●   UNIVENTRICULAR ATRIOVENTRICULAR CONNECTION

Univentricular atrioventricular connection describes a group of cardiac malformations where the atrioventricular connection is completely or predominantly to a single ventricular chamber. Embryologically, this malformation is thought to result from failure of the development of the bulboventricular loop stage. Much debate still exists today on the various subclassifications of cardiac anomalies within this group and what should be included or excluded (1–4). From a clinical point of view, a congenital heart defect with a univentricular atrioventricular connection, single ventricular physiology, describes a heart with one functioning ventricle with inflow from one or both atria. Numerous terms were used to describe this malformation, including univentricular heart, primitive ventricle, common ventricle, single ventricle, cor triloculare biatriatum, cor biloculare, dominant ventricle, and double inlet ventricle (DIV) (3). The classic Van Praagh classification (5), which was later modified by Hallermann et al. (6), described one or two atrioventricular valves that empty into a single ventricle and excluded mitral or tricuspid atresia (TA). Anderson’s simpler classification described a single ventricular mass with or without a rudimentary chamber and allowed for the inclusion of mitral or TA (7, 8). In Anderson’s classification, the rudimentary chamber, if present, should not have an inlet but may have an outlet (7, 8). Within univentricular atrioventricular connection, three subgroups can be identified: double inlet, where two atria connect to a single ventricle through two patent atrioventricular valves; single inlet, where one atrium connects to a single ventricle through a single atrioventricular valve; and common inlet, where both atria connect to a single ventricle through a single atrioventricular valve (1). The morphology of the ventricle is generally a left ventricular morphology with a rudimentary right chamber. On rare occasions, a right ventricular morphology with a rudimentary left chamber, or a ventricle of indeterminate morphology without a rudimentary chamber, can be seen. A single ventricle heart, which results from a surgical repair of a congenital heart anomaly, should not be classified as univentricular atrioventricular connection. Table 19.1 lists several cardiac anomalies that may show a single ventricle on fetal echocardiography. Of those, DIV and TA with ventricular septal defect (VSD) have been commonly classified in the univentricular atrioventricular connection and will be discussed in this chapter. Figure 19.1 represents four-chamber views in fetuses with different cardiac defects and a single ventricle anatomy.

●   DOUBLE INLET VENTRICLE

Definition, Spectrum of Disease, and Incidence

DIV is considered a classic and most common form of univentricular atrioventricular connection (1). It is characterized by two normally developed right and left atria that connect via separate right and left atrioventricular valves to a common ventricle (Fig. 19.2). The most common form of DIV is a double inlet to a morphologic left ventricle, representing about 80%, and the anomaly is also called double inlet left ventricle (DILV) (5). In DILV, a small underdeveloped right ventricle (not shown in Fig. 19.2) is commonly present and connects to the single ventricle with a VSD. This “remnant” ventricle is a small outlet chamber and the septal defect is usually called bulboventricular foramen. The aorta and pulmonary arteries usually arise in D- or L-malposition, and depending on the looping, one or both vessels (double outlet) may commonly arise from the small outlet chamber. In cases where the bulboventricular foramen (septal defect) is restrictive, the corresponding arising vessel(s) from the remnant chamber may be diminutive (pulmonary stenosis or aortic coarctation). Other forms of DIV include a double inlet right ventricle, a DIV of mixed morphology, and a DIV of undetermined or undifferentiated morphology (5). DIV is rare and is found in 0.1 per 1,000 live births (9). The prevalence is more common in fetal series due to the easy detection of DIV on the four-chamber view of the heart.

