It's the moment that made football fans across the country hold their breath.
Buffalo Bills safety Damar Hamlin collapsed on field in a Monday night game against the Cincinnati Bengals.

The Bills confirmed the 24-year-old suffered cardiac arrest.

"It's not a heart attack," said Dr. Julio Panza, director of cardiology at Westchester Medical Center. "Cardiac arrest is situation where for one of many reasons that the heart, which is a pump is not pumping right. Therefore there is no blood circulation head to toe."

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For that reason, Panza said it is crucial to get the heartbeat restored immediately.

"Any second without blood going to the brain is a risk for brain injury," he said.

Within 10 seconds of Hamlin collapsing, team trainers were on the field and began delivering lifesaving CPR before he was taken to the hospital where he is sedated and remains in critical condition.

While the cause of the medical episode is unclear, it happened moments after Hamlin tackled a Bengals wide receiver in the first quarter of the AFC matchup.

Panza said that moderate blow to the chest could have caused what is called "commotio cordis."

"It occurs as a consequence of blunt trauma to the chest that basically sets an unnormal rhythm that is not consistent with the proper function of the heart," he explained.

An incident like this at an NFL game is rare.