Fetal echocardiography was introduced to clinical medicine in the early 1980s when the first studies reported its use for evaluation of cardiac arrhythmias as well as basic cardiac anatomy using M-mode, M-mode-directed real-time, and real-time ultrasound.^1-29 Although the prenatal diagnosis of congenital heart defects was the goal, a logical approach to the problem was not available until investigators in 1985 suggested the concept of using the four-chamber view as an initial screening tool to detect fetuses at risk for structural malformations.^30-36 After DeVore et al introduced the inclusion of color Doppler in the screening protocol to improve detection of congenital heart defects, other investigators reported similar results.^32,37-44 While using the four-chamber view to screen for congenital heart disease seemed promising, it met with varied success.^1,32,45-49 One of the main reasons was that not all major heart defects altered the size, shape, or anatomy of the structures identified in the four-chamber view.⁴⁷

To overcome the limitations of the four-chamber view screening examination, the the American College of Obstetricians and Gynecologists (ACOG), the American Institute of Ultrasound in Medicine (AIUM), and the American College of Radiology (ACR), published between 2003 and 2007 recommendations stating that an attempt, if “technically feasible,” should be made to examine the outflow tracts of the fetal heart during the screening examination.^50-53 In 2013, the AIUM, ACOG, the ACR, and the Society of Radiologists in Ultrasound jointly published recommendations that the outflow tracts should be evaluated in all fetuses undergoing second and third trimester ultrasound, while ISUOG in 2010 stated that it was still optional (Table 47-1).^54,55

The purpose of this chapter is to review the elements of the screening examination of the fetal heart using two-dimensional ultrasound, which is the current diagnostic tool suggested for this purpose. After reviewing this approach for screening, discussion of additional ancillary tools using color/power Doppler ultrasound as well as 3D/4D ultrasound will be discussed.^{37,39,43,56-103}

Two-Dimensional Image Acquisition

The requirements for proper image acquisition of the fetal heart are a high frame rate, an image of the four-chamber view that fills most of the ultrasound screen, and gain settings that have been optimized for cardiac imaging.

The Interdependence of the Fetal Heart Rate and Frame Rate (Hz)

While the image acquisition frame rate has not been agreed upon, experts have suggested that an image acquisition of 80 frames per second (80 Hz) or higher would be preferable to lower acquisition rates.¹⁰⁴ Assuming two fetal cardiac cycles per second, this would translate to 40 frames per cardiac cycle. Table 47-2 lists the range of fetal heart rates from 120 to 180 per minute and the corresponding frame rates (Hz) required to obtain 40 frames per cardiac cycle.¹⁰⁴ Figure 47-1 illustrates a dataset in which 200 fetuses were examined between 20 and 38 weeks of gestation in which 94% of fetuses had image acquisition rates of 80 Hz or higher.¹⁰⁴

Increasing the Image Frame Rate

To increase the frame rate, the examiner needs to decrease the angle and depth used for image acquisition. Figure 47-2A illustrates that as the angle is decreased from 72 to 30 degrees, the frame rate increases from 42 to 91 Hz (Figure 47-2B). Following narrowing of the image field, the depth is next decreased to the minimum value that still enables imaging the four-chamber view on the screen. Figure 47-2C illustrates that as the depth is decreased from 12.9 to 8.5 cm, the frame rate increases from 91 to 130 Hz.¹⁰⁴

Increasing the Size of the Image

Once the aforementioned steps to maximize the frame rate are accomplished, the examiner can use the Box and Zoom feature, as illustrated in Figure 47-2D. This feature increases the size of the four-chamber view to fill the screen, with only a minimum decrease in frame rate from 130 to 121 Hz.

Adjusting the Image Quality

The image quality is an important part of the screening examination of the fetal heart.¹⁰⁵ While there are a number of image adjustments, the image should have a high contrast between the blood pool and the endocardial border of the right and left ventricular chambers. Figure 47-3 compares two images in which the settings have been adjusted to optimize the interface between the blood pool and endocardial border. Table 47-3 lists the settings for the General Electric Voluson E8 and E10 that illustrate the differences between general obstetrical imaging of soft tissues and the optimized image of the fetal heart.

