A potential cause of Buffalo Bills safety Damar Hamlin's jarring collapse and cardiac arrest — witnessed in real time by millions of viewers watching "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, 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 Hamlin 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 has 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 Buffalo Bills has only said that Hamlin suffered a cardiac arrest, when the heart stops beating properly, and is now in critical condition.
It was during the first quarter of Monday night's game between the Buffalo Bills and Cincinnati Bengals when Hamlin, 24, tackled a Bengals receiver, who collided with Hamlin's 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, 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.
"It's not about how hard of a hit it was," said Dr. Comilla Sasson, an emergency medicine physician in Denver and a spokesperson for the American Heart Association. "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 the heart 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 heart beating 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, cardiologist Dr. Mark Link of the University of Texas Southwestern Medical Center said.
It's even more rare for people over the age of 20 because ribs harden with age and are better able to protect against blunt trauma, according to Link, who is a clinical cardiac electrophysiologist specializing in heart rhythm problems and an expert in commotio cordis.
NFL players undergo health screenings, which may include scans of the heart, to identify potential health problems long before they hit the field.
This 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 this 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."
The 24-hour period after such an injury is critical, doctors say. Physicians are 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 sustained following Monday night's collision. This could include an ultrasound of the heart, a cardiac MRI and CT scans of the brain.