What happened to Damar Hamlin's heart?

There is a "Pray for Damar" sign at the Bills’ stadium today.

The team says Hamlin spent the night in intensive care and remains there today in critical condition.

So, what happened to his heart?

It was a heart-stopping moment for millions as they watched the Buffalo Bills' medical team perform CPR on the field to safety Damar Hamlin. Medics also brought an automated defibrillator to his side. So, what may have happened to the 24-year-old football player in Monday night's game?

We turned to Tulane Cardiac electrophysiologist Dr. Eoin Donnellan. He has not treated Hamlin, but is an expert in the field. Signs point to sudden cardiac arrest when the heart suddenly loses its normal rhythm and stops pumping blood.