle. Depending on the type of truncal defect present, the number and position of the pulmonary arteries on the great vessel will vary.32
FIGURE 9–29. Subcostal view of the heart in a fetus with truncus arteriosus showing a single truncal vessel overriding a ventricular septal defect.
The in utero diagnosis of truncus arteriosus may be challenging. The definitive diagnosis can be made only if the origin of the pulmonary artery can be identified arising from the large, single great vessel. Because of the inherent technical factors associated with fetal echocardiography, this may be difficult.
As with tetralogy of Fallot, identification of the overriding great vessel is usually accomplished with slight angulation from either the apical or the subcostal four-chamber view. Several views, including long- and short-axis views of the right outflow tract, must be obtained to confirm the absence of a pulmonary artery. Evaluation of the aortic arch is also important. A right-sided arch has been reported in 15–30% of cases. Interruption of the aortic arch has also been associated with truncus arteriosus.32
Complete Transposition of the Great Arteries
Eighty percent of fetuses with transposition of the great arteries have complete or d-transposition.20 In this setting, the connections between the atria and ventricles are normal, meaning that the right atrium connects through the tricuspid valve to the right ventricle, and the left atrium connects through the mitral valve to the left ventricle. However, the aorta arises from the right ventricle, and the pulmonary artery arises from the left ventricle. This results in two parallel circulations that will only allow mixing of venous and arterial blood through the ductus arteriosus, interatrial, or interventricular connections.
The four-chamber views are often normal in the presence of complete transposition. The diagnosis is made by identifying the aorta arising from the right ventricle and connecting to the aortic arch and descending aorta, and identifying the pulmonary artery arising from the left ventricle and then branching into the left and right pulmonary arteries. From a long-axis view of the great vessels, the aorta and pulmonary artery will appear to run in a parallel fashion (Fig. 9–30). The short-axis view, at the level of the great vessels, is also useful in making this diagnosis. In this view, both the pulmonary artery and aorta appear as circular structures adjacent to each other, instead of their normal relationship of the pulmonary artery draping over the aorta. A ventricular septal defect is present in 20% of cases, so color Doppler should be used to assess the interventricular septum thoroughly.21
FIGURE 9–30. Complete transposition of the great arteries. The aorta and pulmonary arteries are running parallel.
Congenitally Corrected Transposition of the Great Arteries
Congenitally corrected or l-transposition of the great arteries comprises the remaining 20% of transposition cases.20 In corrected transposition, the great vessels arise from the correct sides; however, the left and right ventricles and the left and right atrioventricular valves are transposed. In other words, the right atrium is connected to the left ventricle, and the left atrium is connected to the right ventricle. The aorta then arises from the left-sided right ventricle, and the pulmonary artery arises from the right-sided left ventricle. Blood circulation in this abnormality is in series, as it is in the normal heart; therefore, surgical correction is not required unless associated cardiac anomalies are present.
Sonographic identification of this abnormality can be subtle. Correct identification of the cardiac chambers is crucial in making this diagnosis. In the normal heart, the tricuspid valve insertion is slightly more apical than the mitral valve (Fig. 9–31). The right ventricle also has a prominent moderator band near the apex that is usually seen on fetal echocardiography. If these findings appear to be left sided, the diagnosis of corrected transposition should be considered. As with complete transposition, the great vessels exit the heart in a more parallel relationship than seen in the normal heart. This may be appreciated on a short-axis view of the great vessels but is far more subtle than in complete transposition. It is not uncommon to miss the diagnosis of corrected transposition in utero, particularly when no other cardiac defects are present.
FIGURE 9–31. Corrected transposition of the great arteries. Note how the tricuspid valve is slightly superior to the mitral valve. In the normal heart, the tricuspid valve is more apical.
VSDs have been reported in about 50% of patients with corrected transposition. Pulmonic stenosis and abnormalities of the mitral and tricuspid valves are also common.20
Double Outlet Right Ventricle
Double outlet right ventricle (DORV) is a condition in which more than 50% of both the aortic root and the main pulmonary artery arise from the right ventricle. A ventricular septal defect is almost always present.22
This, again, is one of many cardiac defects easily missed when only a four-chamber view is obtained. The long-axis views of the aorta and pulmonary artery are most useful in identifying both great vessels as arising from the right ventricle (Fig. 9–32). In DORV, the most common relationship of the great vessels is side by side, with the aorta right and lateral to the pulmonary artery. When this occurs, the normal perpendicular course of the great vessels is lost. As with transposition of the great arteries, they will appear parallel to each other. Differentiating DORV from transposition relies on identifying both great vessels as arising from the right ventricle. This can be challenging in utero. As with all congenital cardiac abnormalities, the surgical intervention depends heavily on the presence or absence of other cardiac anomalies. Therefore, a thorough interrogation of the fetal heart must be undertaken.
