The cardiovascular system provides a large volume of information about the well-being of the fetus. It is accessible because of rapid developments in the technology of noninvasive techniques, particularly ultrasonography. The fetus has become the new patient of the decade due to the rapid changes in ultrasonic technologies and other fetal assessment techniques. The fetal biophysical profile is useful to detect changes in fetal well-being, and assesses mainly changes in brain function.^1,2 The decision to deliver a fetus prematurely due to cardiac changes must be made in the context of the risks, both pre- and postnatally. Most associations of how cardiovascular changes correlate with other organ function in the fetus have yet to be defined. Therefore, any assessment demands a coordinated team approach between obstetricians, perinatologists, cardiologists, and neonatologists.^3,4

Cardiac defects are the most common cause of infant death in United States.⁵ They occur in approximately 0.8% of liveborn children, but in a much higher percentage of those aborted spontaneously, stillborn, or those diagnosed early in gestation.⁶ Although the etiology of heart defects is not known, population studies have suggested a number of factors that may increase the risk of heart defects. Approximately 8 in 1000 live births are affected by congenital heart disease. The incidence of severe congenital heart disease that will require heart surgery in the first year after birth is about 2.5 to 3 per 1000 live births. The moderately severe forms of congenital heart disease probably account for another 3 per 1000 live births.⁷

Several factors, like maternal folate deficiency, have been linked to an increased rate of spontaneous abortion as well as conotruncal defects, neural tube defects, orofacial defects, and limb defects.^8-11 Any strategy to prevent congenital heart disease should include preconceptional supplementation of folic acid and a balanced diet.^12,13 Structural malformations such as diaphragmatic hernia, cystic adenomatoid malformation, omphalocele, and atrioventricular malformations, as well as pathophysiological conditions such as intrauterine growth restriction, hydrops, indomethacin tocolytic therapy, and fetal anemia, all can be associated with decreased effective cardiac output.¹⁴ We use the cardiovascular profile score to assess the presence and severity of fetal congestive heart failure in these entities as well.¹⁵

An understanding of cardiac anatomy is fundamental for the proper interpretation of clinical echocardiography. In the normal heart, the morphologic right atrium forms the right front part of the cardiac mass. The external wall of the atrium consists of the venous component (into which the superior and inferior vena caval veins as well as the coronary sinus open) and an anterior part, the appendage that has a triangular shape and extends forward surrounding the right wall of the aorta. Internally it is connected with the smooth-walled venous component of the atrium. Numerous pectinate muscles arise from the terminal crest and travel as parallel ridges along the anterior aspect of the free wall of the right atrium.¹⁶

The atrial septum has an interatrial component (between the right and left atria) and an atrioventricular component (between the right atrium and left ventricle). The interatrial portion is relatively small, and its most prominent feature is the fossa ovalis (oval fossa). During fetal and neonatal life, the valve of the fossa ovalis represents a paper-thin, delicate, translucent membrane. In contrast to the fossa ovalis, the foramen ovale (oval foramen) represents a passageway between the 2 atria. It courses between the anterosuperior aspect of the limbus and the valve of the fossa ovalis and then through a natural valvular perforation, the ostium secundum, and into the left atrium.¹⁶

The left atrium is a midline posterosuperior chamber that receives pulmonary venous blood and expels it across the mitral valve and into the left ventricle. When a left superior vena cava persists, the coronary sinus into which it drains is generally quite dilated, in some cases indenting the left atrial wall, and should not be mistaken for the descending thoracic aorta. As on the right side, the left atrium consists of a free wall and a septum. The free wall includes a dome-shaped body, which receives the pulmonary veins, and a fingerlike appendage. The body of the left atrium contains no pectinate muscles, and there is no crista terminalis.¹⁶

The atrioventricular valves serve to maintain unidirectional blood flow and to electrically separate the atria and ventricles. Each valve has 5 components. The annulus, leaflets, and commissures form the valvular apparatus, and the chordae tendineae (tendinous cords) and papillary muscles form the tensor apparatus. The annulus of each valve is somewhat saddle-shaped rather than being truly planar and represents an ill-defined ring of fibrous tissue from which the leaflets arise.¹⁶

Anatomically, the right ventricle can be divided into inlet, trabecular, and outlet regions. This concept of a tripartite chamber correlates well with the embryologic development of the right ventricle. The inlet portion is associated with the tricuspid valve, and its border is defined by the chordal insertions. Anteroapically, prominent muscle bundles traverse the chamber from septum to free wall and demarcate the trabecular region. The septal band is a Y-shaped structure with a long, broad stem and smaller inferior and anterior limbs. The 2 limbs, in turn, cradle the outlet septum and give rise to the medial tricuspid papillary muscle. Apically, the septal band merges with the apical trabeculations and gives rise to the moderator band, which inserts at the base of the anterior tricuspid papillary muscle.

