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.