It was a scene that shocked players and fans, when 24-year-old Buffalo Bills defensive back Damar Hamlin collapsed in the first quarter of a game in Cincinnati on Monday.
Something was very wrong with his heart. Medical personnel reportedly used CPR and a defibrillator to restore his heartbeat, before transferring him to a local hospital, where he remains sedated and in critical condition, fighting for his life.
Cardiac specialists say it's too soon to know what went wrong, but a rare type of trauma called commotio cordis, also known as cardiac concussion, is among the possible culprits. It's a type of arrhythmia, when the heart cannot work effectively and blood pressure can dramatically drop.
Some doctors believe Hamlin's tackle of Bengals receiver Tee Higgins — hitting just the right spot in Hamlin's chest — moments before could have caused the trouble.
A blow to the chest of sufficient velocity and power, "at the exact right moment in the cardiac cycle," can trigger an arrhythmia, said Dr. Christopher Labos, a cardiologist in Montreal.
Commotio cordis is "exceedingly rare," Labos told CBC News, because the heart is vulnerable to it for mere milliseconds.
WATCH | What happened to Hamlin?
But, "when you have a young, otherwise healthy player who collapses with no heartbeat of a blow to the chest, there's really one diagnosis that comes to mind."
That Hamlin's heart rhythm was reportedly restored with a defibrillator "just further confirms that's what happened," he said.
Commotio cordis occurs "probably 20 times a year" in the U.S. and about 60 per cent of those affected survive, according to heart rhythm specialist Dr. Mark Link of the University of Texas Southwestern Medical Center.
WATCH | Hamlin's on-field collapse renews questions about game's safety:
In 1998, a puck struck NHLer Chris Pronger in the chest during a Stanley Cup playoff game, when the player for St. Louis was 23, and he suffered commotio cordis.
On Tuesday, he tweeted his support for Hamlin "during this incredibly scary time."
Pronger told CBC News that taking the slapshot caused his heart to skip a beat — enough to limit oxygen to the brain "in that one heartbeat" and he passed out.
He was able to continue his career, playing for more than a decade after recovering.
The main worry for survivors is brain damage from lack of oxygen when the heart stops pumping blood, Link said. Doctors can help reduce that risk with deep sedation to give the brain a rest, he said.
For every minute of delay in resuscitating someone with the condition, there is a 10 per cent increase in mortality, Dr. David Angus told the television program CBS Mornings on Tuesday.
Hamlin was down for 19 minutes while receiving medical attention.
Sports writer Matt Parrino, who has covered the Bills since 2018, and before that the UFC, has seen all kinds of injuries. But what he saw Monday "was something different."
"There was an urgency from all parties involved," after Hamlin collapsed, he said.
"I saw an EMT, I believe, on the field that had a walkie-talkie of some kind, trying to get more people out there, before the ambulance had arrived.
"And the look on her face, as she looked down at Damar Hamlin and then back to the walkie-talkie as she was kind of screaming instructions, there was a level of speed and urgency to the whole situation."
The Bills said early Tuesday Hamlin suffered a cardiac arrest following the tackle.
"His heartbeat was restored on the field and he was transferred to the UC Medical Center for further testing and treatment," the team said in a statement.