Cardiac output and blood flow redistribution in fetuses with D-loop transposition of the great arteries and intact ventricular septum: insights into pathophysiology.

OBJECTIVES: Although the postnatal physiology of D-loop transposition of the great arteries with intact ventricular septum (D-TGA/IVS) is well established, little is known about fetal D-TGA/IVS. In the normal fetus, the pulmonary valve (PV) is larger than the aortic valve (AoV), there is exclusive right-to-left flow at the foramen ovale (FO) and ductus arteriosus (DA), and the left ventricle (LV) ejects 40% of combined ventricular output (CVO) through the aorta, primarily to the brain. In D-TGA/IVS, the LV ejects oxygen-rich blood to the pulmonary artery, theoretically leading to pulmonary vasodilation, increased branch pulmonary artery flow and reduced DA flow. In this study, we tested the hypothesis that D-TGA/IVS anatomy results in altered cardiac valve sizes, ventricular contribution to CVO, and FO and DA flow direction.
METHODS: Seventy-four fetuses with D-TGA/IVS that underwent fetal echocardiography at our institution between 2004 and 2015 were included in the study. AoV, PV, mitral valve and tricuspid valve sizes were measured and Z-scores indexed to gestational age were generated. Ventricular output was calculated using Doppler-derived velocity-time integral, and direction of flow at the FO and DA shunts was recorded in each fetus using both color Doppler and flap direction. Measurements in the D-TGA/IVS fetuses were compared with data of 222 controls, matched for gestational-age range, from our institutional normal fetal database.
RESULTS: The LV component of CVO was higher in D-TGA/IVS fetuses than in controls (50.7% vs 40.2%; P < 0.0001), with no difference in the total CVO. Flow was bidirectional at the FO in 56 (75.7%) and at the DA in 24 (32.4%) D-TGA/IVS fetuses. Only 21.6% fetuses had normal right-to-left flow at both shunts. Bidirectional shunting was more common in third-trimester fetuses than in second-trimester ones (P < 0.03). AoV and PV diameters were nearly identical in D-TGA/IVS in contrast to control fetuses, hence AoV Z-score was higher than PV Z-score (1.13 vs -0.65, P < 0.0001) in D-TGA/IVS.
CONCLUSIONS: In fetuses with D-TGA/IVS there is loss of the normal right-sided dominance, as each ventricle provides half of the CVO, with a relatively large AoV diameter and a small PV diameter, and high incidence of bidirectional FO and DA flow. This may support the theory that high pulmonary artery oxygen content reduces pulmonary vascular resistance, thereby increasing branch pulmonary artery flow and venous return, which results in increased LV preload and output. Pulmonary sensitivity to oxygen is thought to increase later in gestation, which may explain the higher incidence of bidirectional shunting. Consequences of these flow alterations include increased aortic and, most likely, brain flow, perhaps in an attempt to compensate for the substrate deficiency observed in D-TGA/IVS.