Little is known about physiology and distribution of ventricular flows in fetuses with transposition of the great arteries and intact ventricular septum (TGA-IVS), especially regarding the risk for immediate postnatal desaturation. Ductal flow is the result of a complex interaction between left and right ventricular outputs as well as pulmonary and peripheral resistances.

Longitudinal hemodynamical assessment of 60 fetuses with TGA-IVS and 150 controls was done 3 times during the pregnancy at 18–22, 28–32 and 35–38 weeks of gestation. We measured the size of the inter-atrial defect and calculated left and right ventricular output as well as systolic and diastolic ductal flow. Fetuses with TGA-IVS were divided into 2 groups depending their postnatal saturation. Fetuses with postnatal saturation<65% at the upper members were included in group 1. Echocardiographic parameters were compared between the 2 groups.

Two fetuses had restrictive ductus arteriosus with high velocity flow and were excluded from the calculations, they all had postnatal saturation<65%. Sixteen additional fetuses had a postnatal saturation<65%. There was no difference in left or right ventricular output between the groups at first evaluation. At 28–32 weeks, inter-atrial defect was smaller in group 1 (3±0.3 versus 4±0.8mm, P=0.012) as well as left ventricular output (221 versus 307mL/min, P=0.05). Ductal flow was lower in group 1 after 35 weeks (56 versus 130mL/min, P=0.007), mainly due to a decrease in diastolic antegrade flow which was retrograde in group 1 (−7.1 versus 22mL/min, P<0.004). A retrograde ductal diastolic flow had a positive predictive value of 89% and a negative predictive value of 86% to predict saturation<65% at birth.

At the third trimester, retrograde diastolic flow in the ductus arteriosus could be secondary to low pulmonary resistances but also to decreased left ventricular output in case of small inter-atrial-defect. Fetuses with this ductal profile remain at risk for postnatal inadequate inter-atrial mixing.