Ultrasound Findings

Gray Scale

The four-chamber view is abnormal in DIV as it shows a single ventricle with a missing ventricular septum (Fig. 19.3). Identifying the morphology of the single ventricle on ultrasound is based on the anatomic characteristic of the morphologic right and left ventricles as discussed in Chapter 5. The left ventricular myocardium appears smooth with fine trabeculations, whereas the right ventricular myocardium is coarse with an irregular surface. Assessment of atrioventricular valve anatomy and/or insertion of papillary muscles cannot be used to determine ventricular morphology in univentricular atrioventricular connection. Occasionally, the rudimentary right ventricle is seen in the four-chamber plane (Fig. 19.4) but in most cases the septal defect (bulboventricular foramen) and the rudimentary right ventricle in DILV are often not visualized in the four-chamber plane but in a more cranial plane, when an attempt to visualize the great vessels is made (Fig. 19.5). The rudimentary outlet chamber in DILV is more commonly located on the left side of the main ventricle (L-looping) but can be located on the right side (D-looping) (2). The great arteries are generally in L-malposition if the small outlet chamber is on the left side of the ventricle. When the small outlet chamber is localized on the right side, the great arteries arise either in D-malposition or are normally related with the pulmonary artery arising from the small outlet chamber (2). Outflow tract obstructions are recognized due to size discrepancy rather than flow disturbances, which may be absent. A narrow pulmonary artery suggests the presence of pulmonary stenosis or atresia, whereas a narrow ascending aorta may be associated with coarctation of the aorta or tubular aortic arch hypoplasia.

Color Doppler

Color Doppler may be misleading since two atrioventricular valves are patent and two color stripes are visualized, thus mimicking the virtual presence of a separation or septum (10) (Figs. 19.3 and 19.6). Diagnosis is typically made on grayscale ultrasound, and color Doppler provides additional information on the patency of the left and right atrioventricular valves, flow across the VSD, and great vessels (Fig. 19.5), especially to detect stenosis or atresia (Fig. 19.7). Restrictive VSD, which may occur in this condition, is better evaluated using color Doppler.

Early Gestation

DIV can be detected in early gestation (Figs. 19.6 and 19.7) by detecting the absence of a ventricular septum on the four-chamber view as well as abnormally arising great vessels.

Three-Dimensional Ultrasound

The combination of three-dimensional (3D) ultrasound with tomographic imaging permits the simultaneous visualization of the abnormality in the four-chamber plane and the demonstration of the rudimentary ventricle with the course of the great vessels. Navigating through the volume in an offline setting may facilitate the evaluation of the spatial orientation of the great arteries. Surface rendering shows the large ventricle with inflow from two atrioventricular valves and a rudimentary outlet chamber (Fig. 19.8) and may help in identifying the spatial relationship of the great vessels.

Associated Cardiac and Extracardiac Findings

Associated malformations in DIV are atresia, hypoplasia or straddling of the atrioventricular valves, pulmonary (or subpulmonic) outflow obstruction, (sub)aortic outflow obstruction, and conduction abnormalities, primarily due to the anatomic disruption of the conduction system (1).

The most important extracardiac abnormality to rule out is the presence of right or left isomerism (see Chapter 30), especially in the presence of a common inlet ventricle (11). The sequential approach to the ultrasound examination of the heart may permit detection of corresponding abnormalities. Chromosome anomalies and other extracardiac anomalies than isomerism are possible but rather unusual.

Differential Diagnosis

Table 19.1 lists several cardiac malformations in the differential diagnosis of DIV. DIV may be missed on prenatal ultrasound in a lateral view of the heart in diastole because the papillary muscles may mimic a ventricular septum in a single ventricle.

Prognosis and Outcome

DIV with patent atrioventricular valves is well tolerated in the fetus. Follow-up ultrasound is important prenatally as outflow tract obstruction may develop or worsen due to reduced flow and lack of vessel growth. The neonatal course of DIV is dependent on the presence of associated malformations, such as obstruction of the great vessels or atrioventricular valve abnormalities. Surgical treatment corresponds to a single ventricular repair. The type of surgical repair (pulmonary artery banding, Fontan procedure, or other) mainly depends on detailed evaluation of the great vessel arrangement and perfusion.

An overall mortality rate of 29% with follow-up up to 25 years of age was noted in an outcome study on 105 patients with DILV and transposed arteries (12). Multivariate analysis showed the presence of arrhythmia and pacemaker requirement as independent risk factors for mortality, whereas pulmonary atresia or stenosis and pulmonary artery banding were associated with decreased mortality (12). Gender, era of birth, aortic arch anomaly, and systemic outflow obstruction were not risk factors for long-term outcome (12). Similar findings were reported on eight fetuses with DILV with L-transposition of the great vessels (13). Of these, four fetuses (50%) had pulmonary atresia, one fetus (12.5%) also had TA and coarctation of the aorta (died), and one fetus had complete heart block and long QT syndrome (died) (13). Overall good outcome was noted in six (75%) infants (13). The outcome of fetuses with DIV is generally good in the absence of associated rhythm abnormalities.

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