Chapter 26

Abnormal Fetal Echocardiography

Sandra Hagen-Ansert

Objectives

• Describe the echocardiographic protocol for evaluating the fetal heart.

• List the various septal defects and their sonographic appearances.

• Differentiate between a complete and a partial atrioventricular septal defect.

• Name the echocardiographic findings in the tetralogy of Fallot.

• List the echocardiographic findings in a fetus with critical aortic stenosis.

• Describe the sonographic characteristics of hypoplastic left heart syndrome.

• Identify the risk factors associated with congenital heart disease.

• Describe how the sonographer may identify transposition of the great arteries.

CLINICAL SCENARIO

A 33-year-old patient who is pregnant with her third child is seen for her first visit with the obstetric service at 24 weeks. Her first child died shortly after birth of hypoplastic left heart syndrome (HLHS). Her second child is 4 years old and is undergoing evaluation for a systolic ejection murmur. The mother has a history of a bicuspid aortic valve and underwent a balloon valvulotomy nearly 20 years ago. Because of this high risk for congenital heart disease, she was referred to pediatric cardiology for further fetal echocardiographic evaluation.

A complete fetal echocardiogram was performed. The aortic valve was thickened with reduced cusp excursion. The ascending aorta measured 2.1 mm, and the pulmonary artery measured 4.1 mm. The left ventricle was thicker and smaller than the right ventricle, measuring 6.7 mm compared with the right ventricular measurement of 8.0 mm. The velocity taken at the root of the aorta was increased at 2.5 m/s. What conclusions may be drawn from the fetal echocardiogram? What is the risk of congenital heart disease in this fetus?

The evaluation of the fetal heart has become a routine addition to the perinatal sonographic evaluation. This chapter provides a guideline for the sonographer to become better acquainted with the cardiologist’s view of the fetal heart. The fetal heart protocol is presented, followed by a brief introduction to some common congenital heart defects (i.e., atrial septal defect [ASD], ventricular septal defect [VSD], atrioventricular septal defect [AVSD], tetralogy of Fallot [TOF], transposition of the great arteries [TGA], and HLHS) the sonographer may encounter during the routine fetal sonographic evaluation. In general practice, once a fetal heart abnormality has been observed, the patient is referred to a specialty center with a pediatric cardiologist experienced in congenital heart disease to provide ongoing care, counseling, and surgical options for the patient.

Incidence of Congenital Heart Disease in the Fetus

Congenital heart disease in the fetus represents the most common major congenital malformation with an incidence of 8:1000 live births. This figure encompasses all types of heart disease (e.g., tiny secundum ASD, mild valvular pulmonary stenosis, patent ductus arteriosus, and small muscular or membranous VSDs).

Risk Factors Indicating Fetal Echocardiography

Specific risk factors indicate the fetus is at a higher than normal risk for congenital heart disease to warrant a structured fetal echocardiography evaluation. These risk factors may be divided into the following three categories: fetal, maternal, and familial risk factors.

Sonographic Evaluation of the Fetal Heart

Fetal Cardiac Rhythm

The fetal heart undergoes multiple changes during the embryologic stages. One of these stages is the progression of the cardiac electrical system, which matures at the end of fetal life to cause a normal sinus rhythm in the cardiac cycle. The pacemaker of the heart is the sinoatrial node, which fires 60 to 100 electrical impulses each minute. From the sinoatrial node, the electrical activity travels to the atrioventricular node, across the bundle of His to the ventricles, and down the right and left bundle branches, before being distributed to the rest of the cardiac muscle. This electrical activity precedes the ventricular contraction of the heart.

During this developmental stage, deceleration of the normal fetal heart rate from 150 beats per minute to a bradycardia stage (<55 beats per minute), or even a pause of a few seconds, is common during the course of a fetal echocardiogram. This deceleration may occur if the fetus is lying on the umbilical cord or if the transducer pressure is too great. The fetus should be given a recovery time to bring the heart rate to a normal sinus rhythm, which is usually done by changing the position of the mother (i.e., rolling to her left side to release pressure from inferior vena cava compression) or by releasing the pressure from the transducer.

Normally, the fetal heart should have a baseline rate of 110 to 160 beats per minute. Some variability may be seen in rhythm and acceleration. Abnormal rate and rhythm may lead to fetal asphyxia, and early recognition of “normal” versus “abnormal” is critical. Short episodes of sinus bradycardia may be common in early pregnancy. However, if the bradycardia (<100 beats per minute) continues for several minutes, it is abnormal. Likewise, sinus tachycardia (>160 beats per minute) is common in later stages of pregnancy and may be associated with fetal movement. This rate becomes abnormal when it exceeds 200 beats per minute or has frequent “dropped” beats (premature ventricular contractions).

