Background

As recently as the early 1990s, less than 10% of infants undergoing cardiac surgery in the first month of life received a diagnosis before birth. Today, reported rates of prenatal diagnosis frequently approach 50%.

A variety of maternal or fetal disorders may place a fetus at increased risk for congenital heart disease (CHD) (Table 2-1). If present, a fetal echocardiogram is indicated, and timely referral is recommended. Combined, approximately 5% of pregnancies are referred for in utero evaluation. Abnormal or unsatisfactory (the inability to establish normal) cardiac views obtained as part of an obstetric anatomic survey account for more than 20% of all referrals for in utero evaluation and lead to more than half of all prenatal diagnoses. The anatomic survey, a nearly universal mid-pregnancy ultrasound scan, includes a four-chamber (4C) view of the heart and, if possible, views of both outflow tracts. A positive family history accounts for another nearly 20% of all referrals. However, these are the source of less than 5% of all prenatal diagnoses.

TABLE 2-1 INDICATIONS FOR FETAL ECHOCARDIOGRAPHY

Cardiac Embryology and In Utero Physiology

Overview

Echocardiography is the main diagnostic modality used to evaluate the fetal heart. The optimal timing for performance of a comprehensive transabdominal fetal echocardiogram is 18 to 20 weeks’ gestation. In select cases, late first trimester evaluation may be possible. Evaluation late in gestation is often complicated by a more “fixed” fetal position, which may limit the available acoustic windows.

A complete fetal echocardiogram includes two-dimensional (2D) evaluation of cardiac anatomy, spectral and color Doppler interrogation, and an assessment of cardiac function and rhythm. The components of a comprehensive evaluation are listed in Table 2-2, although not all may be visualized in every fetus at every examination. Similar to transthoracic imaging, fetal echocardiography depends on the ability to obtain standard views and evaluate structures in orthogonal views. The fetus may be very active, and the examiner may need to piece together many partial images to form a composite picture, particularly in the presence of complex CHD.

TABLE 2-2 COMPONENTS OF THE FETAL ECHOCARDIOGRAM

Study Protocol

2D Images

Fetal Position

Single sweep from the uterine fundus to the cervix to establish fetal position (vertex, breech, or transverse).

Abdominal and Cardiac Situs

Although uncommon as a whole, CHD and abnormalities of laterality (heterotaxy syndrome) are commonly found together. It is essential to establish abdominal and cardiac situs:

• Doublecheck that the “P” is to the right of the screen (this should never change).

• Rotate/move the transducer on the maternal abdomen as necessary to orient the fetus such that the fetal head is to the right of the screen (Fig. 2-1).

• Rotate the transducer 90 degrees clockwise into a transverse cut; the fetus is now oriented with the head into the screen (this is the same orientation as a computed tomography (CT) or magnetic resonance imaging (MRI) scan. Right and left are then established, depending on the position of the fetal spine (up or down), and a reviewer can reliably interpret fetal right from left, utilizing a standard practice approach.

• Find the stomach and the heart and establish cardiac (levo, dextro, or mesocardia) position and abdominal situs.

Figure 2-1 Fetus oriented with the head to the screen right (as the sonographer faces it) for cardiac and abdominal situs determination.

Key Points

• Attention to alignment cannot be overemphasized. Anatomic relationships can easily be visually manipulated.

• Comprehensive cardiac evaluation should include sweeps rather than solely a series of still frame captures. Sweeps allow for mental reconstruction and are essential to understanding complex cardiac lesions.

The Four-Chamber View

The 4C view is the most important in a comprehensive examination of the fetal heart (Fig. 2-2A). The image is obtained in a transverse scanning plane (cross section). Once an acceptable image is obtained, cardiac position, axis, and size are assessed. Cardiac position can be influenced by the presence of extracardiac abnormalities that displace the heart within the thorax. Examples include congenital cystic adenomatoid malformations, diaphragmatic hernia (see Fig. 2-2B), and intrathoracic pulmonary sequestration. Normal cardiac axis can be confirmed by visually drawing a line from the spine to the sternum. The interventricular septum intersects that line at an approximately 45-degree angle (see Fig. 2-2C). Axis deviation can be seen in a variety of congenital heart lesions such as Ebstein’s anomaly of the TV (see Fig. 2-2D). Normally the fetal heart occupies about one third of the thorax. If there is any doubt on visual inspection, the circumference of each can be measured and compared (the cardiac-to-thoracic ratio). A normal cardiac-to-thoracic ratio is 0.55 ± 0.05 (see Fig. 2-2E).

