At 14 minutes and 30 seconds into the attempt, Dr. Potkin starts to get concerned about David's heart rate, which is still at more than 100 beats per minute. "I would have thought he'd be in the 40s or 50s," he says. "Maybe that will be coming later."
Though things aren't going exactly as they planned, Dr. Potkin still has confidence in David's ability and believes he'll break the record. "Maybe he'll go to 18 or 20 [minutes]," he says. "Who knows?"
David's face looks calm, but Dr. Potkin says he's probably experiencing serious abdominal discomfort and diaphragm spasms at this point. "He's having pain, no question," he says.
With just one minute to go, David's heart rate starts to slow significantly. It drops from more than 100 beats per minute to 70...then 60. Dr. Potkin also notices an irregular heart rhythm. "There's low oxygen in the body, and his body's responding," he says. "He's in a survival mode right now."
As the clock passes 16 minutes, David rises to the top of the sphere and rests right below the surface. "Come on," Oprah says. "You can do it. You can do it!"
Oprah and the audience hold their breaths as the final seconds tick down.
Watch the pressure-filled final test!