Hypoplastic left heart syndrome is a group of cardiac malformations characterized by a small or absent left ventricle. Hypoplasia of the left ventricle occurs as a result of abnormalities that limit the flow of blood through the left side of the heart, including stenosis or atresia of the foramen ovale, mitral valve, or aortic valve.

Aortic stenosis or atresia most commonly results from an abnormal aortic valve, but occasionally, the obstruction occurs in the subaortic region of the left ventricular outflow tract or beyond the valve. Not all cases of aortic stenosis or atresia lead to hypoplasia of the left ventricle. In particular, in some cases of aortic stenosis or atresia, a ventricular septal defect is present, allowing enough blood to flow through the left ventricle to prevent the development of a hypoplastic left ventricle. Even in cases of aortic stenosis or atresia without a ventricular septal defect, the left ventricle may not become hypoplastic initially but may instead dilate and develop endocardial fibroelastosis, becoming progressively less contractile. Eventually, however, the dilated, globular, poorly contractile left ventricle will shrink and become hypoplastic.

In general, the prognosis for a fetus with hypoplastic left ventricle is poor. Fetuses may develop hydrops in utero and may die before birth. For infants with hypoplastic left heart born alive, survival has improved in the last few decades as a result of the development of palliative surgical interventions and cardiac transplantation.

The prognosis for aortic stenosis is related to the degree of function of the left cardiac ventricle.

A hypoplastic left ventricle can be diagnosed on a four-chamber view of the heart by demonstrating a small left ventricle (Figure 11.2.1), often with poor contractility. The size of the left ventricular chamber is variable. In some cases, no left ventricle can be identified (Figure 11.2.2). In others, the left ventricle is only somewhat smaller than the right.

With ventricular dilatation and endocardial fibroelastosis as a result of severe aortic stenosis, the left ventricle becomes enlarged and globular, with increased echogenicity along the inner wall and poor ventricular contractility (Figure 11.2.3). As pregnancy progresses, the dilated, globular left ventricle may become progressively smaller in utero until it becomes hypoplastic. The increased echogenicity of the ventricular wall will persist (Figure 11.2.4).

Aortic stenosis is characterized by narrowing of the aortic valve and decreased movement of the valve leaflets. The narrowed aortic valve can be visualized on a long-axis view of the left ventricle and left ventricular outflow tract (Figure 11.2.5). The width of the valve can be measured and compared with norms for gestational age. The stenotic valve is often brightly echogenic and visible throughout the cardiac cycle. The ascending aorta
may be enlarged due to poststenotic dilation. Color Doppler will demonstrate a narrow, high-velocity jet of flow across the stenotic valve (Figure 11.2.6), instead of the normal broad area of flow.

Mitral regurgitation commonly accompanies aortic stenosis when hypoplastic left heart is developing. With color Doppler imaging, the regurgitation appears as retrograde flow across the mitral valve during cardiac systole (Figure 11.2.6).

A hypoplastic right ventricle, which is less common than hypoplastic left heart, is characterized by a small or absent right cardiac ventricle. This most often results from pulmonic stenosis or atresia with an intact ventricular septum but can also result from stenosis or atresia of the tricuspid valve. With any of these, there is obstruction of blood flow either into the right ventricle or out of the right ventricle, leading to shunting of blood across the foramen ovale to the left side of the heart. The left ventricle may be enlarged and hypertrophic. A hypoplastic right ventricle may cause fetal cardiac failure and hydrops.

Pulmonic stenosis is characterized by an abnormal pulmonic valve that obstructs the blood flow through the right ventricular outflow tract. The stenosis may be an isolated abnormality of the heart or a component of a more complex congenital cardiac malformation such as tetralogy of Fallot. With isolated pulmonic stenosis, the right ventricle may be small or large depending on the degree of shunting of blood across the foramen ovale and the extent of tricuspid regurgitation.

A hypoplastic right ventricle is best diagnosed on a four-chamber view of the heart when the right ventricle is smaller than the left ventricle (Figure 11.3.1). The small right ventricle often has thickened walls, sometimes markedly (Figure 11.3.2), and ventricular contractility is usually poor. In rare cases, no right ventricle can be found (Figure 11.3.3). In the latter case, the distinction between hypoplastic right ventricle and hypoplastic left ventricle may be difficult.

With pulmonic stenosis, there is narrowing at the level of the pulmonic valve (Figure 11.3.4). Measurement of the pulmonic valve can be compared with norms for gestational age to assess the degree of narrowing. Poststenotic dilation of the pulmonic artery may be seen in some cases of isolated pulmonic stenosis (Figure 11.3.5). With pulmonic atresia or critical pulmonic stenosis, retrograde flow may be seen in the ductus arteriosus, carrying blood from the aorta to the pulmonary arteries. The reversed flow in the ductus arteriosus is best seen on a transverse color Doppler image of both the ductal arch and the aortic arch and is diagnosed when flow in the ductal arch is in the opposite direction to flow in the aortic arch (Figure 11.3.6). Careful assessment for accompanying cardiac anomalies is warranted, especially looking for abnormalities of the tricuspid valve and ventricular septum.

Ebstein anomaly is an anomaly that involves malformation and malposition of the tricuspid valve. The valve is displaced into the right ventricle and is dysplastic and incompetent, leading to tricuspid regurgitation and enlargement of the right atrium. During atrial systole, blood flows from the right atrium toward the apex of the right ventricle. During ventricular systole, the blood regurgitates from the portion of the right ventricle distal to the tricuspid valve, back across the dysplastic tricuspid valve into the right atrium. The right atrium can become markedly enlarged. Hydrops may develop in utero due to fetal cardiac failure.

The prognosis for this cardiac anomaly, when diagnosed prenatally, is poor, with a mortality of 35% to 40%. Some fetuses die before birth and others die in the neonatal period. The prognosis is particularly poor if hydrops develops in utero or when there is pulmonary hypoplasia as a result of compression of the lungs by the enlarged heart. Long-term survivors of Ebstein anomaly often have persistent cardiac arrhythmias.

With Ebstein anomaly, the four-chamber view of the heart is abnormal. The heart is markedly enlarged, especially the right atrium, and the tricuspid valve is displaced toward the apex of the right ventricle (Figure 11.4.1). Tricuspid regurgitation can be demonstrated with color or spectral Doppler (Figure 11.4.2).

An opening in the muscular or membranous portion of the interventricular septum is called a ventricular septal defect. These defects may be small and clinically insignificant or quite large, causing significant shunting of blood across the defect. Some of the small defects close spontaneously after birth. Defects in the membranous portion of the septum are more common than those in the muscular septum and tend to be smaller. Prenatally, blood flow through the septal defect is typically from the right to the left ventricle. After birth, shunting across the defect changes to be from left to right due to changes in pressure in the cardiac ventricles.