The hit to the chest Buffalo Bills safety Damar Hamlin took during the first quarter of the team’s Jan. 2 game against the Cincinnati Bengals was routine. The result wasn’t. Hamlin dropped to the turf with a cardiac arrest, and his life was saved only by the timely intervention of team doctors who administered CPR and applied an electronic defibrillator. Hamlin spent more than a week hospitalized before fully recovering and being sent home in good health.

Yesterday, Hamlin announced at a press conference that the team had cleared him medically and he intends to return to playing football in the upcoming season. The question that had not been answered between January and yesterday was just what had caused Hamlin’s cardiac arrest, but the answer finally came at the press conference: an exceedingly rare condition known as commotio cordis that is caused by a blow to the chest at a precise moment in the heart’s cycle—and not by any underlying heart condition. Here’s what you need to know about the incident:

• Commotio cordis is the result of a blunt force blow when the cardiac muscle has completed a contraction and its electrical signals are undergoing what cardiologists call "repolarization." Says Dr. Lawrence Phillips, medical director of outpatient cardiology at NYU Langone Health: “Each muscle cell is electrically charged, then that charge gets released and it has to reset. That’s repolarization.”
• Anyone can suffer a case of commotio cordis, regardless of heart health. And doing so requires colossally bad luck: repolarization takes place in just 40 milliseconds or so. Still, there are 30 or so commotio cordis cases in the U.S. each year, often in boys and young men playing sports in which a hard object like a baseball or lacrosse ball strikes the chest.
• Adults are less likely to suffer commotio cordis because the rib cage and sternum thicken with age, providing greater protection to the heart. What’s more, Hamlin was wearing football’s standard shoulder pads, which partially protect the chest. Still, the violence of the hit—also a standard part of an NFL game—was enough to overcome that protection.
• Hamlin is unlikely to be at a risk of another commotio cordis incident. “Commotio cordis is a rare event and is related to trauma to the chest at a very specific moment in the heart’s cycle,” says Phillips. “A person should not be at an increased risk of it occurring again.” Hamlin is betting his career—and his very life and health—on that fact.

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