In the summer of 2017, Jaylen Jasper was in Lake Placid, N.Y., getting ready to go to Bahrain with the U.S. junior national volleyball team. He was a freshman at Stanford, and life was sweet.

In a heartbeat, it wasn’t.

Earlier that summer, his younger brother, Jarren, had undergone a physical to be able to play sports in high school in Annapolis, Md., and was found to have an irregular heartbeat.

During what was supposed to be a routine procedure to fix the irregularity, the 14-year-old had a heart attack. His left ventricle wasn’t functioning. Jaylen — now a sophomore and one of the rising stars on the No. 8-ranked Stanford men’s volleyball team — got a call from his sister, Dallas, that Jarren “was dead for 15 minutes.”

Actually, his mother, Donna, told The Chronicle, his heart stopped for 31 minutes. It took that long to get him on life support. “When they told me things weren’t going as planned, that’s when I started to panic,” she said.

What followed was a terrifying ordeal for the family. Jarren eventually had to undergo a heart transplant and spent 62 days in hospitals.

Jarren’s initial visits to a pediatric cardiologist and an electrophysiologist led to a procedure known as a cardiac ablation, the scarring or destruction of heart tissue that allows incorrect electrical signals to cause an abnormal heartbeat. Catheters are threaded through blood vessels to the heart and then used to map the electric signals.

But things went from bad to worse in a hurry. He had his heart shocked twice. He was moved from Children’s National Hospital in Washington, D.C., to nearby MedStar Washington Hospital Center for more procedures.

“We hoped his heart function would come back by itself,” said Donna, a former basketball player at Hawaii.

After several days, a blood clot formed in one of the tubes going into his body from a treatment known as extracorporeal membrane oxygenation (ECMO). A pump circulates blood through an artificial lung back into the bloodstream.

At one point, a tube was leaking profusely. “He almost bled to death,” his mother said. A doctor yelled, “Get the parents out of here!” The bleeding was stopped, but the terror for the family continued.

That August, the bottom of Jarren’s heart was cut out to implant what’s called a Left Ventricle Assist Device. “You can live a long time with an LVAT, but you can’t even take a shower,” his mother said.

Jaylen said, “They took him off life support to see if his heart would beat. Every time they did it, it didn’t work. Eventually, the possibility of him needing a heart transplant became a reality.”

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On Jan. 28, 2018, a heart was located; the surgery came two days later. Jaylen was getting phone updates practically every five minutes.

“I wanted to be home with my parents,” he said. “But I knew they were going to take care of it. There was not much I could do.”

The family doesn’t know who the donor was but would like to find out. “Jarren would like to thank the family,” Donna said.

He came home last Valentine’s Day. He’ll have to take anti-rejection medicine the rest of his life. “The poor little boy looked like a drug addict with needle marks in his arms,” she said.

His toes on his right foot are curled because of nerve damage, making it painful to walk. He does physical therapy twice a week to help the nerves regenerate.

“It was nice when he got home and was arguing with Jaylen on the phone,” Donna said. “We knew then that things were getting back to normal.”

Jarren calls Jaylen “one of my biggest role models,” although he concedes he gets under his older brother’s skin at times.

Jaylen — who is pursuing a double major in political science and psychology and plans to go to law school — puts it this way: Jarren “has a profound way of getting on my nerves. No one in this world can trigger me like Jarren can.”

In a phone interview, now-15-year-old Jarren said he goes to school on a half-time basis but hopes to go full-time next semester. He plays video games and watches a lot of college basketball, he said. He was rooting for the Saints in the NFC title game, but will root for the Rams in the Super Bowl.

“I’m not the biggest Patriots fan,” he said. “I really like the Steelers because they’re my dad’s favorite team.”

His father, Ivin, is offensive coordinator for the Navy football team. Ivin tried hard to get Jaylen interested in football, even offering him a new car if he’d go out for the sport in high school. “That’s how much I didn’t want to play football,” he said. “I didn’t take it.”

The choice appears to have been a wise one.

The 6-foot-7 Jaylen is an outside hitter for the Cardinal (6-1) and leads the team in kills and is 10th in the country with 4.05 per set. Last week, Stanford knocked off UC Irvine, the No. 2 team in the country before losing in a rematch two nights later.

“Jaylen is a tremendous athlete,” head coach John Kosty said. “There isn’t too much boys volleyball in Maryland, but he has played on youth national teams. He’s had opportunities to play at a really high level, but not over time. That’s what he’s learning now.”

Tom FitzGerald is a San Francisco Chronicle staff writer. Email: tfitzgerald@sfchronicle.com Twitter: @tomgfitzgerald