(CNN) A man undergoing cardiopulmonary resuscitation, or CPR, exhibited signs of conscious awareness for 90 minutes before the medical team stopped the life-sustaining procedure, according to a new case report.

The case was presented Monday at Euroanaesthesia 2018, the annual European Anaesthesiology Congress in Copenhagen, Denmark.

The 69-year-old man was admitted to a hospital in Denmark in 2016 after three days of indigestion and nausea. Shortly after arriving, he became unconscious and went into cardiac arrest, according to Dr. Rune Sarauw Lundsgaard , an anesthesiologist at Herlev Hospital in Copenhagen and an attending physician on the case.

"At this time, paramedics were still in the room, and CPR was initiated immediately. Due to a recent event with another patient, the cardiac arrest team was in the next room, and advanced CPR was initiated shortly after. This means that two paramedics and four hospital porters were shifting in pairs of two at performing the CPR," Lundsgaard wrote in an email.

"The patient had no electrical activity in the heart at any time. The heart only functioned because of the manual compressions," he added.

The team of six continued advanced CPR on the patient for nearly 90 minutes. During this period, the patient would occasionally show signs of conscious awareness, according to Lundsgaard.

"As soon as CPR was initiated, the patient opened his eyes. When CPR was performed, the patient was able to respond to verbal communication by moving eyes, lifting hands and legs and nodding his head. The patient's wife was present and able to hold his hand," Lundsgaard said.

This level of conscious awareness during CPR is extremely unusual, according to Dr. Sam Parnia , director of critical care and resuscitation research at New York University Langone Medical Center and leading author of a 2014 study on the topic.

"It is exceedingly rare for people to have actual awareness with external signs of being conscious as is being discussed in this case report," Parnia wrote in an email. "All the studies of CPR have demonstrated that there is insufficient blood flow to the brain (approximately 15% of baseline blood flow) to allow for the return of brain stem reflexes and consciousness with external signs of being awake.

"It is much more likely to have people waking up during compressions when the additive effect of compressions on an already beating heart raises the blood pressure to a sufficient level to provide enough blood flow to the brain," he added.

CPR is a potentially life-saving technique that helps maintain blood circulation when someone's heart stops beating. It can be performed by bystanders or medical personnel and normally consists of chest compressions at a rate of approximately 100 to 120 per minute. Those who are trained in CPR should also give two "rescue breaths" after every 30 compressions, according to the Mayo Clinic

The survival rate for those who experience cardiac arrest outside of the hospital is approximately 12%. Survival is two to three times higher for those who receive immediate resuscitation in a hospital setting, according to the American Heart Association

According to Lundsgaard, medical personnel usually stop performing CPR when the patient shows signs of consciousness.

"Normally, chest compressions are stopped once the patient shows signs of life or spontaneous breathing. [When] the patient moved, we stopped CPR, and immediately the patient went unconscious due to his nonfunctioning heart. This was done several times with the same result," he said.

Though rare, awareness during CPR may be more common than many people think. In the 2014 study , 2% of cardiac arrest survivors could explicitly recall "seeing" or "hearing" actual events related to their resuscitation.

About 10% also recalled having "near-death experiences" characterized by a variety of sensations such as detachment from one's body, feelings of levitation, warmth and the presence of a light, according to the study.

In this case, the patient's heart never exhibited any spontaneous rhythm and did not appear to move during multiple ultrasound evaluations. Consequently, a surgical intervention was not recommended, according to Lundsgaard.

"Several thoracic surgeons at different hospitals were consulted and they all agreed that the prognosis for an operation was very poor," he said.

About one hour into the episode, an ultrasound scan of the heart also showed an aortic dissection, a potentially lethal condition in which blood is forced between the inner and outer layers of the aorta, Lundsgaard said.

The dissection probably contributed to the original cardiac arrest, according to Parnia.

Other potential causes of cardiac arrest include coronary artery disease, heart attack, valvular heart disease and certain arrhythmias, according to the Mayo Clinic

"As he only just entered the emergency room before his cardiac arrest, no MRI, CT or ultrasound scan had been done before his cardiac arrest. The cause of the aortic dissection is unknown," Lundsgaard said.

The team performed CPR on the patient for 90 minutes -- much longer than normal, according to Parnia.

"Most hospitals will stop at around 20 minutes. However, based on results of studies, it is recommended to continue for at least 45 minutes. This is unusual," he said.

However, the team was ultimately unable to save the patient's life.

"The cardiac arrest team in our case was very affected by the situation," Lundsgaard said. "For me, having to tell the patient that we were unable to save his life ... and that in a minute we will stop chest compressions and you will not survive was a challenging situation."

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The experience also raised some difficult ethical questions for the medical personnel -- including the issue of sedating patients during resuscitation, Lundsgaard said.

"The question of sedation during CPR is not new, but sedation is not routinely performed during CPR," Lundsgaard said. "This is an area that needs further research."

And for those who survive such incidents, the long-term effects on mental well-being are unknown.

"We know from anesthesia that accidental awareness during operations often lead to post-traumatic distress and decreased quality of life. One might suspect that awareness during CPR may be just as stressful," Lundsgaard said.