After 23 minutes of breathing pure oxygen, David takes what will be his last breath for—hopefully—the next quarter of an hour. The last thing he does, Dr. Potkin says, is a technique called "lung packing."
"He's filled up his lungs totally as much as they can be with air. He's forcing air into his lungs from the top of the lungs. They're already filled as much as they can be," he says. "Pulmonary doctors a year or so ago didn't even think this was possible that somebody could increase [his] lung size voluntarily. As he does this, his lungs are re-expanding, and his blood pressure and his heart rate can dramatically change as well."
With the clock ticking away, David remains motionless in his 20,000-pound sphere. Every slight movement uses his oxygen, which is made even more precious by David's high heart rate. Steady at around 100 beats per minute, David's heart rate has dropped slightly, but it's still higher than what Dr. Potkin would like it to be.
After an unbelievable eight minutes without breathing, David gets his first time notification from Mandy-Rae and Kirk, two free divers who have helped him with his training. "Until now, he doesn't really know how long he's been holding his breath," Mandy-Rae says. "Now, Kirk is going to start letting him know every once in a while."