The left ventricle is a left posterior chamber consisting of septal and free-wall components, and its entrance and exit are guarded by the mitral and aortic valves, respectively. The ventricular apex is characterized by small, shallow trabeculations, and the apical one-half to two-thirds of the septal surface is also finely trabeculated.¹⁶

Because of hemodynamic streaming within the right atrium during intrauterine life, poorly oxygenated blood from the superior vena cava is directed toward the tricuspid orifice, whereas well-oxygenated placental blood within the inferior vena cava is directed by the eustachian valve toward the foramen ovale and into the left atrium. Consequently, the most oxygenated blood in the fetal circulation travels, via the left side of the heart, to the coronary arteries, upper extremities, and the rapidly developing central nervous system (Figure 20-1).

During fetal life, the presence of a patent ductus arteriosus is associated with equalization of aortic and pulmonary artery pressures and a state of physiologic pulmonary hypertension. Thus, during fetal and neonatal life, right ventricular hypertrophy is evident, and the thickness of the right ventricle is similar to that of the left. The fetal ventricular pressures are equal except in rare cases such as with fetal ductal constriction where the right ventricle and pulmonary artery pressures exceed the left ventricle pressure. Because the pulmonary and systemic circulations are separate in the fetus, each ventricle has a stroke volume determined by the individual preload, afterload, and contractility of that chamber. Both ventricles are linked by a common heart rate and by the atrial pressures, which are similar due to the presence of the foramen ovale. They are also linked by the ventricular septum, which is shared by each ventricle, and by the common arterial pressure, which is the result of the widely patent ductus arteriosus. The left ventricle ejects into the upper body and cerebral circulation, and the right ventricle ejects into the pulmonary arteries and through the ductus arteriosus into the lower body and the placental circulation. The vascular beds of upper and lower body are connected via the aortic isthmus.¹⁷

The unique feature of the parallel nature of the ventricular ejection is that if there is increased afterload of one ventricle, the output of that ventricle will fall and the output of the contralateral ventricle will increase in a compensatory manner. This leads to the commonly observed feature associated with congenital heart disease of disproportionate growth of the normal side of the heart. As a further consequence of the parallel arrangement of these circulations, ventricular outputs can be different, and in the case of obstruction on one side of the heart, the other side is able to increase its work or even completely supply the whole circulation alone (see Figure 20-1).¹⁷

The effect of heart rate on fetal combined cardiac output is much more pronounced than postnatally. The fetus has a range of heart rates between 50 and 200 beats per minute, at which the stroke volume of the ventricular chambers can adapt to maintain adequate combined cardiac output and tissue perfusion. Outside of this range, heart failure will often result. The major determinant of cardiac output is the afterload of the fetal ventricle. Any influence that raises the impedance to ejection will inversely lower the ventricular stroke volume by the effect on both the systolic and diastolic functions of the heart.

The fetal echocardiogram should not be mistaken for the fetal heart screen procedure. The major differences between fetal heart screen and fetal echocardiogram are highlighted in Table 20-1. Consideration should be given to follow the guidelines of the International Society of Ultrasound in Obstetrics & Gynecology.^18,19

Fetal Heart Screening

The “basic” and “extended basic” fetal cardiac screening examination is the general fetal anatomical ultrasound scan that is performed in the majority of low-risk pregnancies at 18 to 22 weeks of gestation to screen for congenital anomalies. The “basic” cardiac screening examination relies on a 4-chamber view of the fetal heart involving a careful evaluation of specific criteria. An “extended basic” cardiac examination requires the views of the outflow tracts.¹⁸ The personnel responsible include obstetricians, radiologists, and sonographers.

Fetal echocardiography is focused on fetal anatomical details and is performed by health care providers with additional expertise in the prenatal detection and interpretation of congenital heart disease. It is performed when an anomaly is suspected on the basis of history, biochemical abnormalities, or the results of either the limited or standard scan suggesting the presence of fetal pathology.

