What's the longest someone has been clinically dead — but then come back to life?

Most of the time, death is permanent — when someone's heart stops beating, it rarely starts again. But sometimes, first responders can help bring a person back from the dead even after their heart stops.

So what's the longest time that someone has been clinically dead and come back to life?

Generally, spending more than 30 minutes "dead" and coming back without major brain damage is rare. But in some specific circumstances, people have been resuscitated after many hours and made a full recovery.

To understand how this happens, it's first important to define exactly what death is.

"Most of the time when [doctors] say 'clinically dead,' we're talking about cardiac death, and that means your heart is no longer beating," Dr. Daniel Mark Rolston, an emergency medicine physician at Northwell Health in New York, told Live Science.

When someone's heart stops beating, all of the cells in their body — and, most importantly, their brain — no longer receive fresh, oxygenated blood. After about five minutes without oxygen, those cells start to die, a process that can't be reversed.

The other type of clinical death is brain death, which occurs when the brain is so damaged that it can no longer control basic life functions, such as breathing and heartbeat.

How resuscitation works

Cardiopulmonary resuscitation (CPR) is designed to keep fresh blood flowing throughout the body and keep brain cells alive after cardiac death. By manually compressing the chest and giving rescue breaths, first responders can help keep cells oxygenated for a short period of time even when the heart isn't beating on its own. Most of the time, CPR can't restart the heart itself, but it can buy time for other techniques that can.

To get the heart to beat on its own again, first responders use a technique called defibrillation. This applies an external electrical current to the heart, mimicking the natural electrical signals that the heart muscles use to contract. In some cases, these electrical signals can reset the heart and help it beat again.

Under ideal conditions, these life support techniques can be relatively successful. According to the American Red Cross, survival rates following CPR in a hospital are about 20%. Those rates drop when people undergo cardiac arrest outside the hospital, falling to about 10%. That's because outside of healthcare settings, fewer people are trained in CPR and response times are generally slower.

"The earlier you get it, the better the outcomes," Rolston explained.

But even in the best-case scenarios, successful resuscitation after more than half an hour is rare, even if CPR is administered continuously.

"For the vast majority of people with very prolonged cardiac arrest periods, the survival is pretty poor," Rolston said. "If you don't get someone back at 30 minutes, their likelihood of survival is pretty low at that point."

Buying time with hypothermia

There is one notable exception to this rule, though: cases where cardiac arrest is combined with hypothermia. Hypothermia occurs when the core body temperature drops below 95 degrees Fahrenheit (35 degrees Celsius), and on its own, it can be very dangerous, causing the heart and lungs to fail, eventually leading to death.

But if the heart has already stopped on its own, hypothermia can actually have some upsides. Cold temperatures slow down the body's metabolism, protecting the delicate cells in the brain from dying after using up all of their oxygen.

"If you get cold fast enough, it can protect you for a long time," explained Dr. Samuel Tisherman, a professor of surgery at the University of Maryland School of Medicine who studies how hypothermia could be used as a therapeutic measure in cases of cardiac arrest due to trauma. "There are numerous reports of people drowning in very cold water and being underwater for upwards of an hour and surviving."

The longest-known reported case of successful resuscitation after cardiac arrest and accidental hypothermia is of a 31-year-old man who was revived after eight hours and 42 minutes. The man, whose body temperature was already about 79 F (26 C) due to a summer thunderstorm, experienced a cardiac arrest, and people nearby immediately began administering CPR, which was maintained for over three and half hours. Once the man was at a hospital, he was placed on a life support system that maintained fresh blood flow for five hours, and he was eventually warmed and successfully resuscitated. After three months, doctors reported that the man had completely recovered, with no sustained neurological damage.

What about coming back to life from brain death?

While cardiac death has possible roads to recovery, brain death is another story. When a patient is declared brain dead, it means their brain can no longer send signals to the body that control essential functions.

To be declared brain dead, doctors need to identify what medical issue is causing the brain damage and rule out any conditions that could cause symptoms that resemble brain death. This process can involve imaging the brain using MRI, testing basic neurological functions such as pupil dilation reflexes, and checking whether the patient can breathe on their own.

Every now and then, a news story pops up about a patient who was declared brain dead and taken off life support, only for them to "come back to life." So are these patients really rising from the dead?

In all likelihood, probably not. The definition of brain death is that the basic life-supporting regions of the brain are so damaged they won't be able to recover, so brain death isn't a condition that can be reversed. What's most likely happening in cases when someone supposedly recovers is that the original diagnosis of brain death was wrong.

"Errors have been made where people declared brain dead were later found to have spontaneous movement that should not have been possible," Dr. Robert M. Sade, a professor of surgery at the Medical University of South Carolina, told Medscape in 2018. "In virtually all those cases, brain-death determination was not done correctly."