Almost 40% of people who underwent CPR and survived cardiac arrest had memories, dreamlike experiences or some perception even when they were unconscious, a study into near-death experiences suggests.
Researchers also found signs of brain wave activity, suggesting awareness - sometimes up to an hour - before they were resuscitated.
"There's nothing more extreme than cardiac arrest because they're literally teetering between life and death, they're in a deep coma and they don't respond to us physically at all," lead study author Dr Sam Parnia, an associate professor in the department of medicine at NYU Langone Health, told NBC News, Sky News' US partner.
"What we're able to show is that up to 40% of people actually have a perception of having been conscious to some extent."
That perception could just be a vague feeling something is happening around them.
Six patients in the study reported what the researcher called "transcendent recalled experiences of death" - what many think of as a near-death experience.
"They may have had a life review, they may have gone to a place that felt like home, and so on," said Dr Parnia.
Several patients were able to recall aspects of their medical treatment, such as pain, pressure or hearing the voices of doctors.
Others remembered dreamlike sensations, such as being chased by the police or being caught in the rain.
Some had positive memories, such as seeing a light, a tunnel or family member, while others felt intense emotions such as love, tranquility and peace.
However, others had a feeling of separation from the body and a recognition they had died, or had delusions of monsters or faceless figures.
Some consciousness during resuscitation
The study, published in the journal Resuscitation, monitored 567 people who underwent cardiac arrest resuscitation at 25 different hospitals.
Fewer than 10% of patients survived and the researchers were able to interview 28 of the 53 survivors.
Eleven reported having memories or perceptions that suggested at least some consciousness during resuscitation.
Researchers also measured brain oxygen and electrical activity in some patients and found certain brainwave activity, suggesting some mental function during CPR.
"I think that's incredible," Dr Sheldon Cheskes, a professor of emergency medicine at the University of Toronto, who studies cardiac arrest resuscitation and was not involved in the research, told NBC. "You would never have known that without being able to do that brainwave monitoring."
During cardiac arrest, the heart quivers with uncoordinated contractions and flow around the body, including to the brain, ceases. Unlike a heart attack a person in cardiac arrest is always unconscious.
Survivors may have some memories of CPR
The researchers also tested if patients could recall specific sights or sounds, known as implicit learning.
To do so they placed headphones on patients during resuscitation and played three words - apple, pear, banana - and used a tablet to display 10 images.
Only one of the 28 patients interviewed correctly remembered the three-word sequence and none could recall the images.
Dr Katherine Berg, an assistant professor of medicine at Harvard Medical School and chair of the writing group for the 2025 American Heart Association post-cardiac arrest care guidelines, told NBC the main takeaway was that survivors may have some memories of CPR.
"I hope that studies like this one will prompt physicians to ask cardiac arrest survivors about these memories and experiences and assess for any post-traumatic stress or other psychological symptoms that might need to be addressed," she said.