The fetal heart screen relies on evaluation of the cardiac situs, axis and position, 4-chamber view, and atrioventricular valves. Key components of the normal 4-chamber view include an intact interventricular septum and atrial septum primum. Usually, there is no disproportion between the left and right ventricles. A moderator band helps to identify the morphologic right ventricle (Figure 20-2).

A normal heart is no larger than one-third the area of the chest. The ratio of the size of the chest is a useful prognostic tool in structural and functional heart disease at risk to develop heart failure. The circumference of the heart and thorax can be measured and express as a ratio. This can be achieved in a cross-sectional view when the entire thorax is seen and includes a 4-chamber view, at least 1 complete rib, and no abdominal content. The cardiothoracic circumference ratio has a mean value of 0.45 at 17 weeks and 0.5 at term.²⁰ The ratio of the cardiac and thoracic area can also be calculated and is rather constant throughout pregnancy, with a normal value of 0.20 to 0.35 (Figure 20-3). These methods to calculate heart size are a simple tool to evaluate fetal heart cardiomegaly.²¹

The heart is normally deviated about 45 ± 20 degrees (2 standard deviations) toward the left side of the fetus.²² Careful attention should be given to cardiac axis and position. Situs abnormalities should be suspected when the fetal heart and/or stomach is not found on the left side as well. Abnormal axis increases the risk of cardiac malformations, especially involving the outflow tracts. Some hearts are abnormally displaced from their usual position in the anterior left central chest. Abnormal cardiac position in the anterior left central chest can be caused by a diaphragmatic hernia or space-occupying lesion, such as cystic adenomatoid malformation. Position abnormalities can also be secondary to fetal lung hypoplasia.

Both atrial chambers normally appear similar in size, and the foramen ovale flap should open into the left atrium. Pulmonary veins can often be seen entering the left atrium. The lower rim of atrial septal tissue, septum primum, should be present. The moderator band helps to identify the morphologic right ventricle. Both ventricles should appear similar in size without evidence for thickened walls. Disproportion is the best sign of fetal congenital heart disease.²³ Although mild ventricular disproportion with right-sided enlargement can occur as a normal variant, with more severe disproportion, hypoplastic left heart syndrome and aortic coarctation are important causes of this disparity.^23-25

The ventricular septum should be carefully examined for cardiac wall defects. A septal wall defect may be difficult to detect when the transducer’s angle of insonation is directly parallel to the ventricular wall. Under these circumstances, a defect may be falsely suspected because of acoustic “drop-out” artifact.

Two distinct atrioventricular valves should be seen to open separately and freely. The septal leaflet of the tricuspid valve is inserted to the septum closer to the apex when compared to the mitral valve. Abnormal alignment of the atrioventricular valves can be a key sonographic finding for cardiac anomalies such as atrioventricular septal defect.

The aortic and pulmonary artery size should be evaluated, and if there is disproportion of chamber sizes, or if a structure appears abnormal in size, appropriate measurements should be made for comparison with normal ranges. Measurements in two-dimensional images should be made of the internal dimensions (inner to inner walls) in a standard fashion. For normal values, consider the appendix of The Perinatal Cardiology Handbook.²⁶

A normal extended basic examination requires that normal great vessels are approximately equal in size, and they cross each other at right angles from their origins as they exit from their respective ventricular chambers. Failure to confirm these findings in a well-visualized study warrants further evaluation. Evaluation of the outflow tracts can increase the detection rates for major cardiac malformations like conotruncal anomalies such as tetralogy of Fallot, transposition of great arteries, double outlet right ventricle, and truncus arteriosus.¹⁸

Fetal Echocardiogram

A fetal echocardiogram should be performed if recognized risk factors raise the likelihood of congenital heart disease beyond what would be expected for a low-risk screening population. Fetal echocardiography is for pregnancies at risk for structural, functional, and rhythm-related fetal heart disease. Indications for fetal echocardiography can be classified as maternal, fetal, and familial. The indication with the greatest positive yield is a finding of suspected fetal heart disease at routine obstetrical screening examination. Routine obstetrical ultrasound screening is critical in the prenatal detection of fetal/congenital heart disease. Unfortunately, a high proportion of prenatally detectable cases of congenital heart disease occur in patients without any risk factors or extracardiac anomalies.²⁷

Indications for Fetal Echocardiogram

Maternal Indications

• 
  

  Type 1 or 2 diabetes mellitus (preconception)—The incidence is reported to be 2- to 5-fold higher than that of the general population. Major structural defects identified include D-transposition of the great arteries, hypoplastic left heart syndrome, tetralogy of Fallot, and pulmonary and tricuspid atresia. Diabetic hypertrophic cardiomyopathy, which typically does not evolve until late in gestation, is an indication for fetal echocardiography, although the majority of affected infants are clinically well after birth and the pathology regresses spontaneously after birth, but rare neonates will have severe ventricular outflow obstruction.

