What caused Damar Hamlin's cardiac arrest? Doctors say his heart was likely healthy before collapse

A potential cause of Buffalo Bills safety Damar Hamlin's jarring collapse and cardiac arrest — witnessed in real time by millions of viewers of "Monday Night Football" — was immediately recognized by heart experts who also happened to be watching the game.

"I knew exactly what was going on," said Dr. Nahush Mokadam, the division director of cardiac surgery at Ohio State University Wexner Medical Center. "The way he first stood up and then collapsed ... it's not what a concussion would look like."

In a statement released Tuesday, Hamlin's family thanked first responders and health care professionals at University of Cincinnati Medical Center, where he is being treated.

"On behalf of our family, we want to express our sincere gratitude for the love and support shown to Damar during this challenging time," the family wrote.

As of Tuesday afternoon, no briefing had been scheduled at the hospital, NBC News confirmed.

Neither Mokadam nor any other physician interviewed for this story is involved in Hamlin's treatment. In a statement, the Bills said only that Hamlin suffered a cardiac arrest, when the heart stops beating properly, and is in critical condition.

It was during the first quarter of Monday night's game between the Bills and the Cincinnati Bengals when Hamlin, 24, tackled a Bengals receiver, who collided with his chest. Hamlin stood up after the tackle but immediately collapsed.

While there are several potential causes for Hamlin's cardiac arrest, cardiologists suggested that a rare phenomenon called "commotio cordis" was to blame.

In such cases, "there is nothing wrong with the heart," said Dr. Hari Tandri, the director of the cardiac arrhythmia program at Vanderbilt University Medical Center in Nashville, Tennessee. A healthy heart, when hit with blunt force at a specific time, Tandri said, can launch into an abnormal and potentially deadly rhythm.

A spokesperson for the American Heart Association, Dr. Comilla Sasson, an emergency medicine physician in Denver, said: "It's not about how hard of a hit it was. It’s actually about the timing of when the blow happens."

Normally, the heart pumps oxygen-rich blood throughout the body about every second. There is a rhythm to the process, keeping the blood flowing at a healthy pace. But every time the heart beats, there is a tiny moment — less than a fifth of a second — that makes it vulnerable to the force of a projectile, such as a hockey puck or a baseball, that can lead to a chaotic and potentially deadly heart rhythm.

It is in this exact moment, experts say, that a blow to the chest in the exact right place can launch an otherwise healthy person into cardiac arrest. The heart's electrical system malfunctions, and the heartbeat rhythm goes haywire.

Seconds after such an injury are critical to the patient's survival, Sasson said.

"For every one minute that you don't have CPR performed, your chances of survival go down by about 10%," she said. In Hamlin's case, medical personnel on the sidelines rushed in to perform CPR until he was stable enough to be taken by ambulance for further treatment.

It's thought that commotio cordis occurs about 15 to 20 times a year in the U.S., mostly in adolescents participating in sports like baseball, hockey or lacrosse, said Dr. Mark Link, a cardiologist at the University of Texas Southwestern Medical Center.

It's even rarer for people over age 20, because ribs harden with age and are better able to protect against blunt trauma, said Link, an expert in commotio cordis who is a clinical cardiac electrophysiologist specializing in heart rhythm problems.

NFL players undergo health screenings, which may include scans of the heart, to identify potential health problems long before they hit the field.

That suggests that an underlying cardiac condition would be unlikely to go unnoticed, Mokadam said.

But there are several other reasons a person may go into cardiac arrest. Sometimes, a blood vessel within the heart muscle swells and bursts.

Mokadam said that is unlikely to have happened in Hamlin's case. "If it was an aneurysm that burst, he would have needed emergency open heart surgery," he said. "CPR and an AED aren't going to take care of the problem." (AED refers to a form of defibrillation treatment.)

What's next?

The 24 hours after such an injury are critical, doctors say. Physicians are most likely doing a number of tests to make sure there are no underlying heart problems that could have led to Hamlin's collapse or injuries he sustained after Monday night's collision. They could include an ultrasound of the heart, a cardiac MRI and CT scans of the brain.

Follow NBC HEALTH on Twitter & Facebook.