The Fontan surgery that Sam needs is complicated and hard to explain in any detail, so I have found the information for you so you may read what it entails.
Fontan OperationThis involves connecting the veins from the main circulation (SVC & IVC) directly to the pulmonary arteries. Blue blood is thus directed into the lungs rather than to the left atrium. A patch is placed to prevent blood passing from the RA to the LA - though sometimes a small hole (a 'Fenestration') is deliberately left.
Both this description of surgery and the graphics below are from the RCH website.
This is what Sam's Heart looked like at birth before his first surgery in comparison to a normal heart:
This is what Sam's Heart looks like now (after his second surgery)
This is what Sam's heart will be like post Fontan:
here for a highly details and informative explanation.
For a medical outlook on what HLHS means for Sam both now and in the years to come, I found this description most useful and easy to understand:
As he grows: your child’s long-term outlook
Babies with HLHS will need three surgeries in their first few years of life; and children will need follow-up care throughout their lives to ensure that their hearts continue to function adequately. They are never entirely cured of their cardiac defect, and will probably need to take heart medications.
The most positive outcome is that a child can undergo these surgeries and not have any symptoms going forward. On the other hand, there are children whose hearts struggle with their single ventricle.
Children’s cardiologists will follow your child’s recovery and progress, and will provide further treatment, if needed. There is a risk of complications, including arrhythmias, heart failure and blood clots. And the potential does exist for needing a heart transplant in early or mid-adulthood.
Thanks to sophisticated diagnoses and treatment innovations, babies with HLHS who would not have survived a mere 20 years ago now have a good chance for a full life. And the medical science on HLHS and other congenital heart defects continues to evolve and improve. (Source)
All of this information is good to have. It helps me to understand what Sam needs to go through and how I can help him and advocate on his behalf. But it is the promise which God gave me that I hold most dear. Because there are so many variables in medicine, nothing is certain, each case is different, each child is different. And I know that God's promise for Sam to have life in abundance is unchanging and rock solid.
Information is important, faith is essential.