Examination of the fetal heart: making a diagnosis and avoiding pitfalls

Septal Defects

Defects in the atrial septum are not reliably detected in fetal life because oxygenated blood and a proportion of systemic venous return pass through the foramen flap to fill the fetal left ventricle. The contribution to left ventricular filling from pulmonary venous return is low until the third trimester. Fortunately, atrial septal defects (ASD) are relatively unimportant in the perinatal period and if there is a family history of ASD or Holt Oram syndrome has been identified, postnatal echocardiography at about 3 months of age will aid management plans. A so-called ‘primum’ ASD is entirely different and is part of the malformation known as atrioventricular septal defect (Fig. 4.2, shown right).

Ventricular septal defects (VSD) may be muscular, perimembranous or a combination of the two. Large ventricular septal defects are often recognized in the four chamber view. VSDs may be divided into those requiring surgery and those that should close spontaneously during the first few years of a child’s life. The latter are rarely associated with aneuploidy, but published reports suggest large muscular defects (Fig. 4.3, shown left) may have a 20-40% risk of extra cardiac malformation or aneuploidy. ⁶

Perimembranous defects may be isolated or where larger associated with aortic override. Isolated defects may close spontaneously during fetal life or in childhood, but those associated with override will require postnatal treatment. They are more likely to be associated with extracardiac malfor-mations or aneuploidy. The differential diagnosis comprises mal-aligned VSD (sometimes associated with coarctation of the aorta), Tetralogy of Fallot, double outlet right ventricle spectrum, common arterial trunk, pulmonary atresia with ventricular septal defect and rarer conditions, such as override of the pulmonary artery with aortic atresia (Fig.s 4-6). The complete diagnosis is made by a careful assessment of the outflow tracts, often using longitudinal views of the fetus and the three vessel view with an assessment of direction of flow in the arches using colour Doppler.

Atrioventricular septal defect (AVSD) is suspected when there is lack of offsetting of the atrioventricular valves and a ‘rounded’ shape of the heart. Its characteristic is the common atrioventricular junction. The size of septal defects is variable and the condition may be diagnosed by the presence of a common junction even when the septa are intact with no ventricular or atrial defects. ⁷ The shape of the heart is unusual because there is a shorter inlet and a longer outlet portion of the heart compared with normal. In AVSD the aorta is deviated more anteriorly away from its usual central position between the mitral and tricuspid valves - this is sometimes referred to as being ‘un-wedged’. There is characteristically a common atrioventricular junction with a common valve, ideally viewed on short axis views of the heart and there may be separated orifices (Fig. 4.2, shown above right). As with many defects, important additional malformations may co-exist, such as associated coarctation of the aorta, pulmonary stenosis or total anomalous pulmonary venous connections. The important association is with Trisomy 21, present in about 80% of AVSD diagnosed antenatally. Surgical outcome is good (5% mortality) and similar for babies with Trisomy 21. Poorer outcome is associated where there are unbalanced ventricles and significant atrioventricular valve regurgitation early in pregnancy. The defects may be very small, and there may be no ventricular component - the so called ‘primum’ atrial septal defect.

Pitfalls

VSD is often one component of a more complex malformation and the diagnostic process should not stop at the four-chamber view. Lack of off-set of the atrioventricular valves should alert the scanner to the presence of a perimembranous VSD or an AVSD. Careful assessment of the left-sided atrioventricular valve, particularly on short axis views, will help to differentiate AVSD from a perimembranous VSD. In AVSD, the usual ‘fish mouth’ mitral valve is replaced by a tri-leaflet valve, which opens towards the ventricular septum (Fig. 4.2, shown above right).

Where there is an absent AV connection (mitral or tricuspid atresia) the identity of the rudimentary chamber is usually ascertained from its position in the chest with the rudimentary right ventricle lying anterior to the main chamber and rudimentary left chamber lying in a posterior position to the main ventricle.

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