4 DISCUSSION
In fetuses with d-TGA, severe hypoxemia may occur in the neonatal period
due to inadequate blood mixing at the atrial level in the absence of
VSD, making early detection and follow-up essential in experienced
centers. In our study population, the FO diameter and the absence of a
VSD were predictors of urgent BAS in fetuses with d-TGA.
According to definitions by Rudolph et al., 10,11 in
fetuses with d-TGA with an IVS, cardiovascular circulation is
characterized by blood reaching the left atrium via the FO and the left
ventricle carriers oxygenated blood to the pulmonary circulation and
then to the descending aorta. This condition results in restriction of
the DA and increased pulmonary venous return to the left atrium, which
may be associated with narrowing and restriction of the FO owing to the
elevated pressure in the left atrium 12-14. In our
study, the FO diameter was significantly decreased in the urgent BAS
group, compared with the fetuses without urgent BAS (5.1 mm vs 6.3 mm).
This finding is consistent with a report of 45 fetuses with d-TGA and
IVS, in which the median FO diameter was significantly decreased (5.7
mm), requiring urgent BAS 15. Our study group also
included fetuses with both VSD and IVS. In another study, the median FO
diameter was 4.8 mm in the urgent BAS group as compared with 5.9 mm in
fetuses not required urgent BAS, but the differences were not
significant 16.
In ROC analysis, a FO diameter of 6 mm was found to predict urgent BAS
with a sensitivity of 73.3% and specificity of 72.2%. In our
experience, a FO diameter of greater than 6 mm highly reduced the need
for urgent BAS. Similar to our finding, in a study of 60 fetuses, the FO
diameter was significantly smaller in the urgent BAS group and the FO
diameter was found to be the most valuable predictor for urgent BAS,
though their cut off value on the ROC analysis was greater (6.5 mm).
This difference may arise from the incidences of VSD, being 41.7% in
the current study vs 18% in their study group17.
Besides FO, some authors also used the ratio of FO to the total septal
length (FO:TCL), with a FO:TCL of 0.5 yielding a higher predictive value
for urgent BAS with a sensitivity of 99%, specificity of 60%, positive
predictive value of 50% and negative predictive value of 94%18.
In our study, 5 fetuses had restrictive appearance of FO and no
high-flow rate in the DA, suggestive of absence of constriction of the
DA. While one fetus had an aneurysmatic FO with its flap extending more
than 50% to the left atrium, the remaining four had a flat FO flap
swinging less than 30° into the left atrium. In accordance with other
studies, all 5 fetuses underwent urgent BAS immediately after birth16,18. There are also some studies suggesting that the
appearance of the FO flap (flat, fixed or aneurysmatic) was not
associated with urgent BAS 19-22. Inconsistencies
between studies may result from the small number of fetuses included,
the definition of urgent BAS, the lack of echocardiographic examination
after 37 weeks of gestation, or misdiagnosis of FO restriction in normal
fetuses due to breathing movements and physiological right ventricular
hypertropia. In addition, interpretation of the FO may vary due to the
primary or secondary nature of restriction: the former represents an
abnormally narrowed aneurysmatic FO and motility, while the latter may
result from hemodynamic alterations including abnormal blood flow in the
DA and pulmonary venous return 23. In our study, in 13
fetuses requiring urgent BAS, narrowing of the FO was possibly caused by
underlying d-TGA hemodynamics, i.e., the pumping of blood with high
oxygen saturation directly from the left ventricle to the lungs, causing
increased pulmonary venous return to the left atrium.
In our experience, the presence of a VSD appeared to play a protective
role from urgent BAS: only 4 of 20 fetuses with VSD underwent urgent
BAS. In a retrospective single center study, BAS was performed in 64%
of neonates with an IVS and 25% of neonates with
VSD24. In addition to the FO diameter, the absence of
VSD was found to be an independent predictor of urgent BAS (Table 7). In
the ROC analysis, the cut off value for the VSD diameter was 3.2 mm,
with 75% sensitivity and 75% specificity.
BAS can generally be thought of as a safe procedure. In our study, no
complications developed in the neonates who underwent urgent BAS. In a
study of 73 neonates undergoing BAS, the procedural success rate was
98.6%, while hemodynamically significant arrhythmia developed in 4.1%,
tamponade in 1.4%, and catheter-related complications in
2.7%24.
The strengths of this study are the comprehensive analysis of the FO and
VSDs with echocardiographic examinations performed most commonly after
37 weeks of gestation and a single center study. There are two
limitations to this study. First, its retrospective design. Second, we
could not include the FO in ROC and regression analyses in fetuses with
VSD or IVS separately due to the small number of the fetuses.
In conclusion, prenatal echocardiography performed after 37 weeks of
gestation in fetuses with d-TGA provides valuable information about the
dimensions of FO and the absence of VSD to estimate the need for
postnatal urgent BAS that would prevent immediate life-threatening
complications.