The sonographer should evaluate the fetus for normal cardiac structure and signs of cardiac failure (enlarged right ventricle, hydrops, ascites, edema, pleural effusion, or pericardial effusion) and perform a simultaneous M-mode study with the transducer perpendicular through the atrial and ventricular cavities. Every ventricular contraction should be preceded by an atrial contraction. Alterations in the rhythm pattern seen during fetal development may result from premature atrial contractions and premature ventricular contractions, supraventricular tachycardia, tachycardia, or atrioventricular block.

Fetal Echocardiogram

Ideally, a clear redundant view of the cardiac anatomy occurs when the fetus is at least 20 to 22 weeks of gestational age. Although reports in the literature have demonstrated some congenital heart defects with transvaginal sonography in the first trimester,¹ it is difficult to image all of the cardiac anatomy clearly. In addition, some cardiac lesions may not show changes until much later in fetal development (e.g., HLHS).

A fetal echocardiogram always begins with the determination of fetal lie, fetal number, activity level, location and size of the placenta, evaluation of the umbilical cord, gestational age, and fluid assessment. The visceroatrial situs should be determined. A transverse view of the abdomen reveals the aorta anterior and to the left of the spine (closest to the left atrium); the inferior vena cava is anterior and to the right with drainage into the right atrium (Fig. 26-1; see Color Plate 33). The stomach should be anterior and leftward.

FIGURE 26-1 Fetal situs is normal. Heart position is in the left chest, and the apex of the heart points to the left; stomach *(ST)* is to the left; aorta is anterior and to the left of the spine; and inferior vena cava is anterior and to the right of the spine. (See Color Plate 33.)

Four-Chamber View

The four-chamber view provides the groundwork for the cardiac examination. The normal position of the fetal heart is in the left thorax with the apex directed about 45 degrees to the left (Fig. 26-2). The size of the heart should be less than one-third of the area of the thorax, and the heart circumference should be less than half of the thoracic circumference. Four chambers of the heart should be clearly present with the right-sided chambers slightly larger than the left-sided chambers (Fig. 26-3).

Venous Drainage

The venous drainage of the heart is through the coronary sinus, which may be seen coursing from left to right just superior to the mitral groove. Enlargement of the coronary sinus may prompt evaluation for an abnormality known as left-sided superior vena cava. Four pulmonary veins drain into the left atrium (two upper and two lower). These veins deliver oxygenated blood flow from the lungs to the heart after birth. Usually at least two or three of these veins may be identified as they drain into the left atrium (Fig. 26-4). Color Doppler imaging may aid in their visualization.

FIGURE 26-4 **(A),** Fetal circulation. **(B),** Three of the four pulmonary veins *(PV)* enter the left atrial cavity.

Atrial and Ventricular Septa

The four chambers should be divided by the interatrial septum and interventricular septum to separate the right from the left side. In the fetus, a communication exists between the right and left atrial cavities known as the foramen ovale. This is found in the center of the atrial septum, and the “flap” of the foramen moves slightly during the cardiac cycle with greater movement toward the left atrium. The sonographer should be able to identify atrial septal tissue superior and inferior to this foramen ovale flap. The ventricular septum is thicker than the atrial septum. This septum has two anatomic descriptors, the membranous (thin) and muscular (thicker) septa. The ventricular septum is approximately the same thickness as the posterior myocardial wall of the left ventricle. Septal defects may occur anywhere along the ventricular septum with membranous defects slightly more visible than muscular defects.

The right and the left ventricles should extend to the cardiac apex, and both ventricles should squeeze simultaneously during systole (Fig. 26-5). There is a bright linear structure near the apex of the trabeculated right ventricle, which is the moderator band. The blood leaves the right ventricle through the pulmonary artery via the right and left pulmonary arteries to their respective lung. The left ventricle is smooth-walled compared with the right ventricle. The aorta delivers blood from the left ventricle to the rest of the body.

FIGURE 26-5 **(A),** Left ventricle is measured at the level of the mitral anulus. **(B),** M-mode measurements may also be made at the level of the anulus for the right ventricle and left ventricle.

Atrioventricular Valves

The two atrioventricular valves separate the filling chambers (atria) from the pumping chambers (ventricles). The septal leaflet of the tricuspid valve is slightly more apical than the left-sided mitral valve leaflets. These leaflets should be thin, pliable structures that open fully with ventricular diastole and close completely during systole. Doppler evaluation of the atrioventricular valves shows a biphasic motion as the valve opens in diastole and closes in systole (Fig. 26-6).

FIGURE 26-6 Normal Doppler flow patterns in the four-chamber view of the mitral **(A)** and tricuspid **(B)** leaflets. The smaller first peak is the e wave; the second higher peak is the a wave.