In adults, the four-chamber, two-chamber, and three-chamber views are imaged from the apex/axillary window because of limitations related to the rib cage and lungs. The problem with fetal images acquired with this orientation is the potential for echo dropout of the endocardial borders of the lateral and septal walls of the interventricular chambers. The four-chamber view of the fetal heart, however, can be imaged from several orientations as the result of fetal position (Figure 47-4). Table 47-4 lists the combinations of image orientations of the four-chamber view from 70 consecutive fetuses referred for screening ultrasound. The most frequent orientation was when the apex was perpendicular to the ultrasound beam (85.7%) (Figure 47-4B and G) and the least frequent orientation was when the apex was up (21.4%), (Table 47-4, Figure 47-4D). Therefore, since it is preferable to image the full thickness of the right and left lateral walls of the ventricles as well as the interventricular septum, imaging the four-chamber view when the apex is perpendicular or tangential to the ultrasound beam is preferable to when the apex is up or down (see Table 47-4, Figure 47-4).

Screening for Abnormal Position, Shape and Size of the Four-Chamber View

Using linear measurements, the position, shape and size of the four-chamber view can be quickly ascertained during the screening examination. The position of the four-chamber view within the chest is part of the guidelines, while the shape and size of the four-chamber view are recent concepts reported in the literature.^106,107

Orientation of the Heart Within the Chest

According to the AIUM and ISUOG guidelines, the orientation of the four-chamber view within the chest should be 45 degrees (+/-) 20 degrees.^108,109 Deviations from this range are associated with congenital heart disease.³⁴ The easiest method to determine the position of the heart is to draw a line between the spine and anterior chest wall followed by a bisecting line drawn along the length of the interventricular septum (Figure 47-5).

Evaluating the Shape of the Heart Using the Global Sphericity Index

An abnormal shape of the right or left ventricles suggests cardiac dysfunction.^110,111 Evaluation of the global shape of the four-chamber view can be evaluated by measuring the global sphericity index (GSI) (Figure 47-6) from two diameters; the basal-apical (BAL) and the transverse lengths (TL).^106,107 Once measured, the GSI is computed (BAL/ TL). The GSI is independent of gestational age and is not correlated with noncardiac biometric measurements.¹⁰⁶ Table 47-5 lists the mean, standard deviation, 5th, 50th, 95th, and 99th centiles for the GSI. The lower the centile, the rounder the shape of the heart, the higher the centile, the more ellipsoid the shape of the heart. Figure 47-7 illustrates results from 300 sequential fetuses examined prospectively between 17 and 39 weeks of gestation.¹⁰⁶ Fifty-five fetuses (55/300, 18.33%) had an abnormal GSI < fifth centile suggesting the four-chamber view was more globular in shape than ellipsoid. Of the fetuses with an abnormal GSI, 96.36% (53/55) had associated abnormal ultrasound findings (Table 47-6). Two fetuses (2/300, 0.67%) had an abnormal GSI with normal ultrasound findings. Ten fetuses with abnormal ultrasound findings had a normal GSI (10/300, 3.33%) (p <0.001), (see Figure 47-7). The most common abnormal ultrasound finding was an abdominal circumference below the 10th centile. This was followed by intrauterine growth restriction and abnormal fetal heart findings that consisted of functional and/or structural defects (see Table 47-6). Of the 53 fetuses with an abnormal ultrasound, 48% had one or more abnormal findings. The sensitivity of an abnormal GSI for identifying a fetus with abnormal ultrasound findings was 84%, specificity 99%, positive predictive value 96%, and a negative predictive value of 96%. The benefit of the GSI is that it can be quickly measured during the routine ultrasound examination by identifying end-diastole, and computing the ratio between the BAL and TL measurements. If abnormal, further evaluation of fetal cardiovascular and noncardiovascular anatomy should be undertaken.

