The concern is whether, during those long minutes before Mr. Hamlin’s heart rhythm returned, when his heart was not pumping normally, a lack of blood damaged his brain, heart experts said. If a patient in cardiac arrest does not recover right away, doctors will often induce a coma to give the brain a chance to rest, Dr. Mack said. Sometimes doctors also cool the brain to slow its metabolism while it recovers, using cooling blankets — which have coils that circulate chilled water — and headpieces.

“The more concern there is about brain injury, the more aggressive doctors are about sedation and hypothermia,” Dr. Mack added. If Mr. Hamlin remains unconscious from 72 to 96 hours after his cardiac arrest, “there is a real concern,” Dr. Mack said.

Other athletes, like marathoners, have collapsed and even died when their hearts stopped from heart attacks or from arrhythmias caused by underlying heart abnormalities. Some examples, Dr. Mack said, are hypertrophic cardiomyopathy, in which the heart walls are thickened, or long QT syndrome, a heart-signaling disorder.

But, though they can’t rule them out, heart experts doubt that Mr. Hamlin has these conditions. Dr. Rajat Deo, an arrhythmia specialist at the Perelman School of Medicine at the University of Pennsylvania, said the N.F.L. extensively tested its athletes and would be expected to find such problems.

Cardiac arrests differ from heart attacks, which are caused when an artery supplying blood to the heart is blocked, depriving the organ of blood. That can trigger an arrhythmia or even sudden death.

“Could it have been due to a blockage?” asked Dr. Brahmajee Nallamothu, a professor of internal medicine and cardiology at the University of Michigan. “I guess, but probably unlikely in a young guy.”

Ken Belson contributed reporting.