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ST. LOUIS – An unprecedented scene unfolded in front of millions Monday night, as the NFL suspended the prime-time game between the Buffalo Bills and Cincinnati Bengals after Bills safety Damar Hamlin suddenly collapsed after making a tackle.

Players on both teams were seen visibly shaken as medical staff performed CPR on Hamlin.

The Bills said the 24-year-old went into cardiac arrest, but his heartbeat was restored prior to leaving the stadium by ambulance.

“It’s usually from an abnormal heart rhythm from the bottom part of the heart that prevents the blood from flowing around,” Dr. Marc Sintek, a cardiology specialist at Barnes-Jewish Hospital, said. “You think about how many football games, basketball games, soccer games, whatever you’ve watched; it doesn’t happen too often.”

Sintek said cardiac arrest is most commonly due to a disorder that causes an abnormal heart rhythm. But it can also occur due to a structural problem with the heart or by commotio cortis, which is caused by blunt trauma to the chest, resulting in an arrhythmia.

“They have an abnormal heart rhythm because of that hitting to the chest,” he said. “That is a very rare cause of sudden cardiac death in an athlete. It is really outstanding and unlucky that it can happen at that split second.”

Former Illinois football and NFL player Carey Davis said what happened to Damar Hamlin shines a light on how dangerous the game of football can be.

“It is something that we have never seen before or that I have ever seen before in the NFL,” he said.

“This is a game. This is not life or death. This is something that I do to provide for myself. To provide for my family. But I want to be able to walk when I’m done playing this game. I want to be able to talk when I’m done playing this game and spend time with those that are near and dear to me.”

Carey said the NFL community of players is a tight nit group, and hopes the outpouring of support continues for Hamlin, his family, and Bills teammates during an unprecedented time.