Damar Hamlin's shocking midgame collapse has people hoping for the best and looking for answers.
The Buffalo Bills said in a tweet early Tuesday morning that the 24-year-old, who is in critical condition, experienced a cardiac arrest after a hit. The team later stated he was still in the intensive care unit after spending the night there.
The scene horrified players and onlookers alike: Hamlin got up from a tackle and stepped forward before falling backward, and he was then down on the field for some 10 minutes as medical staff gave him CPR.
While Hamlin's team and family have yet to confirm exactly what happened, many of the doctors following his case online have narrowed it down to one likely cause: commotio cordis (kuh-MOH-dee-oh KOR-dis).
That's when a blunt blow to the chest, delivered at a specific point in the cardiac cycle, induces a dangerous, life-threatening heart arrhythmia or cardiac arrest.
It most commonly occurs in sports settings but is rare overall, with only 20 to 30 cases reported each year, according to Dr. Christopher Madias, the director of the New England Cardiac Arrhythmia Center at Tufts Medical Center.
"It has to be a perfect storm of events where there's an impact to the chest wall overlying the heart with just enough force, and what's most critical is the timing," Madias told Morning Edition on Tuesday. "It happens within a critical period within the cardiac cycle. We're talking about 20 to 30 milliseconds within the cardiac cycle that the heart is vulnerable to this."
Madias stressed that the first few minutes after such an event are critically important.
"The biggest issue with survivability of sudden cardiac arrest, sudden arrhythmic arrest, is how quickly CPR and, even more importantly, defibrillation can be initiated," he said.
In Hamlin's case, responders delivered both of those things immediately and were able to restore his heartbeat at the scene.
And though his prognosis remains unclear, Madias says that this medical response and the fact that Hamlin is young and presumably otherwise healthy are "all aspects that could lead to a potential full recovery."
Here's what else to know about commotio cordis.
It's seen most commonly in projectile sports
Commotio cordis is most commonly seen in sports like baseball, hockey and lacrosse, which involve what Madias calls "blunt projectiles that impact the chest wall" — like a ball or a puck.
That's not to say that football isn't dangerous, but the sport is more commonly associated with head injuries than heart issues. The NFL has repeatedly come under fire for its concussion protocols, and Monday's incident has raised questions about the length of time it took for the game to eventually be postponed (the league announced Tuesday afternoon that the game will not resume this week).
The National Commotio Cordis Registry reported 224 cases between 1995 and 2010, according to a 2010 New England Journal of Medicine article, though many cases likely go unrecorded.
Commotio cordis primarily affects young people between the ages of 10 and 18. The New England Journal of Medicine article says that 95% of known victims in the U.S. are boys or men and 78% are white.
It goes on to say that about half of commotio cordis cases have been reported in young athletes competing in amateur organized sports who "receive a blow to the chest that is usually (but not always) delivered by a projectile used to play the game.
Another quarter of commotio cordis events have been caused by recreational activities played at home or on playgrounds, and the final 25% have involved incidents like scuffles, fights and accidents in non-sports settings.
Commotio cordis is "usually, though not invariably, fatal," according to the journal article, which also cites registry data showing that its survival rates have increased over time.
The article attributes that improvement to rising public awareness, increased availability of automated external defibrillators (AEDs) and earlier and faster lifesaving actions by bystanders.
There are strategies to prevent it
Commotio cordis is so rare that Madias says he is not aware of any particular actions the NFL is looking at to try to prevent it.
He says, however, that there have been studies in youth sports looking at protective equipment like padded chest guards.
The New England Journal of Medicine article says that while it's unrealistic to try to eliminate commotio cordis entirely, organized sports hold the most potential to prevent such events and the risk of death from them.
Preventive measures can take several forms. For one, the article says, coaches can teach inexperienced batters to turn away from the ball to avoid "errant" pitches, and they can instruct defensive lacrosse and hockey players to avoid using their chests to block the ball or puck at the goal.
And design improvements in commercial sports equipment — like "safety baseballs" made entirely of rubber or air-filled balls in other sports — could also help. The article's authors caution that chest protectors and vests, at least in the form available at the time, were not necessarily effective and could even provide a false sense of security.
It's also important for players and spectators to be alert and proactive if they see an athlete collapse on the field — and for them to learn CPR.
"Deaths have often been associated with the failure of bystanders to appreciate the life-threatening nature of the collapse and to initiate appropriately aggressive and timely measures of resuscitation," the authors state.
Another key factor is the availability and use of AEDs, which are portable medical devices that can be used after a cardiac event to analyze the heart's rhythm and deliver an electric shock if needed.
Citing national registry data, the article's authors say a public health strategy that makes AEDs available at youth sporting events and in recreational settings is "likely to result in the survival of more young people in the event of commotio cordis."
Several states have either proposed or adopted laws that require AEDs at school athletic events and in gyms, health clubs and other public spaces, according to the National Conference of State Legislatures (AED Brands tracks each state here).
New York — where Hamlin's home team is based — passed a law in 2002 requiring all public schools to have AEDs in their buildings and at sporting events. Louis' Law is named after a 14-year-old student who died of commotio cordis in 2000 while playing lacrosse at his first high school game.
Here are Red Cross guides to performing CPR and using an AED.
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