Hypoplastic right ventricle (HRV) is also known as pulmonary atresia with intact ventricular septum (PA:IVS). The normally formed right ventricle becomes hypoplastic, in association with pulmonary atresia, and occasionally tricuspid atresia. The competence of the tricuspid valve determines the size of the right ventricle, with type I HRV being associated with a competent valve and small ventricle and type II HRV being associated with an incompetent valve and normal or large right ventricle (Romero et al., 1988). Another classification system is based on the degree of severity of right ventricular hypoplasia (Alwi, 2006). Group A patients have only mild right ventricular hypoplasia, with a well-developed infundibulum, beyond which the pulmonary arterial outflow tract is atretic. Group C patients have severe right ventricular hypoplasia, with an atretic infundibulum. Group B patients have a moderately HRV, but may have a reasonably well-developed infundibulum. Patients in Group C tend to have poor prognosis with long-term outcome being dependent on a single-ventricle repair.

In the setting of right ventricular hypoplasia, blood flows from the right to the left atria through the foramen ovale, and the left ventricle supplies both systemic and pulmonary circulations, the latter by retrograde flow through the ductus arteriosus. Additional cardiac malformations that may coexist include atrial and ventricular septal defects and transposition of the great vessels. Additionally, coronary arterial anatomy is frequently abnormal, and there may be varying degrees of tricuspid regurgitation or Ebstein malformation (see Chapter 46) (Daubney et al., 2002).

HRV is extremely rare, accounting for less than 3% of all cases of congenital heart disease diagnosed during the first year of life (Fyler et al., 1980). The overall population incidence is likely to be 1 to 2 per 10,000 livebirths (Hoffman and Kaplan, 2002). In another series, the livebirth prevalence was estimated at 4.1 per 100,000 livebirths (Daubney et al., 1998).

Prenatal diagnosis of HRV is relatively straightforward if there is a significant disproportion in size between the two ventricular cavities (Figure 47-1) (McGahan et al., 1991). However, if the right ventricle is normal in size (due to an incompetent tricuspid valve) prenatal diagnosis may be extremely difficult and may depend on identification of isolated pulmonary atresia (Romero et al., 1988). While the right ventricular cavity is usually very small, the right ventricular wall is often hypertrophied (Grundy et al., 1987). Although adequate visualization of the right ventricular outflow tract may be difficult, Doppler echocardiography may demonstrate absence of flow across the pulmonary valve (Figure 47-2). As with hypoplastic left heart syndrome (see Chapter 48), the diagnosis of HRV may not be clear at an initial second trimester ultrasound examination, but instead may evolve over time so that true hypoplasia may not be visible until late in the third trimester (Hornberger et al., 1996). At fetal echocardiography, detailed evaluation of tricuspid valve annulus size, ratio of right to left ventricular size, and presence or absence of tricuspid regurgitation are essential.