Buffalo Bills safety Damar Hamlin suffered a cardiac arrest after getting hit on Monday night during an NFL game against the Cincinnati Bengals. The play looked routine for a professional football game.

Bengals wide receiver Tee Higgins caught a pass while cutting across the field at full speed. He lowered his shoulders to prepare for Hamlin’s tackle, a bearhug that caught Higgins’ shoulder pads directly in the chest. The whistle blew. Both players hopped up. But after a beat, Hamlin collapsed on the field.

As of Tuesday afternoon, Hamlin remained in critical condition and intensive care after suffering a cardiac arrest, according to a statement released by the Buffalo Bills. The incident raises questions about how the physics of a hard hit could stop a heartbeat on a biological level.

A handful of medical conditions might explain what happened, but two cardiologists who watched replays of the hit said Hamlin may have experienced commotio cordis, an uncommon medical event where blunt force to the chest knocks the heart off its normal rhythm.

“I wasn't involved with his [Hamlin’s] care, but certainly, I've seen in the past very rarely a condition known as commotio cordis,” said Dr. Deepak Bhatt, a director of Mount Sinai Heart, the cardiovascular medicine center of the Mount Sinai Health System in New York City. “All of a sudden that can precipitate a cardiac arrest.”

Commotio cordis mostly happens in kids, during youth sports like baseball or hockey because it usually occurs when a small, hard object hits the chest at just the right angle and just the right time. That makes the condition very rare.

A widely cited medical review published by Tufts University in 2012 stated that about 10 to 20 cases were reported annually to a national registry. The review states that revival after commotio cordis only happens in about a third of cases. Such resuscitation is dependent on using a defibrillator to shock the heart back into its appropriate rhythm.

But the condition stands out in the realm of cardiac arrest because it can happen without any lasting physical injury to the heart.

“It's not that it's damaging the heart at all,” said Dr. Laurence Epstein, a cardiologist with Northwell Health in Nassau County. “Usually, it's purely an electrical phenomenon.”

A perfect storm of bad timing and just the right amount of force

As Epstein explained, the heart is a muscle, and like any muscle in your body, it uses electrical impulses to stimulate the heart cells to contract. These electrical pulses happen in and spread through three key parts of the heart in a precise sequence. This fine-tuned sequence makes up what we know as a heartbeat — where the muscle cells contract and reset (relax) over and over again.

During commotio cordis, a blunt impact essentially interrupts the sequence just as the heart muscle cells are resetting their electrical activity — or repolarizing. The result is ventricular fibrillation, where the lower heart chambers start contracting erratically creating a disrupted rhythm. Blood stops pumping through the heart and lungs and can’t pick up vital oxygen before being launched back to the rest of the body.

“The heart is just kind of wiggling and not pumping any blood,” Epstein said. “So there's no oxygenated blood going to the brain.”

All of that is in contrast to a cardiac or myocardial contusion — where blunt force traverses through the chest cavity and bruises the heart — or worse. Even though the blow doesn’t penetrate the skin, such an impact can damage heart valves or rupture the muscle tissue that makes up the organ.

Epstein said underlying conditions — like genetic disorders — can also weaken the heart muscles, making them more prone to arrest after blunt force trauma. On Twitter, misinformation began circulating soon after Hamlin’s incident, suggesting that COVID-19 vaccines might somehow be involved. Similar misinfo circulated after Danish soccer player Christian Eriksen experienced cardiac arrest during a match in 2021, even though he had reportedly not received COVID-19 vaccines at the time.

“From a scientific perspective, [this misinformation] is just idiotic,” Epstein said. “It is using a tragedy to further one's political causes, and it's really offensive.”

Damar Hamlin of the Buffalo Bills tackles Tee Higgins of the Cincinnati Bengals during the first quarter of a game at Paycor Stadium in Cincinnati, Ohio, Jan. 2, 2023. Hamlin was taken off the field by medical personnel following the play.

Research on commotio cordis, mentioned in the Tufts medical review, found that a ball moving at high speed would need to hit the chest within a very short window during a heartbeat sequence — only 40 milliseconds long — to cause the condition.

The ball also needed to be traveling at a certain speed: 40-mile-per-hour impacts caused ventricular fibrillation 50% of the time while 30-mile-per-hour balls triggered it 30% of the time. (These studies involved pigs, which are often used as stand-ins for humans in cardiac research given similarities in heart physiology.)

Some doctors posit that kids and teens are more vulnerable to commotio cordis because of thinner chest walls. There’s literally less space for force to travel between the point of contact and the heart.

That makes the 24-year-old Hamlin’s case unexpected. His chest just happened to get hit with the precise amount of force at the wrong time. Prior to being drafted, Higgins was clocked sprinting 40 yards in 4.43 seconds — or about 18 miles per hour at full speed. But everything from the wide receiver’s weight to the angle of his pads upon impact with Hamlin likely played a role.

“Too weak of a blow or too hard of a blow won't cause it,” Epstein said, adding that there’s some debate in youth sports about whether too much padding in the chest area could make kids more susceptible to commotio cordis.

CPR and defibrillators can mean the difference between life and death

After Hamlin collapsed, he reportedly received CPR on the field and treatment from an automated external defibrillator, or AED, a machine that can assess a person’s heart rhythm and shock it back to normal.

Bhatt said CPR is crucial immediately after cardiac arrest. Those chest compressions make the heart pump blood and the oxygen it carries through the body.

“Importantly, blood isn't getting from the heart to the brain [during cardiac arrest],” Bhatt said. “That's why consciousness has been lost. That's why the person passed out.” After a few minutes of this interruption, brain damage can be irreparable and ultimately lead to death.

But despite being resuscitated, the Bills stated that Hamlin was reportedly sedated upon arrival, and his marketing rep, Jordon Rooney, said Hamlin was given a breathing tube, which is called intubation.

Both Epstein and Bhatt said intubation, when a patient is connected to a ventilator, is normal after the heart forcefully stops.

“Virtually, everyone after a cardiac arrest would get intubated,” Bhatt said.

In 2020, Damar Hamlin started a community charity, and after suffering cardiac arrest during a game this week, it received an outpouring of donations and support.

It is also routine to put the patient in a coma. The brain consumes a lot of oxygen, so the interruption of blood flow during cardiac arrest can really stress it out. Epstein said brain cells can start dying and inflammation occurs during this period of stress, so one countermeasure centers around inducing a coma. He added that doctors will sometimes cool the body to lower this physical stress and allay the chances of brain injury, too.

“By putting somebody in a coma, you decrease the metabolic demands of the brain… You're hoping to limit the amount of damage,” Epstein said. “They're probably gonna keep him intubated and sedated in a coma for a day or so before trying to wake him up to see how much brain injury there is.”

But both doctors said the incident speaks to the importance of people learning CPR and expanding access to AED machines, especially at sporting events.

“Using an AED is easy, even if you've never seen one before. The machine is pretty smart,” Bhatt said. “You don't need to be a cardiologist. You don't need to be a doctor. The device does the work.”