When she was four, Sara Dumas' parents took her to the emergency room. The girl had been vomiting frequently and her parents could not figure out why.
Doctors discovered a heart murmur. When they performed an ultrasound, called an echocardiogram, to look at the function and structures of the heart, they realized it was worse than a mere murmur: Sara had hypertrophic cardio-myopathy (HCM), a congenital heart disease.
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With HCM, the heart muscle becomes thick, making it difficult for blood to pump out of the heart and forcing the organ to work harder to pump at all. The disease can cause shortness of breath and severe chest pain.
In the worst cases, about 5 percent of the time, HCM sufferers need heart transplants.
Sara, at 16, learned that she was among the 5 percent, a group she would jokingly call the "lucky ones."
A Family Problem
HCM is a genetic condition. When one family member is diagnosed with the disease, typically immediate family members are also tested.
Doctors discovered that Sara's father passed down the disease. He tested positive. So did Sara's brother, Ian, who is two years her junior.
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At first, the Dumases did not understand the seriousness of what they were dealing with.
Most children with HCM can live normally, but are not allowed to exercise strenuously, according to Dr. Elizabeth Blume, Sara's cardiologist. Team sports are usually off-limits, because of the risk of sudden death.
'Boston Med': Reality Sets In
When young athletes suddenly suffer heart attacks on the field, undiagnosed cases of HCM is a common cause.
High-spirited and curious, Sara always tried to cope with her disease as best she could, pursuing the things she loved -- and was allowed to do, such as pursuing her passions for music and science. She also likes shopping with her friends, just like any other teenager.
"As far as I can remember, I've always been limited,'' said Sara. "I don't know anything different."
When Sara was 12, the Dumas family relocated and learned just how serious the disease could be.
"When we moved to Connecticut, and started going to Yale-New Haven Hospital," said Sara's mother Lisa Dumas, "and they were like 'Oh boy, they aren't on any medication? This is really serious. This is really dangerous.'"
Both Sara and Ian immediately were put on medication and became regulars in the office of cardiac specialists.
And as Sara got older, her condition worsened. In July 2008, both Sara and Ian were referred to Children's Hospital Boston in Massachusetts. On a family vacation that year, Sara was in so much pain, she dissolved into tears, her mother said.
"We were doing a lot of walking and even with frequent stopping, she was in a lot of pain," said Lisa Dumas.
By November, both siblings had small life-saving devices, known as implantable cardioverter defibrillators (ICDs). installed in their chests. ICDs instantly shock the heart to restore normal rhythm when any abnormality occurs.
At 16, Sara's heart was so damaged that doctors added her name to the national registry of patients waiting for a donor heart.
In the midst of studying for SATs, applying to college and wondering if she'd make it to her prom, Sara, who still wore braces on her teeth, had to contend with the mind-boggling journey that is waiting for a new heart.
Blume, who is medical director of the Heart Failure/Transplant Program at Children's Hospital Boston, was growing more concerned about Sara.
"Sara's gotten sicker and sicker over the last six months, I worry that if we don't find her heart soon, that she'll die," she said.
As Lisa Dumas fretted over what could happen to her daughter, she also had to grapple with what it would mean to save Sara's life.
"In order for my child -- or my children -- to survive, somebody else's family has to make the ultimate sacrifice in a time when they are going through a lot of emotional turmoil," she said.
Just 2 ½ months after she was added to the list, a donor heart match was found for Sara.
'Boston Med': Hope for a New Heart
Two surgeons from Children's Hospital Boston boarded a small plane on their way to the hospital where the donor was located. The heart was viable, but there were delays.
In pediatric donors, it is common to harvest a lot of organs, adding to the time it takes to get each organ to its recipient. Several hours later, after working through the night, the surgeons returned to Children's Hospital Boston with Sara's new heart.
During surgery, the teen's blood pressure dipped. Doctors grew tense. They could not determine the cause.
As the Dumas family sat for about five hours in the waiting room for Sara to come out of the OR, it was not far from anyone's mind that Ian, then 14, would also undergo the same surgery not too far down the road.
Sara came through her transplant surgery without serious complications and recovered quickly. She was back home two week later.
Among the Lucky of the 'Lucky Ones'
Sixteen months after her surgery, Sara can be counted among the success stories.
"She's had no bumps in the road since her transplant," said her surgeon, Dr. Francis Fynn-Thompson. "I would expect her to do well and go a long time with her new heart."
"In children, the median time a heart lasts is 12 to 14 years,'' said Fynn-Thompson, who specializes in the repair of complex congenital heart disease and the management of end-stage heart and lung disease in children.
"We do sometimes re-transplant patients when their first heart fails but we have patients who are 20 years out with their first transplanted transplant," he said. "It's unpredictable."
With continued improvement of immunosuppressant medications and rejection screening, the doctor said, patients can live even longer with their transplanted hearts.
Now 18, Sara is heading to college in the fall to study biology. She wants to be a cardiologist.
"I'm already taking anatomy in (high) school and it's a little too slow for me right now," she said. "I hope medical school and college will be more challenging than high school classes are."
She enjoys swimming, something she was always afraid to do before, since it caused shortness of breath, and also likes walking and using elliptical trainers, her mother said.
She's also done some running with her friends since her transplant, something her doctors would not permit her to do before.
As for Ian, now 15, he's scheduled to head back to Children's Hospital Boston next month to see if his disease has progressed to the point where he needs to be added to the transplant list.
ABCNews' Kimberly Brown contributed to this report.