What does an increased atrial‐to‐ventricular length ratio mean in fetuses with atrioventricular septal defect?

Although atrioventricular septal defect (AVSD) is named as such, the septal defect is in fact a variable component of this interesting congenital malformation. Depending on the relationship of the bridging atrioventricular valve leaflets to the defective atrial and ventricular septal edges, the shunt through the septal defect can be both interatrial and interventricular, exclusively interatrial (‘primum defect’), or, very rarely, exclusively interventricular1. When the atrioventricular valves are removed, however, those hearts categorized as having AVSDs look alike regardless of the subtype of defect1. The unifying features of various forms of AVSD are: 1) a deficiency of the atrioventricular septum between the left ventricle and the right atrium and adjacent atrial and ventricular septa showing a scooped-out configuration of the ventricular septal crest; 2) a common atrioventricular annulus with the aortic valve in an ‘unwedged’ position; 3) an abnormal ratio between the inlet and outlet dimensions of the left ventricle due to a short inlet and an elongated outlet (Figure 1). Diagrams illustrating unifying features of atrioventricular septal defect (AVSD). (a) Atrial and ventricular septum seen from the left ventricle after removal of the free wall of the left atrium and left ventricle and the mitral valve leaflets. (b) Atrioventricular junction seen from above after removal of the atria and great arterial trunks above the cardiac valves. In a normal heart, the tricuspid valve has a more apical attachment to the septum (solid line in (a), left) than does the mitral valve (interrupted line). The atrioventricular septum (AVS) is present between the right atrium and left ventricle because of this off-set attachment of the tricuspid and mitral valves to the septum. AVSD is characterized by a large defect involving the whole atrioventricular septum and adjacent ventricular and atrial septa, short inlet and elongated outlet with an inlet–outlet disproportion, and a common atrioventricular junction or annulus. The aortic valve (AV) is deeply wedged between the mitral (MV) and tricuspid (TV) valves in the normal heart. Hearts with an AVSD have a common atrioventricular valve annulus regardless of whether the atrioventricular valve orifice is partitioned. The common atrioventricular valve annulus does not allow the aortic valve to have a wedged position. Because of this unwedged position of the aortic valve, the left ventricular outflow tract is elongated. When a normal heart is scanned along a four-chamber plane perpendicular to the gray line in (a), the atrial length is approximately 50% of the ventricular length. When a heart with AVSD is scanned in a four-chamber view, the atrial length is only slightly shorter than the foreshortened ventricular length. The scooped-out ventricular septal crest with apically displaced bridging leaflets makes the ventricular inlet look even shorter in a regular four-chamber view. ABL, anterior bridging leaflet; Ao, aorta; MS, membranous septum; PBL, posterior bridging leaflet; PV, pulmonary valve. The diagnosis of AVSD can hardly be missed at echocardiography in children. Surprisingly, however, fetuses with this not uncommon malformation often escape detection2. In this issue, Machlitt, Heling and Chaoui introduced an increased atrial-to-ventricular length (AVL) ratio as a new fetal sonographic marker for AVSD3. In normal fetuses, the mean AVL was 0.47 with a 95% prediction interval of 0.35–0.63; in fetuses with AVSD, the mean AVL was 0.77 with a range of 0.59–0.99. This increased AVL in fetuses with AVSD is a numeric representation of an important common feature of the malformation: shortening of the inlet that results in inlet–outlet disproportion. Alternatively, the inlet–outlet disproportion could be calculated by measuring the inlet and outlet dimensions seen in the four-chamber and left ventricular outflow tract views, respectively. Shortening of the inlet in AVSD is due to the absence of the atrioventricular septum and adjacent ventricular septum. Shortening can be more obvious in a four-chamber echocardiogram as the defective ventricular septal crest characteristically shows a scooped-out appearance and the bridging leaflets of the atrioventricular valve are displaced towards the ventricles against the scooped-out ventricular septal crest, as Machlitt et al.3 explained in their Discussion (Figure 1, upper panel). The rare exceptions would be those AVSDs in which the bridging leaflets are attached to the atrial septal margin. Unfortunately, measurement of the atrial and ventricular lengths and calculation of the AVL may be cumbersome in daily screening fetal ultrasound, and few will be pleased to have one more parameter to remember. The authors' data, however, provide a firm statistical basis for a very simple message to those who are not fully familiar with AVSD: ‘atrioventricular septal defect should be suspected when the atrial length is more than 50% of the ventricular length in a four-chamber view’. If this simple but important sonographic feature were recognized at fetal screening, few cases with AVSD would escape detection.

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