Pulmonary atresia with ventricular septal defect

A subaortic malaligned ventricular septal defect, aortic valve overriding the defect, infundibular pulmonic stenosis and hypertrophy of the right ventricle; hypertrophy of the right ventricle only develops after birth.

Tetralogy of Fallot is the most common cyanotic heart defect accounting for 5-7 % of structural heart defects.

The essential features of this malformation are: 1. malaligment ventricular septal defect with anterior displacement of the infundibular septum associated with subpulmonary narrowing and overriding aortic root; 2. Demonstrable continuity between the right outflow tract and the pulmonary trunk. In about 20% of cases¹, this continuity is lacking leading to atresia of the pulmonary valve, a condition that is commonly referred to as pulmonary atresia with ventricular septal defect. Hypertrophy of the right ventricle, one of the classic elements of the tetrad, is always absent in the fetus, and only develops after birth. Stenosis of the outflow tract of the right ventricle diverts the blood flow from the pulmonary artery to the ascending aorta, which is usually enlarged. With tight pulmonic stenosis or atresia, pulmonary circulation is supplied via reverse flow in the ductus arteriosus. Tetralogy of Fallot does not cause cardiocirculatory compromise during fetal life, with the exception of cases with absent pulmonary valve, where regurgitation from the massively dilated pulmonary artery into the right ventricle.

Pulmonary atresia rather than pulmonary stenosis can be present in tetralogy of Fallot. With pulmonary atresia, the infant will be ductal dependent and/or have systemic to pulmonary artery collaterals. An even more serious associated anomaly is absent pulmonary valve, which is found in 3-4%. Not only is the fetus at risk for fetal hydrops but postnatally there is usually tracheobronchial malacia from in utero compression by large pulmonary arteries.^{1, 2} Other associated anomalies are atrial septal defects, atrioventricular septal defect and a right aortic arch. Tetralogy of Fallot may be associated with trisomy 18, trisomy 21, microdeletion 22q11, and extracardiac malformations including the VACTERL syndromes, gastrointestinal abnormalities such as omphalocele and diaphragmatic hernia, central nervous system abnormalities and skeletal syndromes. In one prenatal series other abnormalities, chromosomal and extracardiac, were found in 60% of fetuses with tetralogy of Fallot.³ In one clinical study, 7 of 33 infants with tetralogy of Fallot were found to have 22q11 microdeletion, but only 4 had other clinical features associated with 22q11 deletion syndrome.⁴

Tetralogy of Fallot is frequently not detected on prenatal ultrasound if only the four chamber view of the heart is obtained.⁵ The ventricular septal defect in tetralogy of Fallot is not usually visualized in the four-chamber view but rather in the long-axis view of the left ventricular outflow tract. A useful sign that alerts to the presence of this condition is an increased rotation of the cardiac axis, that tends to be significantly increased than normal. Echocardiographic diagnosis of tetralogy of Fallot relies upon the demonstration of a ventricular septal defect in the outlet portion of the septum and an overriding aorta.^{3, 6} There is an inverse relationship between the size of the ascending aorta and pulmonary artery, with a disproportion that is often striking. A large aortic root is indeed an important diagnostic clue.⁷ Doppler studies provide valuable information. The finding of increased peak velocities in the pulmonary artery corroborates the diagnosis of Tetralogy of Fallot by suggesting obstruction to blood flow in the right outflow tract. Conversely, demonstration with color and/or pulsed Doppler that in the pulmonary artery there is either no forward flow or reverse flow allows a diagnosis of pulmonary atresia.⁸ Diagnostic problems arise at the extremes of the spectrum of tetralogy of Fallot. In cases with minor forms of right outflow obstruction and aortic overriding differentiation from a simple ventricular septal defect can be difficult. Indeed, in one of the largest antenatal series of tetralogy of Fallot, in a handful of cases only the ventricular septal defect could be demonstrated at first and aortic overriding and pulmonary stenosis only appeared late in gestation. Conversely, in those cases in which the pulmonary artery is not imaged, a differential diagnosis between pulmonary atresia with ventricular septal defect and truncus arteriosus communis is similarly difficult.

The sonographer should also be alerted to a frequent artifact that resembles overriding of the aorta. Incorrect orientation of the transducer may demonstrate apparent septo-aortic discontinuity in a normal fetus. The mechanism of the artifact is probably related to the angle of incidence of the sound beam. Careful visualization of the left outflow tract with different insonation angles, as well as the use of color Doppler and the research of the other elements of the Tetralogy are useful in these cases. The laterality of the aortic arch is important, as well as the presence and size of the thymus. Both absence/hypoplasia of the thymus^9-11 and right aortic arch^12,13 are associated with a much increased risk of microdeletion 22q11. Atrioventricular connections need to be carefully assessed to rule out the possible association with atrio-ventricular septal defects. Such combination is associated with an increased risk of concomitant autosomal trisomies, Down syndrome in particular,and results per se in a worse prognosis. Abnormal enlargement of the right ventricle, main pulmonary trunk and artery, suggests absence of pulmonary valve. Evaluation of other variables, such as multiple ventricular septal defects and coronary anomalies, would be valuable for a better prediction of surgical timing and operative prognosis. Unfortunately, these findings cannot be recognized for sure by prenatal echocardiography at present. The right ventricular hypertrophy is not evident prenatally.

Search for associated anomalies including fetal karyotype and FISH study for chromosome 22q11 deletion. Isolated tetralogy of Fallot does not alter standard obstetric management. Absence of the pulmonary valve may however cause hydrops and polyhydramnios, either as a manifestation of heart failure or as the consequence of obstruction to fetal swallowing by the very enlarged pulmonary arteries. Anticipation of delivery when fetal maturity is achieved may be an option in these cases.

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