A matter of milliseconds might have been the difference between Damar Hamlin continuing on to the next play rather than collapsing and in need of lifesaving measures.

“If you get hit in the chest,” University of Maryland Medical System cardiologist Dr. Scott Jerome said, “if it happens between heartbeats in a very small window, it can put the heart in ventricular fibrillation.

“The heart stops beating,” said Jerome, an assistant professor of medicine.

Hamlin, a safety for the Buffalo Bills, was in critical condition early Tuesday after suffering cardiac arrest in a collision with Cincinnati Bengals wide receiver Tee Higgins. He got up but then collapsed, and as those in the stadium and a “Monday Night Football” audience watched aghast, medical personnel administered CPR and an automated external defibrillator, or AED, was used on him. He was taken by ambulance to the University of Cincinnati Medical Center.

“Damar Hamlin suffered a cardiac arrest following a hit in our game versus the Bengals. His heartbeat was restored on the field and he was transferred to the UC Medical Center for further testing and treatment,” the Bills said in a statement around 2 a.m. Tuesday. “He is currently sedated and listed in critical condition.”

Jordon Rooney, a family representative who described himself as a good friend of the player, told ABC’s “Good Morning America” on Tuesday that Hamlin’s relatives are in good spirits but going through a lot and need their privacy. He declined to give details on Hamlin’s condition other than to say he is sedated.

“All I can say is he’s fighting; he’s a fighter,” Rooney said.

Jerome, who was not watching the game but looked at replays, said late Monday night that it appeared that Hamlin suffered commotio cordis, Latin for “agitation of the heart,” from being struck in the chest at a particular time in the heart rhythm cycle. That causes an interruption in the heart’s electrical signal, and cardiac arrest.

“It’s a very narrow window,” Jerome said.

The phenomenon has been seen in youth baseball and in lacrosse when balls strike players in the chest, he said.

Young athletes are more vulnerable compared with older, professional athletes who are more capable of absorbing the blow.

“These football players, they’re beefy and have a lot to absorb the hit,” Jerome said.

Commotio cordis is rare, with fewer than 30 cases reported every year, according to an article in the National Library of Medicine.

On the play the 6-foot, 200-pound Hamlin was injured, Higgins led with his right shoulder, which hit the defensive back in the chest. Hamlin then wrapped his arms around Higgins’ shoulders and helmet to drag him down. Hamlin quickly got to his feet, appeared to adjust his face mask with his right hand and then fell backward about three seconds later and lay motionless.

Hamlin was treated on the field by team and independent medical personnel and local paramedics, and he was taken by ambulance to University of Cincinnati Medical Center. Teammate Stefon Diggs later joined Hamlin at the hospital.

About 100 Bills fans and a few Bengals fans gathered on a corner one block from the emergency room entrance, some of them holding candles.

Jeff Miller, an NFL executive vice president, told reporters on a conference call early Tuesday that the league had made no plans at this time to play the game, adding that Hamlin’s health was the main focus.

An ambulance was on the field four minutes after Hamlin collapsed while many players embraced, including quarterbacks Buffalo’s Josh Allen and Cincinnati’s Joe Burrow.

“Please pray for our brother,” Allen tweeted.

Hamlin collapsed at 8:55 p.m., and when he was taken off the field about 19 minutes later in what seemed like an eternity, the Bills gathered in prayer. A few minutes after the ambulance left the field, the game was suspended, and players walked off the field slowly and into their locker rooms where they awaited word on Hamlin and the game.

“I’ve never seen anything like it since I was playing,” NFL executive Troy Vincent, a six-time Pro Bowl cornerback during his career, said in the conference call early Tuesday. “Immediately, my player hat went on, like, how do you resume playing after seeing a traumatic event in front of you?”

Hamlin’s uniform was cut off as he was attended to by medical personnel. ESPN reported on its telecast that Hamlin was also given oxygen.

Vincent said the league took no steps toward restarting the game and did not ask players to begin a five-minute warmup period as ESPN’s broadcasters had announced.

“It never crossed our mind to talk about warming up to resume play,” Vincent said. “That’s ridiculous. That’s insensitive. That’s not a place we should ever be in.”

After Monday’s collapse, Hamlin instantly became one of the biggest stories in sports, and thousands of people found his GoFundMe page.

The result: roughly $3.7 million donated in the first 12 hours. And the number is climbing.

A fundraiser that as of last month had raised $2,921 was up to $3,637,590 by 10 a.m. Eastern on Tuesday — with about 130,000 people going online in that span to donate, on average, about $28. Some of the donations were smaller. Some were more than $5,000. On average, about three donations were being made every second in that initial 12-hour span.

And many came with messages of hope for a 24-year-old player in his second season, sedated in a Cincinnati hospital, listed in critical condition and with some teammates unwilling to return to Buffalo just so they could remain close to him.

“There are moments in life that stop the world,” wrote Michael Lynch, who donated Tuesday morning. “We all pray for two things. Your speedy recovery and that your impact to the world is enhanced by your go fund me.”

The messages poured in from different fan bases, many of the donors letting the world know that they support other teams. One came with a hashtag that read, “we are all Bills fans.”

Hamlin started the GoFundMe in December 2020. He was just wrapping up his college career and getting ready for the NFL draft process. And he wanted to have a toy drive at Kelly and Nina’s Daycare Center in his hometown of McKees Rocks, Pennsylvania, a place with about 6,000 residents along the south bank of the Ohio River.

“As I embark on my journey to the NFL, I will never forget where I come from and I am committed to using my platform to positively impact the community that raised me,” Hamlin wrote when setting up the drive. “I created The Chasing M’s Foundation as a vehicle that will allow me to deliver that impact, and the first program is the 2020 Community Toy Drive. This campaign gives you the opportunity to contribute to our first initiative and positively impact children who have been hardest hit by the pandemic.”

He pulled the first event off with about 10 days of planning. Gifts poured in, some of it clothing donated by Pitt, where he had just finished playing. Hamlin’s upbringing was far from easy: He lost three friends to gun violence while growing up and saw his father incarcerated for about 3 1/2 years for selling drugs. But as soon as he was able, Hamlin wanted to help others.

So, he started the toy drive. And on Monday, the world finally noticed.

Many of the donations came from Bills fans, affectionately known as Bills Mafia, and this is far from the first time they’ve gone online to show support. In recent years, Bills fans have shown support for Miami quarterback Tua Tagovailoa’s foundation after he left a game — also in Cincinnati — with a concussion; for Baltimore quarterback Lamar Jackson’s charity after the Ravens lost a playoff game to Buffalo; and many made donations of $16.88 to the P.U.N.T. Pediatric Cancer Collaborative in western New York following the death this summer of FIU player Luke Knox.

–The Associated Press and Baltimore Sun contributed