  
  
• 
  

  Phenylketonuria—Fourteen percent risk particularly with phenylalanine levels of greater than 600 μmol/L at 0 to 8 weeks, and specific lesions include left heart obstruction and tetralogy of Fallot.

• 
  

  Maternal congenital heart disease—The risk is dependent upon the specific lesion. If the mother has aortic stenosis, the risk of an affected offspring is 13% to 18%, for atrioventricular canal 14%, for tetralogy of Fallot 6% to 10%, for pulmonary stenosis 4% to 6.5%, for ventricular septal defect 6%, for atrial septal defect 4% to 4.5%, and for coarctation of the aorta 4%.^28-30

  
  
• 
  

  Cardiomyopathy—Dependent upon the type of inheritance and age at presentation (most present in adolescence and as such would not be expected to be identified prenatally).

Specifically those associated with anti-Ro and/or anti-La autoantibodies. Fetal atrioventricular heart block (typically present after 17 weeks in mother with autoantibodies with or without clinical autoimmune disease) may result, and late fetal cardiomyopathy with endocardial fibroelastosis can occur in childhood.

• 
  

  Alcohol—Atrial and ventricular septal defects³⁰

  
  
• 
  

  Valproic acid—Atrial and ventricular septal defects, aortic and pulmonary valve obstruction, coarctation of the aorta

  
  
• 
  

  Vitamin A derivatives—Conotruncal abnormalities

  
  
• 
  

  Lithium—Ebstein anomaly of the tricuspid valve

Fetal Indications

• 
  

  Structural cardiac pathology—The indication that has been shown to provide the highest yield for true fetal structural cardiac pathology, which provides support for the critical importance of obstetrical ultrasound screening.

  
  
• 
  

  Hyperechogenic foci

  
  
• 
  

  Functional pathology

  
  
• 
  

  Pregnancies at risk for fetal cardiovascular compromise such as twin–twin transfusion syndrome (recipient twin); maternal viral infection associated with fetal myocarditis or cardiomyopathy due to coxsackie, cytomegalovirus, and parvovirus; high output states including fetal arteriovenous malformations (eg, sacrococcygeal teratoma, vein of Galen aneurysm, placental chorioangioma); agenesis of the ductus venosus; fetal anemia; and hydrops fetalis or polyhydramnios (the latter is a relative risk given that it can be associated with increased atrial pressures and atrial dilation).

  
  
• 
  

  Fetal dysrhythmia (ectopy, bradycardia, tachycardia)

• 
  

  Structural pathology involving other organ systems such as renal dysgenesis, central nervous system pathology (eg, Dandy-Walker syndrome, hydrocephalus, agenesis of the corpus callosum), omphalocele (risk for both cardiac pathology and aneuploidy) 20% to 30% risk, diaphragmatic hernia (15% to 25%), duodenal atresia, esophageal atresia, or tracheo-esophageal fistula

  
  
• 
  

  Increased nuchal translucency in the first trimester (a cutoff value of 3.5-5 mm is quoted for increased risk of CHD)

  
  
• 
  

  Findings suggestive of chromosomal abnormality or aneuploidy or documented abnormal fetal karyotype

  
  
• 
  

  Trisomy 21—Risk of ventricular and atrial septal defect, atrioventricular septal defect (50%)

  
  
• 
  

  Trisomy 18—Risk of atrial and ventricular septal defects, dysplastic atrioventricular and semilunar valves, tetralogy of Fallot, double-outlet right ventricle with mitral atresia or stenosis

  
  
• 
  

  Trisomy 13—Risk of atrial and ventricular septal defects, atrioventricular septal defect, and tetralogy of Fallot

  
  
• 
  

  Turner syndrome—Risk of left heart obstructive lesions including bicuspid aortic valve coarctation of the aorta, aortic valve stenosis, and hypoplastic left heart syndrome

Familial Indications

• 
  

  Paternal congenital heart disease

  
  
• 
  

  Sibling with congenital heart disease—The recurrence risks for future siblings are 2% to 6%.⁷

  
  
• 
  

  Mendelian syndromes

  
  
  
  
  • 
    

    Holt-Oram syndrome—Autosomal dominant; risk of atrial and ventricular septal defects; recently TBX5 gene also associated with hypoplastic left heart syndrome.