Evaluating the Size of the Heart Using Two Diameters

A recent study reported the computation of the area and circumference of the four-chamber view using the same measurements (BAL and TL) for computing the GSI.¹⁰⁶ The benefit of computing the area and circumference is if the heart appears to be enlarged, assessment can be easily accomplished. Most ultrasound machines allow the examiner to measure the BAL and TL and subsequently compute the GSI, area, and circumference online. Once measured, these computations can be compared to gestational age, as well as noncardiac biometric measurements.¹⁰⁷ Figure 47-8 illustrates the 10th and 90th centiles for the circumference and area of the four-chamber view at end-diastole using noncardiac biometry and gestational age.¹⁰⁷

With the recent introduction of the electronic 4D probe, real-time and STIC volumes can be obtained of the fetal heart for analysis. The benefit of the 4D real-time volume is that there is no artifact in the volume from fetal movement since the data in the volume are acquired at one time. However, the resolution is less than a two-dimensional image acquisition of a single plane. The electronic STIC volume has higher resolution and acquires sub-volumes, which almost eliminate the artifact when STIC volumes is acquired using the mechanical probe.^63,67,75 Using 4D STIC volumes acquired with the electronic probe, the anatomy of the four-chamber view will be discussed in the following sections.

Anatomy of the Right Ventricular Chamber

The right ventricle lies beneath the anterior chest wall, with the apex oriented toward the left side of the sternum (see Figure 47-5). The contour of the right ventricle is more globular in shape than the left ventricle. The length of the right ventricle, when compared to the left ventricle, is shorter and the width longer.^112,113

Imaging the Right Ventricle When the Apex is Perpendicular to the Ultrasound Beam

Although the ISUOG and AIUM guidelines describe the presence of the moderator band (MB) as a distinguishing feature to differentiate the right from the left ventricle, it is difficult to identify when the interventricular septum is perpendicular to the ultrasound beam (Figure 47-4B and G). Figure 47-9 illustrates the four-chamber and short-axis views obtained using 4D STIC tomographic imaging.⁷⁵ The reference line is placed through the short-axis view of the ventricles at the level of the tricuspid and mitral valves at end-diastole (Figure 47-9A) and at the level of the right ventricle anterior papillary muscle (Figure 47-9B). The anterior papillary muscle can be observed originating from the apex of the right ventricle (Figures 47-9C and 47-10) and inserting into anterior tricuspid valve leaflet. The location of the right ventricular anterior papillary muscle is unique to the right ventricle and differentiates it from the left ventricle. One leaflet of the tricuspid valve inserts lower on the interventricular septum than the mitral valve leaflet (Figures 47-9C and 10A). These landmarks identify the center of the right ventricle when imaged in the four-chamber view.

Imaging the Right Ventricle When the Apex is Parallel or Tangential to the Ultrasound Beam

The right ventricle has three papillary muscles (anterior, posterior, and septal) that attach to the tricuspid valve via the chordae tendinae. However, when the apex is tangential or parallel to the ultrasound beam, these structures are not clearly identified because they are small, and run parallel to the ultrasound beam (Figure 47-11). Therefore, the distinguishing structure that is unique to the right ventricle is the moderator band that courses from the interventricular septum to the lateral wall of the right ventricle and is clearly imaged when the apex of the ventricle is parallel or tangential to the ultrasound beam (see Figure 47-11). The tricuspid valve leaflets are observed to insert lower on the interventricular septum than the mitral valve leaflets (see Figure 47-11).