Figure 2-2 A, Four-chamber (4C) view of a normal fetal heart. All four chambers are visible with relative symmetry in size between the ventricles and atria. B, This 4C view is notable for abnormal cardiac position, within the right chest, secondary to a large left congenital diaphragmatic hernia. C, Normal cardiac axis. D, Ebstein’s anomaly of the tricuspid valve leading to massive cardiomegaly. E, Abnormal cardiothoracic ratio confirms visually apparent cardiomegaly.

Key Points

• The 4C view is the most informative of all views.

• It is essential to be able to obtain this view quickly and to be thoroughly familiar with the features that denote normality.

Take time to assess all components of the 4C view. This view is abnormal in at least 50% of complex congenital heart disease.

• Symmetry in size of the RA and LA and RV and LV: Late in gestation, the RV may be slightly larger than the LV.

• Determination of left versus right ventricular morphology: The RV, in its normal position, is posterior to the sternum and identified by the presence of the moderator band (Fig. 2-3A). Postnatally, the trabeculation pattern may be quite distinct between the RV and LV; prenatally, this may be less conspicuous. Notice the slightly more apical location of the tricuspid annulus (see Fig. 2-3B) in comparison with the mitral valve.

• Valve morphology and function: The valve leaflets should open fully and appear thin. Leaflets should show complete coaptation.

• Intracardiac shunts: The presence of the foramenal flap in the LA as well as the coronary sinus. Evaluation for myocardial dropout suggestive of a ventricular septal defect. Pertinent to evaluate in a short axis view perpendicular to the interventricular septum, the thin membranous septum is subject to dropout artifact when imaged solely parallel to the septum. Color Doppler may be helpful. Atrial septal defects (ASDs), in particular secundum defects, can be quite difficult, if not impossible, to detect because of the normal patency of the FO.

• Pulmonary venous return: The pulmonary veins (PVs) are often best evaluated in the 4C view (see Fig. 2-3C and D).

• Pericardial effusion (PE).

Figure 2-3 A, The right ventricle is identified by the presence of the moderator band and increased myocardial trabeculations compared with the LV. The descending aorta is seen directly behind the left atrium (LA). B, Note the slight apical offset of the tricuspid annulus in comparison with the mitral valve. C, Pulmonary veins are well seen entering the LA from the right and left lung fields. D, Color Doppler confirms two-dimensional imaging of the pulmonary veins.

The Outflow Tracts

Combined with the 4C view, visualization of both the right ventricular outflow tract (RVOT) and left ventricular outflow tract (LVOT) identifies well more than one half of all congenital heart lesions.

• Establishing the origin of the main PA from the RV and the aorta from the LV is mandatory.

• The LVOT can be obtained by rotating the transducer from the 4C view and angling very slightly toward the fetal right shoulder, similar to the transition by transthoracic imaging in a pediatric patient. Once the LVOT is opened up, rocking the transducer slightly anteriorly (and cranially) will allow visualization of the main PA arising from the RV in normally related great arteries.

• The main PA and ascending aorta should crisscross.

• In transposed great arteries, the aorta and PA are oriented parallel to each other as the PA arises from the LV and courses posteriorly before bifurcating.

• If the great arteries are not clearly seen crossing, the possibility of a conotruncal malformation, specifically transposition of the great arteries, should be investigated.

• The outflow tracts can also be evaluated in a short axis image similar to the pediatric parasternal short axis. In this image, the PA is seen wrapping around the aorta, which is visible en face.

Key Point

• Identifying two semilunar valves is only half of the task. One must identify the branching of the PAs and the brachiocephalic arteries to be confident that the great arteries are normally related.

