Cause of normal pulmonic velocity in fetal tetralogy of Fallot

In 10 fetuses with tetralogy of Fallot, 9 lacked high velocity across the obstructed right ventricular outflow tract, which we conclude was due to the parallel flow circulations created by the combination of ventricular septal defect and arterial duct. The normally high resistance in the fetal pulmonary vascular system substantially exceeds the resistance of the systemic vasculature, resulting in the maintenance of right-to-left ductal flow in all 10 fetuses who had only moderate pulmonary obstruction.