    
    
  • 
    

    DiGeorge syndrome (22q11.2 deletion)—Risk of conotruncal cardiovascular pathology. Deletions were found in 50.0% with interrupted aortic arch, 34.5% of patients with truncus arteriosus, and 15.9% with tetralogy of Fallot.³¹

    
    
  • 
    

    Noonan syndrome—Autosomal dominant; risk of dysplastic pulmonary valve, pulmonary stenosis, and hypertrophic cardiomyopathy.

    
    
  • 
    

    Williams syndrome—Autosomal dominant; supravalvar aortic stenosis, peripheral pulmonary artery stenosis.

    
    
  • 
    

    Ellis-van Creveld syndrome—Autosomal recessive; risk of atrial and ventricular septal defects and atrioventricular septal defect.

    
    
  • 
    

    Familial cardiomyopathy—Most common autosomal dominant inheritance for both hypertrophic and dilated forms.

Responsible Personnel for Fetal Echocardiogram

Responsible personnel include (1) pediatric cardiologists and their sonographers with expertise in fetal and pediatric echocardiography and knowledge about the nuances of congenital heart disease including the medical and surgical outcomes, and knowledge about structural, functional, and rhythm-related fetal heart disease; and (2) obstetrical or radiology personnel with training in fetal echocardiography but with assistance from pediatric cardiologists for interpretation of the findings and prenatal counseling.

Fetal Heart Echocardiographic Technique

The fetal echocardiogram relies on two-dimensional ultrasonography that includes Doppler echocardiography as well. This includes detailed assessment of the cardiac anatomy, blood flows, heart rate and rhythm, and function, and is performed in pregnancies at risk for or with suspected structural, functional, or rhythm-related fetal heart disease. Fetal echocardiography relies on the assessment of the heart using key cardiac views that include apical 4-chamber, parasternal long axis, parasternal short axis, subcostal, and arch views. It is important to establish the situs to confirm that the inferior vena cava and aorta are in correct relationship. The aorta should originate from the left ventricle and is in continuity with the mitral valve. By comparison, the pulmonary artery arises from the right ventricle and branches into its pulmonary branches. Aortic and ductal arches with the head and neck vessels arising from the aorta should be identified. Dimensions of the aortic and pulmonary branches are important, as it is to identify left- or right-sidedness of the aortic arch and patency of the ductus arteriosus, and pulmonary venous connections to the left atrium, inferior vena cava, and superior vena cava connecting to the right atrium, and tricuspid, and mitral valves of similar size, with the tricuspid inserting slightly more apical than the mitral.^23,25,32 The integrity of interventricular and interatrial septa should be evaluated, as well as the foramen ovale bulging to the left atrium with flow from right to left. Effusions, ascites, or hydrops should be assessed. Two-dimensional imaging should include visceral and atrial situs, 4-chamber view assessment, evaluation of the outflow tracts, and the 3-vessel view.

The outflow tracts are usually obtained by angling the transducer toward the fetal head from a 4-chamber view when the interventricular septum is tangential to the ultrasound beam. Another method for evaluating the outflow tracts has also been described for the fetus when the interventricular septum is perpendicular to the ultrasound beam. This approach requires a 4-chamber view of the heart where the probe is rotated until the left ventricular outflow tract is seen. Once this view is obtained, the transducer is rocked cephalad until the right ventricle arterial outflow tract is observed in a plane that is perpendicular to the aorta.^33,34

Moving cranial to the aortic valve, the pulmonary valve can be seen lying anterior, leftward, and cranial to the aortic valve. The ductal connection to the descending aorta, aorta in cross-section, and superior vena cava are seen in this view, which has been termed the 3-vessel view (Figure 20-4).^35,36

The left ventricular outflow tract view confirms the presence of a great vessel originating from the left ventricle (Figure 20-5). Continuity should be documented between the anterior aortic wall and ventricular septum. The aortic valve moves freely and should not be thickened. When the left ventricular outflow tract is truly the aorta, it should even be possible to trace the vessel into its arch, from which 3 arteries originate into the neck. The left ventricular outflow tract view may help to identify ventricular septal defects and conotruncal abnormalities that are not seen during the basic cardiac examination alone.

