The echocardiographic method of diagnosing cardiac abnormalities prenatally generally follows the same rules as in postnatal life. For example, in tricuspid atresia, no patent valve is seen in the normal position between the right atrium and right ventricle. The associated right ventricular hypoplasia and ventricular septal defect are also appreciable (Figure 4.6). The connections of the great arteries in this condition must be identified because this will influence the prognosis. Figure 4.7 shows a common atrio-ventricular valve, closed during systole, in a complete atrio-ventricular septal defect. This is one of the commonest forms of heart disease seen in prenatal life (Machado etal., 1988), representing almost 20% of foetal series, in comparison with the expected rate of around 5% found in postnatal life. This type of defect is found prenatally mainly in association with trisomy 21
but also occurs with other complex cardiac anomalies and atrial isomerism. An isolated defect can occur in non-isomeric and chromosomally normal children, but it is uncommon. When the diagnosis of an atrio-ventricular septal defect with normal atrial situs is made, a foetal karyotype is recommended. Factors that influence the prognosis for corrective surgery, such as atrio-ventricular valve regurgitation or hypoplasia of either ventricle, can also be identified. In pulmonary atresia, the pulmonary root is not found in its usual position, or it is small in relation to the aorta. Pulmonary atresia with intact ventricular septum, of the form most commonly seen postnatally, is characterized by a small and hypertrophied right ventricle. However, pulmonary atresia may also be found prenatally with a dilated right ventricle and severe tricuspid incompetence. In any form of pulmonary atresia, progression of the severity of valvar obstruction has been observed as pregnancy has advanced. Thus, a stenotic pulmonary valve with high-velocity flow may become completely atretic with no forward flow. In aortic atresia, the aorta is tiny and the left ventricle small. Prenatally, the left ventricular cavity can be impossible to find, or the left ventricular cavity can be small, thick-walled, and echogenic. An example is seen in Figure 4.8. In addition to the four-chamber view abnormality, the aorta is hypoplastic from its origin to the site of entry of the duct, where it becomes larger. Flow can be seen to be reversed on pulsed Doppler or colour flow-mapping of the transverse arch. It is the most common form of CHD recognized prenatally, representing about 20% of foetal abnormalities detected, which is more than twice the expected rate of detection of this defect in infants. The high detection rate of this obvious four-chamber view abnormality indicates the success of the four-chamber view screening programme. In transposition of the great arteries, the normal positional arrangement of the arteries is lost. The aorta arises anteriorly to the pulmonary artery and parallel to it (Figure 4.9),
instead of being at right angles to the pulmonary artery at its origin in the normal foetus. The anterior vessel gives rise to the aortic arch, whereas the posterior vessel branches into the duct and pulmonary arteries. The aorta is connected to the right ventricle and the pulmonary artery to the left ventricle. Transposition is increasingly recognized during routine scanning as the ultrasonographer becomes more skilled at great artery evaluation in addition to four-chamber imaging. Aortic override can be recognized prenatally by the anterior displacement of the aorta and the presence of an outlet ventricular septal defect. The possible diagnoses, when aortic override is identified, of Fallot's tetralogy and truncus arteriosus, are differentiated by examination of the pulmonary outflow tract. In Figure 4.10, a great artery is shown arising astride the ventricular septum; just cranial to this view, the main pulmonary artery is seen to be smaller than the aorta. The size and confluence of the pulmonary arteries can be examined in order to predict prognosis in tetraology, as can the morphological features and competence of a truncal valve. In prenatal life, tetralogy of Fallot is frequently associated with extracardiac, particularly chromosomal, anomalies, much more than is anticipated from postnatal practice. In double-outlet right ventricle, both great arteries can be seen arising from the right ventricle anterior to the ventricular septum. An example is illustrated in Figure 4.11. In this case, the anterior artery proved to be the aorta. The small size of the aorta relative to the pulmonary artery raises the suspicion of coarctation as an additional lesion.
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