Incredibly Rare Hit May Have Felled Bills Player

Serious injuries in the NFL these days often involve the head. But in the case of Monday night's shocking injury to Damar Hamlin of the Buffalo Bills, it involved the heart. The 24-year-old suffered cardiac arrest and required CPR and the use of a defibrillator on the field. He remained in critical condition as of Tuesday morning. A look at what happened:

• The play: It looked pretty routine, notes the AP. As Hamlin was tackling Cincinnati's Tee Higgins, the latter "led with his right shoulder, hitting Hamlin in the chest." (Nobody is faulting Higgins.) Hamlin got up, but then collapsed back to the field.
• Cardiac arrest: Hamlin suffered cardiac arrest, defined by the Mayo Clinic as "the abrupt loss of heart function, breathing, and consciousness." It "usually results from a problem with your heart's electrical system, which disrupts your heart's pumping action and stops blood flow to your body." As the Washington Post explains, the blow to Hamlin's chest appears to have caused that disruption.

• Rarity: Heart doctors (not involved in Hamlin's treatment) say it's possible the hit triggered a rare condition known as commotio cordis. "This is a traumatic injury to the anterior chest and it has to happen at the exact time when the heart is getting prepared to beat again," Dr. Anthony Cardillo told CNN, via KHOU. The window in which this can happen is only 40 milliseconds long, cardiologist Gregory Marcus tells the Post. "It's almost like getting struck by lightning," adds Cardillo, per CNN. "It's that rare. There has to be acute trauma to the anterior chest just at that right moment."
• More on that: Commotio cordis usually happens in athletes ages 8 to 18 in sports where projectiles (balls, pucks, etc.) can strike them in the chest at the worst possible moment, even at low impact, per an explainer at the University of Connecticut. To be clear, no official diagnosis has been revealed about Hamlin, and it's possible something else could have caused his heart to stop functioning, including an aneurysm triggered by the collision or an undiagnosed preexisting condition.
• Key question: Regardless of the cause, the big question now is how long Hamlin's brain went without oxygen. The CDC notes that most who suffer cardiac arrest outside a hospital die, but Hamlin received expert treatment quickly. Plus, he's a 24-year-old athlete, also in his favor. The next 24 hours will be critical, with the hospital expected to wean him off a ventilator to see whether he can breathe on his own, Cardillo tells CNN.

(More Damar Hamlin stories.)