The stroke that Pennsylvania Lt. Gov. John Fetterman had on Friday was caused by a clot from his heart, which was out of rhythm for too long, the U.S. Senate candidate said Sunday in a statement.

That condition, called atrial fibrillation, is a common cause of strokes, but what exactly does it mean?

When the heart beats normally, the left atrium, the top left chamber of the heart, beats smoothly and rhythmically, 70 times a minute, and sends blood to the chamber below, the left ventricle.

» READ MORE: John Fetterman suffered a stroke just days before Pa. Senate primary but says he’s recovering well

“In fibrillation, the atrium’s just wiggling, like a bag of worms, and it’s not really squeezing,” said Mark Victor, a cardiologist with Cardiology Consultants of Philadelphia who is not involved in Fetterman’s treatment. “The blood just drains from the atrium to the ventricle literally just by hydraulic force,” rather than being pumped.

What’s dangerous is when blood in an appendage off the atrium starts to congeal, forming a clot.

“Every so often, that’s where the clot sneaks out to the atrium, down into the ventricle, up into the brain,” Victor said.

Fetterman’s statement, which included a video of him and his wife, Gisele, said doctors at Lancaster General, which is part of Penn Medicine, removed the clot, reversing the stroke. A Fetterman spokesperson said the procedure lasted less than an hour. Penn, acting on Fetterman’s request, provided no additional information and did not make the doctors who treated the 52-year-old Fetterman available for comment.

There are two ways to remove a clot, said Erol Veznedaroglu, a cerebrovascular surgeon with Global Neurosciences Institute, a company that treats stroke victims and other patients at numerous hospitals in the Philadelphia region. For many patients, a clot-busting drug is sufficient, he said.

That alternative, for large clots that get into an artery in the brain, is to go “in through the groin and put a catheter all the way up to the clot in the clogged artery in the brain” where a stent can be used to capture the clot allowing it to be pulled out, Veznedaroglu said.

After watching the video of Fetterman and his wife, Veznedaroglu said he expected Fetterman to make a full recovery.

Victor, the cardiologist, said typically people in their 50s with atrial fibrillation would not be treated for the condition unless they had other risk factors, such as diabetes, heart failure, kidney disease, or hypertension.

“If he had nothing else wrong with him, he just had lone atrial fibrillation … he wouldn’t typically be considered at high risk for a stroke,” Victor said, “but that doesn’t mean it doesn’t happen. It just means it’s much rarer than if he were 71.”