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For exactly 23 minutes before he starts holding his breath, David inhales pure oxygen. "As his lungs fill up with pure oxygen and wash out the air, it allows his breath-holding time to be much longer," Dr. Potkin says.

After 12 minutes of pure oxygen, Dr. Potkin says David's heart rate remains higher than he'd like. When it hits 106 beats per minute, Dr. Potkin says David's heart usually begins to slow to around 50 beats at this point. "That's going to be a problem if he doesn't settle down because he's using up more oxygen," he says. "This whole thing is about slowing the heart rate down. I expect that David's heart rate's going to be in the 40s, 30s, maybe even down to the 20s."

Dr. Potkin says most doctors would be highly concerned about letting someone's heart rate drop so low. "We talked about what level we should pull him out for medical reasons, and David says he wants to push to the limit—down to 10 beats per minute," he says. "There aren't many doctors who can sit around and watch somebody with a heart rate of 10 beats per minute."
FROM: David Blaine Risks His Life to Break a World Record
Published on June 02, 2008