Please read my post “D-transposition (Complex Transposition) of the Great Arteries”, as well as my posts on morphology of pediatric heart to gain a better understanding of pediatric heart function and physiology.
This is a procedure that may be done on a fetus (intra-uterine), or a neonate that displays or will display severe cyanosis after birth. This procedure is considered if there is a severe pulmonary venous inflow obstruction such as hypoplastic left heart syndromes, or in the case of D-transposition (complex transposition), or in certain cases of severe pulmonary hypertension (PHTN).
If a fetus has a hypoplastic left heart or mitral stenosis, this procedure is done to reduce the stress on the left atrium and therefore  the entire pulmonary venous system.
In other words, if venous outflow from the lungs enters the left atrium and cannot get out of the left ventricle, it has to go somewhere, usually back into the lungs. This severely inhibits systemic outflow and also mixing of the blood at any capillary level.
The object of the repair is to open the atrial septum in order to decompress the left atrium and allow mixing of systemic and venous blood to occur so that SaO2 levels will increase and damage to the lungs will be minimized.
It is important to note that once pulmonary hypertension sets in, it is very difficult to reverse. Additionally, the lungs are the last organs to develop during gestation and it is very important to protect them from any damage.
If there is a D-transposition, then the atrial septostomy is the palliative procedure of choice (especially if there is no VSD) soon after birth. This procedure allows the neonate to stabilize so that further surgical repairs can be done.
In the case of children who exhibit severe right sided cardiac failure, Eisenengers syndrome or severe pulmonary hypertension may benefit from this procedure. Puncture of the atrial septum often relieve the symptoms of severe pulmonary hypertension.
Surgical ProcedureÂ
This is a minimally invasive, palliative procedure that introduces a balloon tipped catheter into either the left or right atrium via the lateral chest wall. The catheter punctures the foramen at the atrial septal level, and then the balloon is inflated. It is then pulled back through the septum in order to create a hole that will allow mixing of the right and left circulations. This immediately reduces symptoms of cyanosis and allows the  neonate (or fetus) to stabilize. This hole may or may not be permanent, and subsequent septostomies may be required.
Outcomes for this procedure are good, even when done intra-uterine. In the case of D-transpositions and other defects that severely limit left heart function, it is almost essential to stabilize the infant prior to further surgery, which often must be done within the first few weeks after birth.
Ken Heiden RDCS