On Monday night, Buffalo Bills player Damar Hamlin collapsed after making a tackle during a football game, and according to cardiac experts, a "pretty unusual" event may have been the cause.

What may have caused Hamlin's collapse

Following his collapse, medical personnel administered CPR to Hamlin for nine minutes. After his heart rhythm was restored, he was taken to the University of Cincinnati Medical Center where he is still in critical condition.

Cardiac experts said what likely caused Hamlin's collapse was an erratic heart rhythm known as arrhythmia, specifically a phenomenon called commotio cordis, which according to Mariell Jessup, chief science and medical officer at the American Heart Association, "is a cardiac arrest after a blunt force trauma to the chest."

"It is thought that when an object, usually a ball, hits the chest, it hits in a vulnerable cycle of the heart so that it begins an electrical instability and an arrythmia, which we call ventricular fibrillation," Jessup said.

According to Javid Moleshi, a cardiologist at the University of California San Francisco School of Medicine, there's only a 20-millisecond interval in the heart's cycle where a strong blow could cause an arrhythmia.

Aaron Baggish, a sports cardiologist at Harvard Medical School who has worked with the New England Patriots, said that for an arrythmia to occur, there has to be "a perfect storm," meaning the chest has to be struck at the exact right place at the exact right time when the heart is relaxing after squeezing out blood.

Baggish added that a blow to the chest "is definitely a front-runner" to explain what happened to Hamlin but cautioned that "it is premature to be definite."

"It's not rare, but it's pretty unusual," said Michael Mack, chair of cardiovascular services at Baylor Scott & White Health. Most incidents of commotio cordis have happened when a baseball player was struck in the chest by a ball. Incidents have also occurred in hockey and lacrosse, and in all cases, chest protectors didn't prevent the event from happening.

Baggish said he wouldn't count on football pads being able to prevent the arrythmia from happening either, adding that, to his knowledge, "no commercially marketed pads for any sport guarantee complete protection." Mack also noted that "the force of a blow can go right through" a pad.

Experts say the concern now is whether the lack of blood in Hamlin's body during the minutes before his heart rhythm returned damaged his brain. According to Mack, if a patient in cardiac arrest doesn't recover immediately, doctors may induce a coma to allow the brain to rest. Sometimes doctors will cool the brain with cooling blankets as well in an effort to slow its metabolism while it recovers.

"The more concern there is about brain injury, the more aggressive doctors are about sedation and hypothermia," Mack said. If Hamlin remains unconscious for 72 to 96 hours following his cardiac arrest, "there is real concern," Mack added. (Kolata, New York Times, 1/3; Zakrzewski/Weber, Washington Post, 1/3; Hutto, MedPage Today, 1/3)