MRI shows limited mixing between systemic and pulmonary circulations in foetal transposition of the great arteries: a potential cause of in utero pulmonary vascular disease

Objectives: To investigate the relationship between foetal haemodynamics and postnatal clinical presentation in patients with transposition of the great arteries using phase-contrast cardiovascular magnetic resonance. Background: A severe and irreversible form of persistent pulmonary hypertension of the newborn occurs in up to 5% of patients with transposition and remains an important cause of morbidity and mortality in these infants. Restriction at the foramen ovale and ductus arteriosus has been identified as a risk factor for the development of pulmonary hypertension, and this can now be studied with magnetic resonance imaging using a new technique called metric optimised gating. Methods: Blood flow was measured in the major vessels of four foetuses with transposition with intact ventricular septum (gestational age range: 35–38 weeks) and compared with values from 12 normal foetuses (median gestational age: 37 weeks; range: 34–40 weeks). Results: We found significantly reduced flows in the ductus arteriosus (p<0.01) and foramen ovale (p=0.03) and increased combined ventricular output (p=0.01), ascending aortic (p=0.001), descending aortic (p=0.03), umbilical vein (p=0.03), and aorto-pulmonary collateral (p<0.001) flows in foetuses with transposition compared with normals. The foetus with the lowest foramen ovale shunt and highest aorto-pulmonary collateral flow developed fatal pulmonary vascular disease. Conclusions: We found limited mixing between the systemic and pulmonary circulations in a small group of late-gestation foetuses with transposition. We propose that the resulting hypoxia of the pulmonary circulation could be the driver behind increased aorto-pulmonary collateral flow and contribute to the development of pulmonary vascular disease in some foetuses with transposition.