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Atrioventricular septal defects (AVSDs) are a common anomaly, occurring in 7.5% of liveborns with congenital heart defects1. Furthermore, they are the most common cardiac malformation associated with aneuploidy, in particular trisomy 212. In our experience, AVSD is usually only diagnosed if either extracardiac anomalies and soft markers of trisomy 21 are present or if amniocentesis has revealed an abnormal karyotype. The detection rate for isolated AVSD is likely to be even lower.
In the fetus, AVSD may potentially be detected by visualizing the four-chamber view3, 4. Compared to hypoplastic left heart or univentricular heart defects, major distortions are not necessarily apparent in AVSD5. In a pediatric cardiology study in the UK, only 38% of 579 children with AVSD had the anomaly diagnosed during fetal life, whereas detection rates for hypoplastic left heart and univentricular heart defects were much higher at 66% and 60%, respectively6. Difficulties in analyzing the four-chamber view correctly may be the cause of these differences5.
The aim of our study was to investigate whether measurements of fetal heart length, ventricular length, atrial length or corresponding ratios in the four-chamber view will discriminate between hearts with AVSD and normal cardiac anatomy.
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Heart length, ventricular length and atrial length increase with advancing gestation. Results of regression analysis in the controls are shown in Table 1. In all three distances a quadratic term was found to be significant.
Table 1. Regression equations for measured distances in the four-chamber view (y) with gestational age (x), residual SD and R2 in the control group (n = 123)
|Heart length||−18.79 + 2.34 x − 0.017 x2||2.41||0.96||123|
|Ventricular length||−12.53 + 1.58 x − 0.011 x2||1.86||0.94||123|
|Atrial length||−6.24 + 0.77 x − 0.006 x2||1.24||0.90||123|
When normalized for gestational age, comparison between the study group and controls showed no differences for heart length; however, in fetuses with AVSD, ventricular lengths were shorter (P < 0.001) and atrial lengths were longer (P < 0.001) than those in controls (Figure 2).
The AVL ratio was found to be independent of gestational age. The mean AVL ratio in controls was 0.47 (95%CI, 0.46–0.48) and the 95% prediction interval (in which 95% of the expected measurements will lie) ranged from 0.35 to 0.63.
In the study group with AVSD, measured values of the AVL ratio ranged from 0.59 to 0.99. In 24/29 fetuses the AVL ratio was above the 95% prediction interval. None of the cases of AVSD in our series showed an AVL ratio below the normal mean (Figure 3). Selecting a cut-off for normal AVL ratio at 0.6 would detect 86.2% of AVSDs at a 5.7% false-positive rate, and a cut-off of 0.58 would include all AVSDs and 7.3% of the controls.
Figure 3. Values of the atrial-to-ventricular length (AVL) ratio in the study group (▪) and controls (○). The solid line represents the mean AVL ratio in controls and the dashed lines mark the 95% reference range.
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In the study group there were 13 fetuses with trisomy 21, two with trisomy 18 and one case with Robertsonian translocation. Twelve fetuses had a normal karyotype and in one fetus the karyotype remained unknown. In the cases with AVSD, the AVL ratio was higher in euploid fetuses than in aneuploid fetuses (P = 0.008). The mean value of the AVL ratio was 0.73 (95% CI, 0.67–0.78) if trisomy 21 was present and 0.83 (95% CI, 0.75–0.91) in euploid fetuses, respectively.
The AVSD was isolated in 12/29 cases in the study group. In the remaining 17 cases there were five with coarctation, three with tetralogy of Fallot, three with right atrial isomerism, three with additional ASD or VSD and one each with hypoplastic aortic arch and pulmonary hypoplasia. Partial AVSD was found in one fetus with an AVL ratio of 0.71. A total of 9/13 fetuses with trisomy 21 had isolated AVSDs, whereas only 3/12 euploid fetuses showed isolated defects. Partial AVSD was found in one fetus with an AVL ratio of 0.71.
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The AVL ratio determined in the four-chamber view discriminates with great accuracy between normal cardiac anatomy and the presence of an AVSD. If 0.6 is used as cut-off value, 86.2% of AVSDs are detected at the expense of 5.7% false-positives. A 100% detection rate is achieved at a rate of only 7.3% false-positives. These results suggest that integration of the AVL ratio into routine ultrasound screening may increase the detection rate of AVSD.
