Recently, I wrote about the dark side of CPR. Despite a common misperception that CPR can rescue almost anyone from the brink of death, most people that receive it don't survive. Of those that do, many sustain devastating neurological injury and may never wake up. CPR often causes additional injuries that can add pain and indignity to the final moments of life, and can sometimes be traumatizing to the healthcare providers that deliver it.

I heard from many nurses, doctors, EMTs, and paramedics who were grateful that I had brought attention to the difficult reality that CPR may often cause more harm than good.

But I also heard from people who owed their lives to CPR. Nick Sakes, an avid cyclist from Minneapolis, was 58 when he collapsed on a ride at a busy intersection. A nurse in a nearby car saw him go down. He didn't have a pulse, and she performed CPR until paramedics arrived. Using a defibrillator, they found that he was in an abnormal heart rhythm called ventricular fibrillation, a common cause of cardiac arrest that is often responsive to electric shocks.

After three shocks, Sakes' heart reverted to a normal rhythm. He had a pulse again. He regained consciousness the next day, and was startled to find a team of doctors looking down at him. Apart from sore ribs, he suffered no significant injuries from his cardiac arrest, or from his resuscitation. "I haven't had any problems," he told me. He still rides his bike just as much as before. "I feel exactly the same," he said.

Henry Jampel, a professor of ophthalmology at Johns Hopkins, told me a similar story. Twenty three years ago, when he was forty four, he collapsed after a morning swim. A few months earlier he had completed the Ironman World Championship triathlon in Hawaii. He wasn't breathing, and didn't have a pulse. His workout partners started CPR.

After twenty-seven minutes, paramedics arrived. They found that he, too, was in ventricular fibrillation. After three shocks, he was back in a normal rhythm, with a pulse. He woke up in the hospital later that day, with no memory of what had happened. Six weeks later he was back at work as an eye surgeon, with no lasting injuries. He became an advocate for CPR and the widespread use of automated defibrillators; he's now the board chair of the Sudden Cardiac Arrest Foundation.

Jampel, Sakes, and other survivors of cardiac arrest who wrote to me all shared a similar worry. "Our concern about what you wrote is that a bystander might come across an individual who had collapsed," Jampel said, "and has in their mind, 'this is futile, this is hopeless, I'm not going to get involved.'" Even though CPR is not as effective as many people believe, sometimes it can mean the difference between life and death. How do we know who will benefit, and who won't?

We can't know, but we can make an educated guess. After studying CPR for sixty years, physicians have a sense of which factors tend to be associated with survival. The first is age. I wrote before that older patients do worse with CPR, on average. But that relationship cuts both ways; younger patients sometimes do much better. In 2017, researchers studying a group of about 2,000 patients in Austria found that survival after cardiac arrest at thirty days was around 25% for patients under age 65, but only 4% for patients over 65. A study conducted in Toronto of patients aged 2 to 45 with cardiac arrest found a survival rate of 21%, while average survival for all age groups from cardiac arrest tends to be about 10%.

Another factor is chronic illness. In 2014, researchers examined the effects of diseases like heart failure, cancer, cirrhosis, and kidney failure on the odds of survival in patients that received CPR. Patients with chronic illnesses were significantly less likely to survive to hospital discharge than those without them. The more severe the illness, the less likely was survival. And among the survivors, patients with a chronic illness tended to live just a few more months, while healthier patients often lived for several years.

The particular cause of a cardiac arrest matters too. Cardiac arrest can happen for a few reasons. Abnormal rhythms, like ventricular tachycardia or ventricular fibrillation, can cause sudden cardiac arrest, with no apparent warning, and can often be restored by a shock from a defibrillator. Acute illnesses like overwhelming infection, kidney failure, massive bleeding, or pulmonary embolism, on the other hand, can cause cardiac arrest which is unresponsive to shocks, and often more difficult to reverse.

Although CPR is the first step in attempting resuscitation after cardiac arrest, it's not the most important. Chest compressions circulate blood to the brain and other organs, but they don't address the cause of the arrest. As I wrote before, CPR is a bridge, not a treatment. Compressions are meant to buy time until the underlying cause can be identified and potentially reversed.