Anatomy of the Left Ventricular Chamber

The left ventricle lies beneath the right ventricular chamber, with the apex oriented toward the left side of the sternum (see Figure 47-5). The contour of the left ventricle is more ellipsoid than the right ventricle and has a smoother endocardium. The length of the left ventricle, when compared to the right ventricle, is longer and the width smaller (Figures 47-9 and 47-10).^112,113

Imaging the Left Ventricle When the Apex is Perpendicular to the Ultrasound Beam

Although the ISUOG and AIUM guidelines describe the presence of the moderator band (MB) as a distinguishing feature to differentiate the right ventricle from the left ventricle, they do not provide additional details describing the left ventricle other than it is less trabeculated.^53,109 Figure 47-9 illustrates the four-chamber view and short-axis views using tomographic imaging.⁷⁵ The reference line is placed through the short-axis view of the ventricles at end-diastole at the level of the tricuspid and mitral valves and the papillary muscles (Figure 47-9A and B). The full width of the left ventricle is best obtained when the plane of the ultrasound beam is directed between anterior and posterior papillary muscles, bisecting the tricuspid and mitral valves (see Figure 47-9A and B). The left ventricle in the four-chamber view plane has a longer apex-forming chamber than the right ventricle (Figure 47-9C). The left ventricle may also demonstrate a portion of the posterior papillary muscle lying along the lateral wall. The septal mitral valve leaflet inserts higher on the interventricular septum than the septal tricuspid valve leaflet (Figures 47-9C and 47-10).

Imaging the Left Ventricle When the Apex is Parallel or Tangential to the Ultrasound Beam

The apical forming LV is easier to see in this view as well as the insertion of the mitral valve leaflets (Figure 47-11) when compared to when the apex is perpendicular to the ultrasound beam (see Figure 47-10).

Anatomy of the Interventricular Septum

The interventricular septum is triangular in shape, with the base of the triangle at the apex of the heart (Figures 47-10 and 47-11). The portion of the interventricular septum that is imaged in the four-chamber view consists of only a small portion of the septum (Figure 47-12). Therefore, ventricular septal defects (VSD) cannot be excluded by just imaging the four-chamber view (see Figure 47-12). The importance of orientation of the interventricular septum to the ultrasound beam is as follows:

1. 
   

   The full width of the interventricular septum is best observed when the septum is perpendicular (see Figure 47-10) rather than when it is tangential or parallel to the ultrasound beam (see Figure 47-11). The reason for this is because of the properties of axial resolution. However, when the interventricular septum is parallel to the ultrasound beam, lateral resolution is predominant, resulting in less resolution of the walls of the septum (see Figure 47-11).

   
   
2. 
   

   Because of the properties of lateral resolution of the ultrasound beam, smaller ventricular septal defects may not be identified when the interventricular septum is perpendicular to the ultrasound beam because the resolution separating the interventricular septal structures is not sufficient (Figure 47-13). Detection of ventricular septal defects may only be accomplished if color or power Doppler is activated showing the shunt across the septum, or if the interventricular septum is parallel or tangential to the ultrasound beam (Figure 47-13).^32,37-44

   
   
3. 
   

   When a ventricular septal defect is suspected, the interventricular septum should be imaged parallel, tangential, and perpendicular to the ultrasound beam using two-dimensional grayscale, color, and power Doppler ultrasound. This can be accomplished by directing the ultrasound transducer over the surface of the maternal abdomen, thus changing the orientation of the ultrasound beam in relation to the interventricular septum (Figure 47-14).

After identifying the anatomy of the right and left ventricular chambers and interventricular septum, attention should be directed to the size and movement of the endocardial walls of the right ventricle, left ventricle, and the interventricular septum during ventricular systole. Understanding movement of these structures will enable the user to subjectively identify disorders of ventricular contractility by just observing wall motion. Movement of these structures can be illustrated with 2D-Speckle Tracking.¹⁰⁴

Movement of the Right and Left Ventricular Lateral Walls During Ventricular Systole

Figure 47-15 illustrates displacement of the right and left ventricular endocardium during systole. Sections 1r to 3r of the right ventricle demonstrate that most of the movement is longitudinal, from the base toward the apex, while in the same sections of the left ventricle the movement is both longitudinal and toward the center of the ventricular chamber. However, there is more longitudinal movement in the lateral wall of the right ventricle than the left ventricle. Sections 4r to 6r of the right ventricle demonstrate movement both longitudinal and medially toward the center of the ventricular chamber, as do the same sections of the left ventricle.