"If the family was unaware the child had any congenital heart defects, then an autopsy may find something unrecognized, but not directly related," he said.
An estimated 800,000 to 1 million Americans are living with cerebral palsy, according to Blackman. It is caused by an injury sustained during a pregnancy, the birth process or after a head injury or stroke when a young child's brain is starved of oxygen.
"This kind of heart problem can happen out of the blue to any child -- at a Little League game when someone is totally well," he said. "This was a special child, for sure. To think his sibling was on the plane, as well, it must have been a rough ride for everyone on the plane, especially the family."
Volunteers sprang into action when Zachary's heart began to fail, according to passenger volunteers.
Dan Goslin, a registered nurse, said that he, a doctor and a midwife helped the airline crew, as they attempted to resuscitate Zachary.
"He was having trouble breathing. We tried oxygen," he told ABC affiliate WSBTV. "That didn't work. We tried to move him out of the chair and get him to another position. That didn't work and everything we tried just really didn't work."
The flight was met by emergency paramedics who also tried successfully to revive the teen.
Leahy, who's organizing the gofundme campaign, said Delta Airlines "went above and beyond to assist in saving Zach's life."
Zachary's body has already been returned to Georgia and a funeral has been scheduled for Dec. 22, according to Leahy. He said his sister and her family were en route, driving by car to Georgia.
"He was a great kid, full of life and fun to be around," Leahy said of his nephew. "We just related to him like a normal kid."