In the case of ventricular tachycardia and fibrillation, which together cause about a third of cardiac arrests, and are the most likely prove reversible, this process doesn't even require a hospital, or a doctor. Modern automated defibrillators, which are accessible in many public places, are able to identify shockable rhythms as well as deliver a shock, and don't require medical training to operate. In one study of over 13,000 patients with cardiac arrest, survival was as high as 35% in patients with one of the "shockable" rhythms, and less than 2% in patients with a non-shockable rhythm. It's these devices, more than CPR itself, that can save lives in cardiac arrest caused by a shockable rhythm.

But many people "have walked by a defibrillator in an airport fifty times and have no idea what it is," Jampel pointed out. It's a crucial link in the chain of survival that is often neglected. "We want people to be able to recognize a cardiac arrest, call 911, ask someone to find a defibrillator, and start chest compressions," Jampel told me.

Taken all together, we know that a young or middle-aged person without significant medical problems who experiences sudden, unheralded cardiac arrest—which is more likely to be caused by a shockable rhythm—has a better shot at recovery than an older person with several medical problems that suffers a cardiac arrest in the context of an acute illness like severe pneumonia. For bystanders or friends and family with CPR training, there's no reason to hesitate in offering CPR to someone meeting that first description.

Damar Hamlin, the 25 year old safety for the Buffalo Bills who suffered a cardiac arrest in the middle of a game in January, is a perfect example of CPR at its best. He was young and fit, with no known medical problems. He collapsed suddenly, suggesting a rhythm problem. Medical personnel witnessed the collapse, and began CPR immediately. And—most importantly—they applied a defibrillator, found that he was in ventricular fibrillation, and shocked him out of it.

"I doubt there are many healthy 27-year-old athletes who would say, if they collapse, I'm done, just let me go," Jampel said, and he's right. These are the patients in whom doing everything makes the most sense. If I were to collapse suddenly, in cardiac arrest, it's what I would want for myself, despite everything I know about the downsides of CPR.

But the odds of a good outcome like this decrease as we age, and as our bodies begin to accumulate medical problems. By the time we're in our seventies, or eighties, or nineties, the odds of CPR helping us get slimmer. It's a spectrum, and as we move along it, over the course of our lives, we draw ever closer to a place where death is inevitable, no matter what medical technology is at hand.

So how to decide when to opt out, and when not?

Thinking about the factors we've outlined above can help. When you're young, it might make sense to choose everything, CPR and all. As you age, if you value life above all else, then perhaps you may still opt for CPR, defibrillation, intubation, and everything else a hospital can do when your heart stops, regardless of the odds of futility, or even harm.

The harm can be considerable. As I wrote in May, CPR can cause bleeding in the lungs, lacerations to the liver, and fractured ribs or sternum. Many survivors of CPR sustain damage to their brains, and may never be quite the same again. All of these outcomes become more likely with age, frailty, or chronic illness – and the likely harm of CPR may begin to outweigh its potential benefit.

If instead you hope for a gentler, quieter death at the end of your life, with minimal medical interventions, then CPR might not be for you. Because CPR is the default action for cardiac arrest regardless of age or illness in every hospital, where most of us will die, it's critical to talk about these preferences early, or better yet, put them in writing. If you receive a new medical diagnosis, or are admitted to the hospital for an acute problem, that's a good time to think about these preferences, and to discuss them with your family. Or, absent a change in your health, entering a new decade of life can be an opportunity to consider what you would or wouldn't want should your heart stop suddenly.

As I spoke with Sakes about this, he told me about his mother, who is eighty. Her health was starting to change, he said. She had a heart attack last year, and it's getting harder for her to walk. "I'm experiencing both sides of this right now, as we're talking," he said. "I'm like, hell yes I want CPR! But my mom would probably say, hell no!"

"I think I'm going to talk to my mom about this," Sakes said, as our conversation was ending.

It was exactly what I'd hoped to hear.