A view of the right ventricular outflow tract documents the presence of a great vessel from a morphologic right ventricle with a moderator band (Figure 20-6). The pulmonary artery normally arises from the right ventricle and courses toward the left of the more posterior ascending aorta.

The pulmonary arterial valve moves freely and should not be thickened. Pulmonary valve stenosis, for example, may be associated with right ventricular wall hypertrophy, with decreased chamber volume secondary to increased right-to-left shunting across the foramen ovale, with right ventricular hypertrophy or with increased chamber volume if tricuspid regurgitation occurs.³⁷ The distal pulmonary artery normally divides toward the left side into a ductus arteriosus that continues into the descending aorta. The right side is the right pulmonary artery.

In addition to information provided by the basic screening examination, a detailed analysis of cardiac structure and function may further characterize visceroatrial situs, systemic and pulmonary venous connections, foramen ovale mechanism, atrioventricular connections, ventriculoarterial connections, great arteries relationships, and sagittal views of the aortic and ductal arches.^18,25,38

Today, no fetal echocardiogram exam would be complete without the use of color-flow Doppler sonography. The addition of color-flow Doppler sonography adds further information concerning the function of the atrioventricular and semilunar valves, inferior vena cava, and ductus venosus (Figure 20-7), exclusion of abnormal flows (particularly atrioventricular valve regurgitation), as well as flows within important fetal flow pathways such as the ductus venosus, foramen ovale, ductus arteriosus, and umbilical venous pulsations (Figure 20-8).³⁹

The principle of color Doppler echocardiography is that information on blood flow velocity is computed, color encoded, and simultaneously superimposed on the two-dimensional echocardiographic image (Figure 20-9). The two-dimensional echocardiographic system assigns different colors to the blood flow, depending on the direction and velocity of the blood flow. Most commonly, flow moving toward the transducer probe is displayed in shades of red and flow moving away from the transducer is in shades of blue. Color Doppler echocardiography provides a display of mean flow velocities with higher velocities being shown in brighter shades and the lower velocities in darker shades. Turbulent blood flow can be displayed as a mosaic with green superimposed on the other colors. Thus, color Doppler sonography gives information regarding direction, velocity, and turbulence of blood flow to provide visualization of blood flow patterns in the fetus. Color Doppler sonography is an important adjunct to cross-sectional scanning. The correct direction of flow throughout the cardiac chambers and arch vessels can be demonstrated. It can be used to exclude regurgitation at any valve. Unaliased color flow throughout the heart indicates that the flow is at normal velocities. If color shows aliasing at any point in the heart, pulsed Doppler sonography should be used to obtain an accurate velocity. The peak velocity at the atrioventricular valves is 30 to 60 cm/s and is fairly constant throughout gestation (Figure 20-10). The peak velocity of flow at the arterial valves is approximately 25 cm/s at 12 weeks, increasing to 60 to 100 cm/s by term. Turning on color-flow Doppler sonography helps to exclude a significant ventricular septal defect when the ultrasound beam is perpendicular to the ventricular septum. Color Doppler sonography is essential to confirm normal pulmonary venous connections. The ultrasound beam must be positioned “in-line” with the flow under observation when using pulsed or color-flow mapping.³⁵

The fetal aortic arch is seen on a sagittal section as a narrow curvature arch passing to the descending aorta with a “candy-cane” appearance. The most specific feature of the aortic arch is the origin of the head and neck vessels including the left carotid artery and left subclavian artery from the aortic arch (Figure 20-11).

In the normal fetus, the ductus arteriosus arises in a more anterior position in the chest compared with the aortic arch, as it is connected with the right ventricle, the heart’s most anterior structure. The ductus arteriosus has a wider curvature than the aortic arch, having the appearance of a “hockey-stick” on fetal echocardiography. The peak velocity of this vessel reaches the highest value of the entire fetal circulation and increases toward the end of pregnancy. The pulsatility index is more constant throughout gestation, varying between 1.9 and 3.0.

Ductus arteriosus patency is fundamental for fetal survival. Premature ductal constriction or closure is associated with cyclooxygenase inhibitors like indomethacin, used frequently to stop preterm labor, or other anti-inflammatory agents that inhibit the production of prostaglandins. Suspicion of ductal constriction on two-dimensional images, should be confirmed with pulsed or continuous Doppler assessment. With ductal constriction the systolic velocity is greater than 1.40 m/s, the diastolic velocity is greater than 0.35 m/s, and the pulsatility index is less than 1.9 (Figure 20-12). Prenatal ductal constriction can lead to progressive right ventricle dysfunction, with secondary tricuspid regurgitation (Figure 20-13), congestive heart failure, and fetal death if the foramen ovale is restrictive.