FIGURE 9–32. Double outlet right ventricle in a fetus. The aorta and the pulmonary artery are both arising from the right ventricle.
Double outlet left ventricle, in which both the aortic root and the main pulmonary artery arise from the left ventricle, has also been reported, but it is exceedingly rare.23
Total Anomalous Pulmonary Venous Connection
Total anomalous pulmonary venous connection (TAPVC) is an anomaly in which all of the pulmonary veins drain either directly into the right atrium or into channels that terminate in the right atrium.24 In the normal heart, venous return is to the left atrium. TAPVC is rare and, as with many other congenital cardiac anomalies, is a difficult diagnosis to make in the fetus.
The diagnosis relies on the inability to identify any pulmonary veins entering the left atrium and the identification of all four pulmonary veins entering the right atrium or abnormally converging and entering the superior vena cava, inferior vena cava, portal vein, or ductus venosus.
If any pulmonary veins are seen entering the left atrium, or if all pulmonary veins are not seen entering an ectopic structure, the diagnosis of TAPVC is excluded. Partial anomalous pulmonary venous connection may be present in this setting, but cannot be definitively ascertained in utero.
The pulmonary veins are best identified in either a subcostal or an apical four-chamber view. Usually only the two superior veins are identified in utero (Fig. 9–33), adding to the difficulty of making this diagnosis. Enlargement of the right ventricle and pulmonary artery may be secondary signs of TAPVC.24
FIGURE 9–33. Apical four-chamber view showing the two normal superior pulmonary veins appropriately entering the left atrium.
When enlargement of these structures is present and the normal pulmonary veins cannot be identified as they drain into the left atrium, the possibility of TAPVC should be entertained. Using color Doppler to identify an abnormal convergence of veins posterior to the right atrium may also be useful.
Cardiac Axis and Position
As stated previously, determining cardiac axis and position is one of the first steps in performing a fetal echocardiogram. Abnormal cardiac axis or position may be an important clue that a structural defect is present.14
Normal cardiac axis is termed levocardia, meaning the apex of the heart points to the left side of the fetal chest. Even if a heart is levocardic, if its axis is > 45 ± 20° to the left, an abnormality may be present. Cardiac anomalies that result in severe levocardia are usually those that cause an enlarged right atrium, such as Ebstein anomaly (Fig. 9–34). It is thought that this enlargement causes the heart to shift and lie more horizontally. Mesocardia occurs when the apex of the heart points midline. Mesocardia is uncommon but has been associated with transposition of the great vessels14 (Fig. 9–35).
FIGURE 9–34. Severe levocardia in a fetus with Ebstein anomaly. The apex of the heart (arrowhead) is angled too far to the left chest.
FIGURE 9–35. Four-chamber view showing mesocardia, with the apex of the heart pointing midline.
The terms dextrocardia or dextroversion refer to the apex of the heart pointing abnormally to the right (Fig. 9–36). Isolated dextrocardia is associated with a structural cardiac abnormality in 95% of cases.14 Dextrocardia associated with abdominal situs abnormalities carries a lower risk. Dextroposition is present when the apex of the heart points normally to the left side of the fetal chest, but the heart itself is positioned in the right chest (Fig. 9–37).
FIGURE 9–36. Dextrocardia of the fetal heart. The apex of the heart (arrowhead) is pointing incorrectly to the right chest.
FIGURE 9–37. Dextroposition of the fetal heart. The apex of the heart (arrowhead) is pointing correctly to the left chest; however, the entire heart is being displaced into the right chest by the mass in the left chest (calipers).
When dextroposition is present, two possibilities should be considered. Either the heart is being displaced to the right by a left-sided thoracic defect such as a diaphragmatic hernia or a cystic adenomatoid malformation, or the heart is filling a potential space in the right thorax. This may be indicative of an absent or hypoplastic right lung.