Aortic and Ductal Arches

The aortic and ductal arches can be assessed in either the long axis (LAX) view or by sweeping cranially from the 4C view.

• In the long axis view, the aortic arch takes a tight curve as it courses from ascending to descending, often described as a “candy cane” appearance (Fig. 2-4).

• The ductal arch stretches from the anterior chest all the way back to its entry into the descending aorta, commonly referred to as having a “hockey stick” appearance (Fig. 2-5A and B).

• Alternatively, starting from the three-vessel view (described later), one can follow the course of the aorta from the rightward ascending to the leftward descending aorta. Additionally, the continuation of the main PA as the ductus arteriosus, which then joins the leftward descending aorta, is obtained.

Figure 2-4 In the long axis view, the aortic arch follows a tight curve, resembling a “candy cane.” In this image, the brachiocephalic vessels are identified along the transverse arch, confirming the great artery is the aorta.

Figure 2-5 A, In the long axis view, the ductal arch follows a distinctly different course from that of the aorta; its course is more angular as it connects the pulmonary artery (PA) with the descending aorta. B, Color Doppler evaluation of the ductal arch.

Systemic and Pulmonary Venous Return

• The entry of the superior vena cava (SVC) and IVC into the RA is documented in an image identical to that of the transthoracic bicaval view.

• In addition, the three-vessel view (Fig. 2-6A) is obtained to evaluate for the presence of a persistent left-sided SVC (see Fig. 2-6B). This view also confirms the normal relationship of the great arteries. It is obtained by sweeping cranially from the 4C view to the level of the superior mediastinum. From anterior to posterior and in decreasing size from left to right, the main PA, ascending aorta, and SVC are seen.

• The PVs are often best evaluated in the 4C view. By 2D imaging, PVs can be seen entering the LA; this can be confirmed by color Doppler.

Figure 2-6 A, The normal three-vessel view. From anterior to posterior and left to right in decreasing size are the main pulmonary artery (PA), the ascending Ao, and the right SVC. B, The three-vessel view in the presence of a persistent left SVC (LSVC). Note the presence of a fourth vessel to the left of the main PA.

Rate and Rhythm Assessment

Once a comprehensive assessment of the cardiac anatomy is complete, the HR and rhythm are documented.

• The rate and rhythm of the fetal heart are evaluated by mechanical surrogate events, specifically, the movement of the atria and ventricles or blood flow across valves.

• The HR is typically obtained using a Doppler tracing obtained with the sample volume just distal to the aortic valve (Fig. 2-7). The time from onset of flow from one beat to the next is obtained and then using the following conversion a rate in beats per minute (bpm) is calculated.

• Average HR is 140 bpm at 20 weeks’ gestation, decreasing to 130 bpm by term gestation.

• Fetal bradycardia is defined as a persistent HR of less than 80 bpm.

• It is important to differentiate between HRs out of the normal range but occurring as a physiologic response (sinus bradycardia and sinus tachycardia) from those that arise because of a disturbance in the rhythm-generating mechanism within the heart.

• Rhythm disturbances are fairly common; the majority are benign and self-limiting. Most arrhythmias will be immediately obvious on the 4C view (too fast, too slow, or irregular); however, the M-mode and PW Doppler provide additional information.

• The majority of both benign and serious arrhythmias occur in structurally normal hearts.

• Normal fetal rhythm is regular with a 1 : 1 atrial to ventricular relationship.

• Sinus rhythm is established by measuring the time from the beginning of the mitral A wave corresponding to atrial contraction to the beginning of aortic outflow. This is defined as the mechanical PR interval (Fig. 2-8) and is the mechanical equivalent of the electrocardiographic PR interval.

• A mechanical PR interval of greater than 200 m is considered prolonged.

Figure 2-7 The fetal HR is calculated by measuring the time from the onset of aortic flow in successive beats.

Figure 2-8 Normal sinus rhythm is established by documenting an atrial contraction before each aortic Doppler profile and then measuring the time from atrial contraction to aortic outflow, the mechanical PR, a surrogate for the electrocardiographic PR interval.