A Web exclusive by Drs. Benoit and Copel, and only available online in conjunction with the authors' article, "Antenatal screening with fetal echocardiography: when and how," in our September 2003 issue.
The goal of routine screening of the fetal heart is establishing that cardiac structure and function meet certain criteria (Figure 1). Attention first is focused on confirming that the heart lies on the fetuss left side and on the same side as the stomach. Completing the four-chamber view allows for assessment of cardiac position, size, axis, chamber structure, septal integrity, and ventriculoarterial connections. The aortic and ductal arches also are assessed.
Both operator skill and image quality affect the accuracy of comprehensive cardiac assessment. Optimal fetal cardiac assessment with ultrasound (U/S) provides a high level of detection of serious cardiac malformations.1 Even when optimal, however, fetal cardiac scanning has limitations, particularly in detecting small septal defects. In addition, certain cardiac abnormalities may develop later in pregnancy, including pulmonary and aortic stenosis, cardiomyopathy, and cardiac tumors.2 Thus is remains important for each examiner to develop a routine that fosters complete assessment of the fetal cardiac structure and function, in standard fashion, with each exam (Figure 2).
Evaluation of fetal situs, the first step in assessing the fetal heart, requires knowledge of fetal position. Situs solitus is characterized by the location of both the heart and stomach on the left side of the fetus.
Most of the fetal heart should be located in the anterior half of the fetal chest. With a transverse view of the fetal chest, the most posterior aspect of the heart should lie in the right hemithorax, midway between the anterior and posterior sides of the fetus. For the most part, both ventricles are situated in the left anterior quadrant of the chest, with the exception of a small portion of the right ventricle. The atria may be partially located in the right thorax.
Cardiac size, also an important component of the exam, can be evaluated with the four-chamber view. The heart area should be approximately one third of the total chest area at the level of the four-chamber view.3
The cardiac axis refers to the position of the fetal heart in the chest, and can be determined by measuring the angle formed between the interventricular septum and a line from the spine to the anterior chest wall that divides the fetal thorax in half. The fetal heart typically lies at a 45-degree angle from the midline, and 95% of normal hearts are between 30 and 60 degrees of axis.
The four-chamber assessment includes apical, basal, and long-axis views (Table 1 and Figure 3). The apical view, with the U/S beam parallel to the interventricular and interatrial septa, allows evaluation of chamber size and of the atrioventricular valves. The long-axis view, with the U/S beam perpendicular to the septa, allows for evaluation of septal integrity.
Heart in the left chest
Atria equal size
Ventricles equal size
Heart 1/3 of chest area
Left atrium posterior
Foramen ovale flap in left atrium
Apical offset in tricuspid valve
Intact interventricular septum
Moderator band in right ventricle
Axis 45-60 degrees
The right and left atria and ventricles of the fetal heart typically appear nearly equal in size, although the right ventricle may become larger than the left near term. The ventricular walls also should appear nearly equal in thickness. The moderator band may be seen in the right ventricle (trabeculated appearance), whereas the left ventricular wall appears smooth. The foramen ovale may be visualized in the left atrium, along with the pulmonary veins. The atrioventricular valves may be demonstrated, with the septal leaflet of the tricuspid valve inserting more apical than the mitral valve. Cardiac function is assessed by observing the contractility of the heart and valvular motions. In the long-axis view, the interventricular septum should be seen as continuous from the apex to the atrioventricular valves.
Transverse and longitudinal views of the ventriculoarterial outflow tracts are performed. The posterior leaflet of the aortic valve should be seen in continuity with the anterior leaflet of the mitral valve (Figure 3). The anterior wall of the aorta also should be continuous with the ventricular septum and the superior vena cava should be visible. Most importantly, the great artery tracts should cross each other and the vessels should be of normal size and not show regurgitation on color-flow Doppler (Figure 3). To complete the examination, visualize the aortic and ductal arches in transverse or longitudinal views (Figure 4].
1. Achiron R, Glaser J, Gelernter I, et al. Extended fetal echocardiographic examination for detecting cardiac malformations in low risk pregnancies. BMJ. 1992;304:671-674.
2. Yagel S, Weissman A, Rotstein Z, et al. Congenital heart defects: natural course and in utero development. Circulation. 1997;96:550-555.
3. Paladini D, Chita SK, Allan LD. Prenatal measurement of cardiothoracic ratio in evaluation of heart disease. Arch Dis Child. 1990;65:20-23.
Richard Benoit. How to perform basic antenatal screening with fetal echocardiography.
Sep. 1, 2003;48.