Surgical operations involving general anaesthetic may damage mental ability by starving the brain of oxygen, researchers fear, after conducting tests on climbers as they scaled Mount Everest.
Up to one fifth of people develop hypoxia, or oxygen deficiency, after surgery, which can lead to symptoms such as wheezing, confusion, high blood pressure and even heart failure and death.
But research from City, University of London and University College London suggests that the condition can also have a significant impact on brain function, with the effects lasting for at least 11 days after oxygen deprivation.
The landmark study, which monitored 198 climbers who scaled Everest, where there is only one third the amount of oxygen that there is at sea level, found significant cognitive decline after the teams descended.
In particular, tasks associated with speech and language, learning, planning, focusing and organising were severely affected, with performance falling by nearly 20 per cent on tests taken before and after the ascent. The effects were still present when the climbers returned to Kathmandu, 11 days later.
Researchers believe the results may partially explain why far more climbers die coming down from the Everest than they do going up.
Asked whether the same impact might be seen after surgery, the study leader Professor Stanton Newman, dean of the School of Health Sciences at City, said: "Absolutely. What we found is that there was a clear relationship between levels of hypoxia and brain function which has not been established before.
"It was possible to group people into three groups, some of whom had mild effects, and some whom it was significant. We noticed that it was worse for older people.
"We know that a number of people who go back to work after surgery and find that they have cognitive problems but it was unclear what was causing it.
"We also noticed that the declines happened even when people were given oxygen and their oxygen levels returned to normal."
The team used the climbers to study the effects of hypoxia because it would be unethical to deprive humans of oxygen in trials.
Participants received a series of neuropsychological tests assessing memory, language, attention, and executive function.
These were administered at sea level in London, 3475m at Namche Bazaar in Nepal, and at 5180m at Everest Base Camp.
Tests were also conducted upon return to 1280m in Kathmandu. This group was also compared with a control group.
Hypoxia can also be caused by lung diseases such as emphysema, bronchitis and pneumonia where sufferers struggle to take in enough air. The research was published in the journal PLOS One.