Whenever a fetal echocardiogram is performed, special attention should be paid to identifying cardiac axis and position. Any deviation from normal may be indicative of an underlying intra- or extracardiac defect.
The normal fetal heart rate is regular and between 100 and 180 beats per minute (bpm). A dysrhythmia is present if the fetal heart rate is noted to be abnormally fast, slow, or irregular. Dysrhythmias are detected in approximately 1% of fetuses.30
Most dysrhythmias are benign; however, in a small number of cases, they may be life threatening. M-mode is the most useful method of assessing the type of dysrhythmia present. As stated previously, the M-mode cursor should be placed simultaneously through a structure in the fetal heart that represents an atrial beat (atria wall or atrioventricular valve) and the ventricular response (ventricle wall or semilunar valve).
Premature atrial contractions (PACs) are the most common dysrhythmia encountered in the fetus (Fig. 9–38).14 They have been associated with a redundant foraminal flap, as well as maternal use of caffeine, cigarettes, or alcohol.33 Rarely, PACs may evolve into a sustained tachycardia; however, most resolve around the time of delivery and seldom present a problem in the newborn. Tachycardias are the second most common dysrhythmia seen in the fetal population. Tachycardias are classified as:
• Supraventricular tachycardia (SVT)—heart rate of 180–280 bpm, with atrioventricular concordance (Fig. 9–39)
• Atrial flutter—atrial heart rate of 280–400 bpm, with variable ventricular response (Fig. 9–40)
• Atrial fibrillation—atrial heart rate of >400 bpm, with variable ventricular response
FIGURE 9–38. M-mode tracing of premature atrial contractions in a fetus. Normally spaced atrial beats (arrows) can be seen followed by a premature beat (arrowhead).
FIGURE 9–39. M-mode tracing of supraventricular tachycardia in a fetus. Both the ventricular (v) and atrial (a) rates were 240 beats per minute.
FIGURE 9–40. M-mode tracing of a fetal heart with atrial flutter. The atrial (a) rate was 352 beats per minute, while the ventricular (v) rate was 180 beats per minute.
Sustained SVT can result in fetal hydrops or death and represents a fetal medical emergency.34 SVT is associated with structural heart disease in 5–10% of cases.35
The treatment of SVT in utero is difficult. Immediate medical therapy should be implemented if there are signs of fetal compromise. Digoxin has been the initial drug of choice when treating fetal SVT; however, several other medications are available and may be used in place of or in combination with digoxin.
Bradycardia may also be encountered in the fetus. Transient bradycardia is often encountered during the course of an ultrasound examination secondary to pressure from the transducer. The bradycardia is resolved when the transducer is removed. This entity should not be confused with pathologic bradycardias that result in a sustained slow heart rate.
Ninety-six percent of fetuses with sustained bradycardia will have second- or third-degree heart block.14 Second-degree heart block is commonly referred to as a 2:1 or 3:1, etc., heart block, referring to the fact that the ventricular rate will be a submultiple of the atrial rate. In other words, two atrial contractions will occur for every one ventricular contraction, or three atrial contractions will occur for every one ventricular contraction (Fig. 9–41).
FIGURE 9–41. M-mode tracing of a fetal heart with a 2:1 heart block. The atrial rate is 120 beats per minute, whereas the ventricular rate is 60 beats per minute.
Third-degree, or complete, heart block is present when there is complete dissociation between the atrial and ventricular rates, with the atrial rate being faster. Approximately 50% of fetuses with complete heart block have significant structural heart disease, specifically, atrioventricular septal defects, corrected transposition of the great arteries, cardiac tumors, or a cardiomyopathy.14 Complete heart block associated with an atrioventricular septal defect is highly suggestive of polysplenia syndrome.36 In fetuses with complete heart block without structural defects, there is a high association with maternal connective tissue diseases such as lupus.37
Second- and third-degree heart block are difficult to treat in utero. Increasing fetal heart rate through the maternal administration of sympathomimetic agents and placement of an in utero pacemaker have been attempted, but with dismal results.38 Administration of maternal steroids has also been reported.39
The prognosis for fetuses with complete heart block and structural heart disease is poor. In fetuses without structural heart disease, outcome is dependent on the atrial and ventricular rate and the presence of fetal hydrops.
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