Running the recent Los Angeles Marathon was a triumph for thousands of people who made it through 26.2 miles of city streets. But for one runner, Jay Yim, the day didn't exactly go as planned.

Yim, 21, suffered a heart attack around mile 18 and was rushed to the Ronald Reagan UCLA Medical Center via ambulance in what appears to be the worst casualty of the race (it was reported that about 30 other runners were hospitalized). Yim is listed in good condition and is expected to make a full recovery.

Here's how things went down: After Yim collapsed in West L.A., LAPD motorcycle officer Joshua Sewell, who cut his vacation short to volunteer at the race, was one of the first people to come to Yim's aid. Yim was about the fourth person he'd seen hit the pavement that day, he said, from exhaustion or dehydration. But when he tried to revive Yim he got no response and found no pulse. Sewell yelled for someone to call an ambulance and recruited an LAPD bicycle officer to help administer CPR. "I did CPR in the [police] academy 15 years ago but not since then," he said. The lapse didn't seem to matter -- the routine kicked in and chest compressions and mouth-to-mouth resuscitation were done with precise timing.

Also on the scene was Dr. Charles Chandler, chief of surgery at Santa Monica-UCLA Medical Center, who was watching the marathon from near his home and saw Sewell running by. "When I got there Jay was in the middle of the street -- completely still, and his pupils were dilated and he wasn't moving any air." Chandler helped out with CPR, eventually getting a pulse, and called UCLA's emergency room to tell the staff the ambulance was on its way.

After undergoing tests, it was discovered that Yim had suffered some seizures as well, possibly caused by the cardiac arrest, said Dr. Paul Vespa, director of Neurocritical Care at UCLA, who treated Yim. An MRI showed some brain swelling, and fearing brain injuries hypothermia was induced. In that process, the body is cooled to 32 degrees Celsius [Update: An earlier version said Fahrenheit. The 32 degrees Celsius equals 89.6 degrees Fahrenheit] (the procedure is also used for some cardiac arrest cases) and is in a coma. The process, still somewhat controversial, basically brings on hibernation, Vespa said, causing a metabolism shutdown. "When you have a brain injury," he said, "a whole number of bad pathways get activated, and that can lead to cell death and damage. Hypothermia blocks those pathways." He added that hypothermia can also put the body at higher risk for infection, since the immune system is suppressed.

Yim's body was warmed after about 48 to 72 hours, and he is now awake and talking. He's undergoing physical therapy, and while Vespa said it's too soon to tell if Kim will ever do another marathon, his overall prognosis is excellent. What caused his cardiac arrest still isn't known, and although it's unusual for someone his age and good health to suffer a heart attack, dehydration or inadequate nutrition during a marathon or other physical activity can trigger such catastrophic events.

But Yim, a USC pre-med student originally from Phoenix, has some incentive to run again. Sewell, who ran the marathon in 2006, said he promised he'll finish the last 8.2 miles with Yim when he's able. "I told him that, and he got a big old smile on his face," Sewell said, adding that he's been spending a lot of time with Yim and his family. "I got a little emotionally attached to this one."

"Everything was in Jay's favor," said Yim's older brother Roy, a law student from Florida. "It's weird how it all turned out."