Namine had her third and final heart surgery, called the Fontan, back in 2013. When all was said and done, the surgeon left a hole, called a fenestration, to relieve blood pressure around her heart. Because of this, the procedure Namine had is referred to as a “fenestrated Fontan.”
Since the fenestration is essentially a hole in the heart, it can’t remain there indefinitely. It eventually has to be closed, either naturally or surgically. In some cases, the fenestration will close on its own; this is what Namine’s cardiologist believed happened, since her blood-oxygen level had stabilized nearly a year later.
Now, however, Namine’s blood-oxygen level is lower, and the only explanation the doctors have is that the fenestration might not be closed, after all. For reference, a physio-typical person will have a blood-oxygen level of 98-100. A year after the Fontan procedure, Namine’s O2 averaged around 94-96. Now, her O2 has dropped to around 92. It’s not so low that it poses a danger to her, but if it drops below 90 and stays that way, then she may need to have the procedure to close the fenestration.