The findings published on Wednesday could one day lead to better care for patients with heart and lung ailments in intensive care units, premature babies and others suffering from similar low-oxygen levels, known as hypoxia.

"We want to understand why humans respond differently to low oxygen levels," Mike Grocott, a researcher at University College London, who led the study, said in a telephone interview.

"The problem with studying critically ill patients is there are so many other things going on it makes it difficult to single out the effects of any one variable."

The study took place in the so-called "Death Zone" on Everest -- the area above 8,000 metres where thin air limits the amount of oxygen getting into the lungs and can lead to organ failure and coma even for the fittest individuals.

The team of eight doctors reached the 8,850-metre summit via the South Col route but carried out their experiment about 400 metres lower because of severe weather at the top.

After finding an area that provided some shelter, four of the team members removed their gloves, unzipped their down suits and had blood drawn from the femoral artery in the groin.

"The disadvantage is you have to uncover a little bit more of yourself than you would like at those temperatures," Grocott said. The outside temperature at the time was about minus 25 C.

An analysis showed what doctors had long suspected -- that high-altitude climbers have incredibly low levels of oxygen in their blood, normally seen only in patients close to death.

The readings were also the lowest-ever recorded in humans, as low as 2.55 kilopascals. This compares to a normal level of 12-14 kilopascals for healthy people and a level of eight kilopascals for critically ill patients, Grocott added.

A kilopascal is a unit of pressure.

"We did an extensive search and couldn't find any measurements in humans that were lower and not many that were even close," he said.

Grocott and colleagues, who published their findings in the New England Journal of Medicine, believe a build-up of fluid in the lungs due to the high altitude might have contributed to the low oxygen levels.

While further research is needed, the results suggest that some critically ill patients may be able to tolerate lower oxygen levels before receiving oxygen or other interventions that run the risk of side effects, Grocott said.

"What is interesting about this is there may be some patients who can tolerate lower levels and get less harm from these other interventions," Grocott said.