Cardiac rate and regular rhythm should be confirmed (Figure 20-14). The normal rates range from 120 to 160 beats per minute. Mild bradycardia is transiently observed in normal second trimester fetuses. Fixed bradycardia, especially heart rates that remain below 110 beats per minute, requires timely evaluation for possible heart block (Figure 20-15). Repetitive heart rate decelerations during the third trimester can be caused by fetal distress.⁴⁰ Occasional skipped beats are typically not associated with an increased risk of structural fetal heart disease. However, this finding may occur with clinically significant cardiac rate or rhythm disturbances as an indication for fetal echocardiography.⁴¹

Mild tachycardia (>160 beats per minute) can occur as a normal variant during fetal movement. Persistent tachycardia, however, should be further evaluated for possible fetal distress or more serious tachydysrhythmias.

Rhythm Abnormalities

1. 
   

   Intermittent (<12/24 hours) supraventricular tachycardia or atrial flutter: In several series, intermittent tachycardia did not progress to sustained tachycardia during fetal life. Continued follow-up is recommended, as some may become sustained after birth.

   
   
2. 
   

   Intermittent accelerated ventricular rhythm at rates less than 200 beats per minute: A benign rhythm, usually seen in the neonate, that responds to short-term postnatal beta-blocker therapy.

   
   
3. 
   

   Isolated atrial or ventricular ectopy: Only 0.5% to 1% develop sustained supraventricular tachycardia or atrial flutter. We recommend checking fetal heart tones weekly until ectopy resolves, and obtaining a postnatal 12-lead electrocardiogram (ECG).

   
   
4. 
   

   Atrioventricular block with a structurally normal heart and negative maternal Sjögren (SSA/SSB, Ro/La) antibodies: This is rare. If the fetus has type 2, 2-degree atrioventricular block without SSA/SSB antibodies, monitor closely for ventricular tachycardia as this presentation and sinus bradycardia are consistent with congenital long QT syndrome. Postnatal follow-up is essential to exclude long QT syndrome. Long QT syndrome has been associated with fetal, infant, and childhood sudden cardiac death.

   
   
5. 
   

   If the fetus has sustained arrhythmia (other than ectopy) and is 35 weeks of gestation or more with minimal or no hydrops, delivery and postnatal therapy may be the best option. Slower than normal heart rate or persistent rapid heart rate leads to cardiomegaly. The time frame of the onset of the arrhythmia may therefore be estimated by the effect on the cardiac size. For example, an intermittent arrhythmia that has appeared recently would not be expected to cause cardiac enlargement.

Significance and Effectiveness of Prenatal Cardiac Diagnosis

Prenatal cardiac diagnosis, by facilitating anticipatory use of prostaglandin E₁ to prevent closure of the ductus arteriosus in neonates with critical impairment of systemic or pulmonary blood flow, allows affected fetuses to avoid neonatal acidemia.⁴²

Fetuses with lesions reparable into 2 ventricular systems appeared to enjoy a significant enhancement of survival prospects. Among the individual cardiac lesions for which prenatal diagnosis has been suggested to impart a survival advantage are transposition of the great arteries,⁴³ coarctation of the aorta,⁴⁴ and hypoplastic left heart syndrome.⁴⁵

Prenatal diagnosis, by providing time, may alter survival through alteration of the site of delivery and subsequent surgery. It is possible, if not likely, that the latter will exert a profound impact of prenatal cardiac diagnosis on postnatal survival.

Cesarean delivery may be indicated in rare situations in which the coordinated care of the neonate requires the skills of multiple specialists who are assembled specifically at the time of delivery, and in fetuses with cardiac rhythm disturbances that preclude effective intrapartum fetal heart rate monitoring. Such arrhythmias may include chaotic rhythms that confound the logic of external fetal heart rate monitors that calculate heart rate from instantaneous measurement of R-R intervals or regular tachyarrhythmias or bradyarrhythmias such as atrial flutter or complete heart block, where the heart rate may not vary with the alterations in sympathetic and parasympathetic tone that are associated with uterine contraction.⁴⁶

Limitations of Fetal Echocardiography

Fetal position and fetal size are critical factors. As the gestational age increases, the ossification increases, making it harder to see intrathoracic structures (Figure 20-16). Image resolution is also an important factor. As a result of such limitations, ventricular septal defects—even atrioventricular septal defects and more subtle conditions such as tetralogy of Fallot—with milder right ventricle outflow obstruction may be missed.

