Damar Hamlin’s Cardiac Arrest: What We Know and Don’t Know

Five cardiac experts said that a freak accident may have caused the cardiac arrest that sent Hamlin to the ground.

It is not known what caused Mr. Hamlin's heart to stop. A blow to his chest during a collision with a receiver on the opposing team could be a sign of an arrhythmia.

While a blow to the chest is definitely a front-runner to explain what happened on Monday, it is premature to be definite. Mr. Hamlin will most likely have extensive testing done to make sure there was no other reason.

In order for arrhythmia to occur, the chest needs to be struck with the right amount of force and the heart needs to relax.

Dr. Michael Mack is the chairman of cardiovascular services at a Dallas hospital. Baseball, lacrosse, and hockey have all had players hit in the chest by balls. The incidents wouldn't have happened if chest protectors had prevented them.

An arrhythmia can be caused by a 20-millisecond interval in the heart's cycle.

Medical personnel at the game tried to resuscitate Mr. Hamlin for nine minutes as they tried to keep his blood flowing. The University of Cincinnati Medical Center is where Mr. Hamlin is in critical condition.

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Mr. Hamlin was put on a respirator. He said that if he is still on the respirator, he has not woken up.

The concern is if a lack of blood damaged Mr. Hamlin's brain when his heart rhythm was not normal. Dr. Mack said that if a patient in cardiac arrest doesn't recover immediately, doctors will often induce a coma to give the brain time to rest. Doctors sometimes use cooling blankets and headpieces to cool the brain while it heals.

The more aggressive the doctors are, the more concerned they are about brain injury. There is a real concern if Mr. Hamlin is unconscious for 96 hours after his cardiac arrest.

Marathoners have collapsed and died when their hearts stopped from arrhythmias. Hypertrophic cardiomyopathy is a heart-signaling disorder in which the heart walls are thin.

They can't rule them out, but experts don't think Mr. Hamlin has them. The N.F.L. extensively tested its athletes and would be expected to find problems.

When an arteries supplying blood to the heart is blocked, it's called a cardiac arrest. A sudden death can be triggered by that.

A professor of internal medicine and cardiovascular disease at the University of Michigan asked if it was due to a blocked arteries. It's unlikely in a young guy.

Ken Belson was a contributor.