Historical Perspective

The first real-time cardiac heart images and quantitative data were published by the Lange, Sahn, and Reed group in Tucson, Arizona, in 1980.2,4,8,18 Lindsey Allan published echo/anatomical correlates in the same year,^2,4,8,18 describing systematically real-time normal and abnormal ultrasonic anatomy of the fetal heart and laying the foundation for the field of fetal echocardiography. Using ultrasonic equipment available at the time, real-time cross-sectional study and diagnosis of fetal cardiac anomalies in utero in the second trimester was possible.

Improvements in diagnostic capabilities over the past 30 years have had tremendous impact on fetal cardiac diagnosis. The use of direct Doppler interrogation of fetal intracardiac flow was first demonstrated in 1985.2,4,8,18 Use of Doppler color flow mapping in the assessment of fetal cardiac malformations and particularly in a screening situation was started shortly after.^2,4,8,18 The use of color Doppler has become indispensable in the diagnosis of more complicated cardiac malformations. By the late 1990s, the diagnostic accuracy of the nature of complex cardiac malformations in utero was as high as 95%.^2,4,8,18

Overview of Fetal Circulation and Cardiac Adaptation at Birth

The fetal myocardium has significant differences from the adult and pediatric myocardium. It is composed of a greater proportion of noncontractile elements (60% versus 30%), and fetal cardiomyocytes can divide, whereas adults’ cardiomyocytes can only hypertrophy. In addition, the removal of calcium from troponin C is slower in the fetus, resulting in slower muscle relaxation. The right ventricle handles more volume, its radius is greater, the radius-to-wall thickness is greater, and it hypertrophies to maintain appropriate wall tension. As a result, the wall thickness of the right ventricle is approximately equal to the left ventricle in fetal life.

These differences result in increased stiffness and impaired relaxation of the fetal heart, as demonstrated in the Doppler pattern across the atrioventricular (AV) valves. In the fetus, passive ventricular filling is impaired, and active atrial filling is responsible for emptying the atrium. As a result, the right ventricle is more sensitive to changes in preload and shows signs of dysfunction before the left ventricle. The result of increased preload, as seen in anemia, viral illness, and significant arteriovenous malformations (AVMs), results in fetal hydrops. There is a gradual change from the “fetal heart” to an “adult heart” that progresses throughout the neonatal period to adulthood. These changes can be easily demonstrated by echocardiography.

The fetus has a unique physiology consisting of various shunts to promote the circulation of oxygenated blood to the brain and deoxygenated blood to the placenta (see Chapter 32, Figure 32-1). The foramen ovale is designed to allow higher oxygenated blood from the placental veins preferentially to the left atrium. The increased oxygenated blood from the placenta travels through the umbilical vein to the ductus venosus. The blood then travels to the inferior vena cava (IVC) and is directed across the foramen ovale by the eustachian valve. Lower oxygenated blood flow from the fetal brain is preferentially directed from the superior vena cava (SVC) to the right ventricle and eventually across the ductus arteriosus to the placenta. The ductus arteriosus is responsible for carrying most of the cardiac output from the “pulmonary circulation” to the descending aorta and the placenta. The flow pattern is typically a predominant systolic peak with continuous low velocity diastolic flow. Continuous diastolic flow toward the descending aorta is a result of low-resistance placental circulation. Elevated diastolic flow in the ductus arteriosus is a sign of ductal constriction or lower body arteriovenous malformations. The aortic isthmus has a distinctive wave form in the fetus. The aortic isthmus is located between the left subclavian and the insertion of the ductus arteriosus. It is unique in that it straddles two different output systems (the “systemic” output of the left ventricle and the “pulmonic” output of the right ventricle that is directed toward the placenta). Flow is normally toward the placenta in both systole and diastole at the level of the aortic isthmus. Left ventricular outflow tract obstruction or significant left ventricular dysfunction results in reversal of flow toward the head in systole. Reversal of flow may also be seen in decreased upper body vascular resistance (AVMs or stressed fetus).

In addition to a full fetal echocardiogram, Doppler assessment of extracardiac structures can provide clues as to the fetus’s well-being. Assessment of the middle cerebral artery (MCA) helps assess effects of pathophysiologic states on the cerebrovascular flow. Normally, resistance in the MCA is relatively high and diastolic velocity is low. In high output disease states such as anemia, the peak velocity of the MCA is frequently elevated. In states of low cardiac output, the cerebral vascular resistance decreases in response to stress, resulting in increased diastolic velocity so as to preserve adequate blood supply to the brain, the so-called “brain sparing effect.”

A major difference in the fetal circulation compared with the postnatal circulation is the inclusion of the placental circulation. Typically, the placenta is a low-resistance circuit. The umbilical arterial flow is in part dependent on the placental resistance. The placenta has the lowest vascular resistance of any structure in the fetal circulation and therefore is the major contributor to umbilical arterial flow. The two vessels that arise from the iliac artery and travel to the placenta carry a large amount of blood. The pulsatility is low and progressively decreases during pregnancy. Reversal of diastolic flow indicates flow toward other vascular regions in the fetus where resistance is low, such as an AVM or severe elevation in placental resistance. The ductus venosus connects the umbilical vein with the inferior vena cava (IVC) as it enters the right atrium (RA). Flow is generally phasic in the IVC toward the heart. Phasic periods with absent forward flow or reversal of flow are markers of impaired relaxation of the right ventricle or right atrium secondary to decreased compliance. This can be the result of a cardiomyopathy, ductal restriction, and/or severe volume overload.

Indications for Fetal Echocardiogram

Indications for a fetal echocardiogram fall within three categories, which are listed in Table 82-1 with examples from each category.^2,4,8,16,17 However, if all high-risk fetuses meeting these conditions have a fetal echocardiogram performed, only 20% of CHD will be detected. This suggests that better screening mechanisms and indications must be determined to help facilitate bringing these fetuses and families to a center capable of advance cardiac care.

Performance of a Fetal Echocardiogram

Each fetal echocardiogram must be individualized, depending on the nature of the suspected cardiac lesion, as well as the status of the mother and fetus. At a minimum, it must involve a thorough examination of the four-chamber view, both arterial outflow tracts, three vessels and trachea view, and an assessment of pulmonary venous return. There is broad variation in anatomy in different congenital heart diseases, including situs inversus, ventricular inversion, transposition of the great arteries, heterotaxy syndromes, and a host of other complex configurations of the heart. Therefore, the examiner should confirm anatomic relationships and functional flow characteristics through a systematic analysis of the following areas:

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