Acquired cardiac lesions that become apparent later in life, even those of genetic origin such as Marfan syndrome and hypertrophic subaortic stenosis, are not generally detectable by prenatal ultrasonography.¹⁸

Safety Issues

Diagnostic ultrasound studies of the fetus are generally considered safe during pregnancy. The lowest possible ultrasonic exposure setting should be used to gain the necessary diagnostic information under the as low as reasonably achievable (ALARA) principle.⁴⁷ The operator must be aware of the acoustic output of instruments used, and the thermal and mechanical indices must be within recommended levels. These indices provide information on the relative risk of producing thermal or cavitation effects. Risks may be greater during embryogenesis (<10 weeks conceptual gestational age). For late first trimester assessments, use short duration of scans and limited duration of Doppler interrogation. Reassessment is recommended in the midtrimester given the image resolution issues in the later first trimester and potential for progression.

The ability to diagnose prenatal cardiovascular disease has improved dramatically due to education and improved ultrasound technology. As our ability to image increases, so does the potential for generation of new knowledge concerning the fetal cardiovascular state. Sophisticated imaging techniques allow us to assess previously puzzling physiologies at earlier and earlier points in gestation. It is crucial that the natural history of congenital cardiac disease be mapped out accurately before we can appropriately undertake a strategy of perinatal cardiac intervention for specific lesions.⁴⁸

The color Doppler sonographic findings in specific heart defects⁴⁹ are summarized in Table 20-2.

Left Heart Defects

Hypoplastic Left Heart Syndrome

Incidence

Hypoplastic left heart syndrome makes up to 4.8% to 9% of congenital heart disease. The incidence is between 0.1 and 2.7 per 1000 live births. It is considered to be the most common cause of cardiac death during the first week of life.^45,50

Embryology/Pathophysiology/Genetics

The fetal left ventricle is predominantly filled with oxygenated blood that returns from the placenta and traverses the foramen ovale.⁵¹ If blood flow across the foramen ovale is diminished or reversed, the combined cardiac output is redistributed to the right ventricle and pulmonary artery, resulting in enlargement of the right heart structures and creating less impetus for normal growth of left heart structures, possibly evolving into hypoplastic left heart syndrome. Perhaps the most well-recognized mechanism for decreased flow or reversal of flow through the foramen ovale in utero is the presence of severe aortic valve disease.^45,52,53 With significant aortic valve stenosis, alterations in left ventricular compliance may occur, either secondary to the development of left ventricular hypertrophy or secondary to the development of left ventricular dilation and dysfunction. Endocardial fibroelastosis, a poorly understood phenomenon where the endocardial lining of the left ventricle becomes fibrotic, may also be present. As the disease state progresses, with subsequent elevation in left atrial pressure, flow across the foramen ovale becomes bidirectional and eventually left to right, the result of which may be the cessation of left ventricular growth.⁵⁴

Although a multifactorial mode of inheritance is likely, the recurrence risk among siblings suggests transmission via an autosomal recessive mode. In addition, pedigree analyses have demonstrated a 12% prevalence of cardiac abnormalities involving the left ventricular outflow tract in first-degree relatives of patients with hypoplastic left heart syndrome.⁵⁵

Hypoplastic left heart syndrome is associated with chromosomal anomalies in about 2% of cases, particularly Turner syndrome (XO), trisomies 13 and 18, and rarely microdeletion of chromosome 22q11.³¹

Diagnostic Features

Recent advances in two-dimensional and Doppler echocardiography have made it feasible to diagnose all forms of congenital heart disease in the fetus. Hypoplastic left heart syndrome is one of the most common structural lesions diagnosed prenatally, as a screening obstetric ultrasound will preferentially identify lesions that dramatically alter the 4-chamber view (Figure 20-17).^49,56,57 The prenatal diagnosis of hypoplastic left heart syndrome is made when a small, muscle-bound left ventricular chamber is identified.

Clues to fetal sonographic diagnosis are a small echogenic left ventricle or absent left ventricle (Figure 20-18), a small left atrium, and left-right atrial shunt (Figure 20-19). There is a hypoplastic ascending aorta and enlarged pulmonary artery with retrograde flow in the aortic arch (Figure 20-20). Left ventricle to coronary fistulous connections may be present.