Before 1958, there was no such thing as CPR. If someone’s heart stopped because of cardiac arrest, that person fell to the ground, with no pulse, no breathing. And they were simply declared dead.

All of that changed when doctors at Johns Hopkins Hospital in Baltimore discovered cardiopulmonary resuscitation by accident when studying how to shock a heart. When they pressed defibrillator panels to the chest of a lab dog, they saw a blip in blood pressure. They pressed harder and discovered they could push blood through the body. And thus was born CPR.

The problem is that only a tiny fraction of people who have a cardiac arrest and are not at a hospital get CPR. That, said Dr. Benjamin Abella, medical director of the Center for Resuscitation Science at the University of Pennsylvania, is “a national tragedy.” Bystander CPR can be the difference between life and death, as the case of Damar Hamlin demonstrated during a Buffalo Bills-Cincinnati Bengals football game earlier this month.

CPR is not as daunting as it seems; it can be learned in minutes. At the University of Cincinnati Medical Center, where Mr. Hamlin was treated, Dr. Jason McMullan, an emergency medicine physician, has taught CPR to everyone from Cub Scouts to people in nursing homes.

Here is what you need to know about what it is and how to do it.

What is a cardiac arrest and how do you know if someone is having one?

A cardiac arrest, also known as sudden cardiac death, occurs when the heart suddenly stops beating.

The person collapses to the ground and is unconscious, appears lifeless, has no pulse and is not breathing. There can be many causes.

Why do you need to do CPR? Why not just call 911?

“You should call 911 but it can take several minutes or more for the ambulance to arrive,” said Dr. Sumeet Chugh, director of the Center for Cardiac Arrest Prevention at Cedars-Sinai in Los Angeles. “In the meantime, with every minute that goes by the chances for a successful revival decrease by ten percent.”

How do you perform CPR?

It used to be more complicated — you had to alternately press on the person’s chest and give mouth-to-mouth resuscitation. But about a decade ago, medical experts realized that mouth-to-mouth was not making things better, was difficult for the public to do and was a disincentive for people to learn CPR.

“Kissing dead people is gross,” Dr. McMullan said.

Research studies suggested that survival is the same with or without mouth-to-mouth. Most people whose hearts stop have enough oxygen in their blood to survive if blood can be pumped through their bodies and, especially, to their brain.

Now CPR is “hands only.” Aim for the center of the chest over the breastbone, or sternum.

“Join your hands together and push hard and fast on the chest 100-120 times a minute to the beat of the song “Staying Alive,” by the Bee Gees, Dr. Chugh said.

There are videos showing how to do it.

And, Dr. McMullan said, even if you don’t push hard enough or fast enough, “any CPR is better than no CPR.”

It is possible — though unusual — to break the person’s ribs. But when that happens it is “very unlikely to have any clinical consequences,” Dr. Abella said. “Being tentative about ribs can mean worse CPR and worse survival,” he added.

Why do you have to push so fast in CPR?

“You want to maximize the output from the heart,” Dr. Abella said. “CPR is not very efficient in moving blood. The heart is a better pump, so to make up for that you go faster. You can’t hurt someone. They are already dead.”

How long do I keep doing CPR?

You should keep it up until an ambulance arrives or until another rescuer can take over for you.

What about using a defibrillator?

An automated external defibrillator, or A.E.D., shocks the heart and resets it. When you open the glass box that holds an A.E.D., the device should speak to you, telling you to place two pads on the person’s body according to a diagram. It analyzes the heart’s rhythm and, if a shock is appropriate, tells you to stand back, press a button and shock the heart.

“For the cardiac arrests that are shockable it is almost miraculous,” Dr. Abella said. “I have seen patients in cardiac arrest who, by the time they are in the emergency room, wake up and wonder why they are there.”

But defibrillators have to be applied quickly, and so it is important to have more than one rescuer — one to do CPR while the other gets the defibrillator.

“If you are doing CPR you should shout and shout for help,” Dr. Abella said. “If you are alone you should run and get an A.E.D.,” if you know one is nearby.

Finding a defibrillator is not always easy. Airports often show their locations on terminal maps.

“The problem is, if you are not in an airport, what do you do?” Dr. Abella said. “For example, I am in a parking structure in the hospital. I don’t know where the nearest A.E.D. is and I am a doctor in a hospital. I have no idea.”

If you spend a lot of time in a public place, it could pay off to learn the location of the nearest A.E.D.

What if you forget all of this when someone collapses?

Many cities have dispatch-assisted CPR. When you call 911, the operator will walk you through the steps.

What if I do it wrong? Can I get sued?

“If they are unresponsive and there is no breathing or pulse beat, they are going to die without CPR and it is highly unlikely that you will make the situation worse,” Dr. Cheung said. There are good Samaritan laws in all 50 states that protect bystanders, he added. And the federal Cardiac Arrest Survival Act, signed into law in 2000, provided civil immunity for users of A.E.D.s. When someone is unresponsive, consent to CPR is implied.

“There has never been a successful lawsuit against someone who has provided CPR with the intent of saving an individual’s life,” Dr. Chugh said.