Semilunar Valves and Great Arteries

The pulmonary anulus should be at least the same size, if not slightly larger, as the aortic anulus (Fig. 26-7; see Color Plate 34). The long-axis view is best to image the base of the aorta, ascending aorta, and sometimes the arch. The normal aorta should be seen in the “center” of the heart, arising from the base of the left ventricle. Continuity between the anterior wall of the aorta with the interventricular septum is normal; continuity between the posterior wall of the aorta and anterior leaflet of the mitral valve should be present. The pulmonary artery is anterior and to the right of the aorta as it arises from the right ventricular cavity. The main pulmonary artery bifurcates into right and left pulmonary artery branches that drain into the lungs. The semilunar valves should be thin, lunar-shaped cusps that open fully in systole.

FIGURE 26-7 **(A),** Four-chamber view. **(B),** Aortic outflow is shown in red in this four-chamber view. (See Color Plate 34.) **(C),** Long-axis view of the left ventricular outflow tract shows the crescent left ventricle *(LV)*, the interventricular septum and its continuity with the anterior wall of the aorta *(Ao)*, and the posterior wall of the aorta with its continuity with the anterior leaflet of the mitral valve. The fetal echocardiogram was imaged at early diastole, with the mitral valve open fully and aortic valve closed. **(D),** High, short-axis view of the great vessels. The right ventricular outflow tract wraps around anterior to the aorta. The pulmonary artery arises from the right ventricle and bifurcates into right and left branches. The ductal insertion occurs midway between the bifurcation of the pulmonary vessels. The great vessel diameters may be measured at the level of the cusps. *Desc Ao,* Descending aorta; *LA,* left atrium; *LPA,* left pulmonary artery; *LV,* left ventricle; *PV,* pulmonary vein; *RA,* right atrium; *RPA,* right pulmonary artery; *TV,* tricuspid valve.

Ductal Arch

The ductal arch may be seen best on the high short-axis view or sagittal view (Fig. 26-8). This short-axis view demonstrates the trifurcation of the main pulmonary artery into the right pulmonary artery (which wraps around the aorta in the short-axis view), the left pulmonary artery, and the large ductus arteriosus. The sagittal view shows the ductus resembling a “hockey stick” as it drains into the descending aorta.

Congenital Heart Anomalies

The most common type of congenital heart disease is VSD, followed by ASDs and pulmonary stenosis. Environmental factors may influence the development of congenital heart disease. Chromosomal abnormalities also have a higher association with congenital heart disease (e.g., fetuses with trisomy 21 have a 50% incidence rate of congenital heart disease, specifically AVSDs).

Atrial Septal Defects

ASDs are usually not recognized during fetal life, unless a large part of the atrial septum is missing. The embryologic development of the atrial septum evolves in several stages; interruption of any of these stages could lead to the formation of an ASD. A natural communication exists between the right and left atria during fetal life. This communication (foramen ovale) is covered by the flap of the foramen ovale that remains open in the fetal heart until after birth, when the pressures from the left heart become greater than the pressures in the right heart to force the foramen to close completely. After birth, a remnant of the foramen ovale, now closed, is a thin depression known as fossa ovalis. Failure of the foramen to close may result in one type of ASD, the secundum defect.

The area of the foramen ovale is thinner than in the surrounding atrial tissue; it is prone to signal dropout during sonographic evaluation, particularly in the apical four-chamber view when the transducer is parallel to the septum. Any break in the atrial septum in this view must be confirmed with the short-axis or perpendicular four-chamber view (subcostal view) in which the septum is more perpendicular to the transducer. Because of beam-width artifact, the edges of the defect may be slightly blunted and appear brighter than the remaining septum.

In utero, the natural flow is right to left across the foramen because the pressures are slightly higher on the right. A small reversal of flow may be present. The flap of the foramen should open into the left atrial cavity. The flap should not be so large as to touch the lateral wall of the atrium; when this redundancy of the foramen occurs, the sinoatrial node may become agitated in the right atrium and cause fetal arrhythmias. The sonographer should sweep inferior to superior along the atrial septum to identify the three parts of the septum: the primum septum (near the center or crux of the heart), the foramen ovale (middle of the atrial septum), and the septum secundum (base of the heart, near the right upper pulmonary vein and superior vena cava).

Secundum Atrial Septal Defect

Secundum ASD is the most common atrial defect and occurs in the area of the foramen ovale (Fig. 26-9; see Color Plate 35). Usually an absence of the foramen ovale flap is noted, with the foramen ovale opening larger than normal. The size of the normal foramen should measure at least 60% of the aortic diameter (i.e., if the aorta measures 4mm, the foramen should be at least 2.4mm).