Restricted Atrial Septum

The mortality rate is higher if the opening between the left and right atria (callout 4 in illustration below) is either very small or absent. This situation occurs in about 6% to 11% of fetuses / infants with HLHS and is known as HLHS with highly restrictive or intact atrial septum (RAS). After birth of these babies, oxygenated blood returning to the left atrium has great difficulty crossing into the right side of the heart and out to the body. Thus, blood oxygen levels in these babies are usually very low, and the babies are very sick and unstable immediately after birth. Persistently high mortality rates, despite surgery within the first few days of life to open the septum, have led to efforts to develop fetal interventions for HLHS with RAS.

Hypoplastic Left Heart Syndrome / HLHS

1. Hypoplastic ascending aorta and aortic arch.
2. Hypoplastic left ventricle.
3. Large patent ductus arteriosus supplying the only source of blood flow to the body.
4. Atrial septal defect allowing blood returning from lungs to reach the single ventricle. 

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SOURCE and for more info: http://www.cincinnatichildrens.org/service/f/fetal-care/conditions/hlhs/