To our knowledge, there is no prenatal ultrasound study that quantifies the dimensions of atrial and ventricular septum lengths in relation to complete heart length. In AVSD, precise data describing the position of the common atrioventricular valve in the four-chamber view are lacking. Our data confirm that the ventricular length is smaller when there is an AVSD. This is supported by previous studies in children and adolescents with AVSD, which found that the defect resulted from a greater deficiency of ventricular than atrial septal tissue9, 10. In these studies it was shown that in all cases of complete and partial AVSD, septal deficiency extends the level of the common AV junction in the ventricular direction. In addition to a smaller ventricular length, we found a greater atrial length in our study group, which is in contrast to previous postnatal studies9, 10. This discrepancy can be explained by the fact that atrial length determined by our method was the distance from the atrial border to the common AV valve. It thus comprises both atrial septal length and part of the AVSD.
Fetal cardiac measurement studies have been reported since the 1980s using M-mode and two-dimensional ultrasound11–13. In our study, heart length measurement is comparable to that recorded in previous studies13, whereas owing to different measurement techniques, ventricular length and atrial length are not. In our study, ventricular length corresponded to the distance between the crux cordis and apex cordis in the controls and between the closed AV valve and apex cordis in hearts with AVSD. Therefore, studies that determined internal ventricular length in the left and right ventricles separately provided shorter measurements14–16.
In the last 25 years, biometry of fetal heart dimensions has not become part of routine ultrasound screening because it is rather time consuming and, with few exceptions17, ineffective in detecting heart anomalies. Measurements of internal chamber dimensions are suitable to compare corresponding chambers in cases of suspected hypoplasia or dilatation. In the present study we have introduced the AVL ratio as a new parameter to be measured during routine screening ultrasonography. Measurement of the AVL ratio will enable the detection of at least one major cardiac anomaly with good reliability. Measurement of the AVL ratio is both simple and effective when standardized as proposed in our protocol.
As we have learned from nuchal translucency (NT) studies, standardized protocols that obtain, document and quantify a certain cross-sectional plane have numerous positive side effects: they urge the examiner's attention to crucial regions. Abnormal measurements may identify a group at high risk for certain anomalies and which require further examination. The application of ultrasound equipment functions such as zoom and cine-loop may improve the quality of examination in general, and in particular in fetal echocardiography5. With adequate training and audit, even distances of a few millimeters may be obtained in the fetus with great accuracy as shown in NT and nasal bone studies18, 19.
AVSDs are frequently associated with chromosomal abnormalities and heterotaxy syndromes20. In subgroup analysis we found a high rate of aneuploidy when the AVL ratio was in the upper normal range (0.59–0.63). Very high values of the AVL ratio were more likely to be associated with complex heart defects. These results suggest that in cases with isolated AVSD, which are commonly found in trisomy 21, distortion of the four-chamber view is minimal. In contrast to hypoplastic left or right heart defects, the four-chamber view in hearts with an AVSD may on first sight appear normal5. This may explain the low antenatal detection rates in the case of this particular heart defect. We assume that a more careful cardiac examination in cases with an AVL ratio of 0.6 and above will permit detection of additional cases with AVSD.
In the study group with AVSD, the lowest value for the AVL ratio was found to be 0.59. These data suggest that AVSDs are unlikely to be present if the AVL ratio is below the mean value of 0.47. Since this preliminary study included a small number of pregnancies, conclusions about the efficacy of incorporating this echocardiographic measurement into screening should be drawn with caution until data from larger prospective studies are available. Our study was performed by examiners experienced in fetal echocardiography who may have been biased by the fact that they had identified this condition.
Our findings imply limitations regarding applicability in the first trimester of pregnancy. We report on normal values from 10 gestational weeks onwards. However, in our study only one AVSD was found before 16 weeks. In this 13 + 5-week fetus, the AVL ratio was 0.61, which is in the upper normal range. Due to the limited number of cases, we cannot exclude a possible need for a lower cut-off in early pregnancy.