The CURRENT researches are not rare on the reference values in normal fetal heart. Measurements of normal cardiac anatomical dimensions, including ventricular diameter, aortic diameter, and so like, have been proposed by many multicenter studies, most of which are positively correlated with gestational age (GA) [12–14]. The normal reference range of fetal heart establishes a quantitative relationship between the cardiac dimensions and the GA, furthermore, it provides an essential reference for clinical evaluation of fetal development. However, compared with the ventricular diameter, the atrial diameter is more susceptible to change in blood volume . Therefore, there are few researches in reference values of RA. The fetal four-chamber heart is symmetrical in normal circumstances, with the ratio of RA/LA close to 1: 1 or slightly larger . Thus, in this study, the ratio of RA/LA was applied to reflect the variation in right atrial diameter.
Right atrium dilation is frequently detected in routine screenings or fetal echocardiograms that can easily draw the examiners’ attention. In our center, four transverse planes have been used in routine obstetric sonography since 2005. Theoretically, most cardiac anomalies could be detected if all the planes could be obtained and the image data interpretations could be made correctly, according to the AIUM or ISUOG guidelines [16, 17]. However, the underlying causes behind the enlarged atrium are still not easy to determine for sonographers. Furthermore, when the fetal abnormalities had been diagnosed, the severity could be judged by the extent of RA dilatation.
We have summarized the cases of RA dilation in recent ten years. RA dilation were classified into four types: RA volume overload, RA pressure overload, displacement of tricuspid valve (Ebstein’s anomaly), and physiological enlargement. Each of the four types can cause different extent of RA dilation. In addition, RA dilation usually accompanies with various degrees of tricuspid regurgitation [15, 18–20]. Generally, RA pressure overload could always lead to worse tricuspid regurgitation with higher velocity. At the same time, both RA volume overload and physiological dilation usually lead to tricuspid valve dilation and tricuspid regurgitation with lower velocity.
It is meaningful for sonographers to make an accurate diagnosis of some fetal congenital heart disease through finding out the main cause of the RA dilation. First, we need to clarify the reasons of fetal RA dilation based on the abnormal cardiac hemodynamics and structural anomalies. As everyone knows, the foramen ovale (FO) and the ductus arteriosus (DA) are the two peculiar anatomical structures during intrauterine development. Normally, the FO facilitates oxygen-rich IVC blood to enter the LA, decreasing the volume of blood in the right atrium (right heart) . If its opening were restrictive, the blood volume of the RA would increase, leading to the RA dilation. The DA imports about 90 percent of the blood flow from the pulmonary artery into the descending aorta, reducing both the volume and pressure load of the right heart .
When compared with the right atrium, the right ventricle contains much of myocardial tissue, with less effect on the diameter when right heart volume was overload. Due to the lack of muscle fibers in the fetal atrium, the RA would expand when the right blood volume increases, which means the atrium is intolerant of volume overload. Compared with other three types, the RA dilation caused by volume overload is more apparent. Restrictive FO is one of the main causes for the RA volume overload. In our research it has been found that restrictive FO would cause the most obvious expansion of the RA. Certainly, other conditions could also lead to RA volume overload such as tricuspid dysplasia, anomalous pulmonary venous drainage and so on. But the extent of RA dilation may not be as severe as that of restrictive FO since the shunt through FO may increase compensatively in these conditions.
The right heart pressure overload could also lead to RA dilation. In this circumstances, the right atrial blood would be obstructed through the tricuspid valve into the right ventricle which may results in an increase of blood volume and RA dilation. The main causes of right heart pressure overload during intrauterine development are pulmonary stenosis/atresia or premature closure /contraction of DA. Tricuspid regurgitation is a common complication on the right heart pressure overload. When not accompanied with valve diseases, the higher systolic pressure in the right ventricle, the higher VTR would be. When comparing the pulmonary stenosis/atresia with the closure of/constriction of ductus arteriosus, it showed that the VTR in DA disease was higher than that in pulmonary stenosis/atresia. At the same time, the change of right atrial diameter was quite different among the diseases. One of the reasons is the pulmonary disease is chronic while the contractility of the right ventricle may have decreased. Besides, massive shunt through foramen ovale would balance atrial pressure so that the right atrial dilatation would not be obvious. On the contrary, premature closure of DA/ DA contraction usually occurs late in pregnancy. In this condition, RV systolic pressure would elevate tremendously in a very short term, leading to severe TR with extremely high velocity.