Survival after stage 1 of palliative surgery for hypoplastic left heart syndrome (HLHS) was first reported by Norwood in 1983; since then there have been numerous modifications to all aspects of the management of these infants resulting in significant improvement in survival. However, for the subgroup of infants presenting with a closed or severely restrictive foramen ovale (FO) morbidity and mortality remain high. The majority of infants with HLHS are now diagnosed in utero; identification of this subgroup prenatally is important in terms of both parental counselling and management of pregnancy, delivery and the early neonatal period. A restrictive FO is one of the most critical risk factors in determining outcome both before and after surgery, for palliation as well as for transplantation.1

In the majority of cases of HLHS the FO is patent but in up to 22% it is restrictive and in 6% the atrial septum is intact;2 these neonates have profound hypoxaemia, acidosis, low cardiac output and haemodynamic instability associated with persistently high pulmonary vascular resistance. They require urgent left atrial decompression.

In a healthy fetus blood returning from the placenta via the ductus venosus and inferior vena cava is preferentially shunted from right atrium (RA) to left atrium (LA) via the FO. In the presence of left heart obstruction, flow at atrial level is reversed and due to the anatomical properties of the flap valve there is potential for restriction of the FO. In HLHS all or virtually all (depending on …