By Michelle Roberts
BBC News health reporter
Dr Grocott and the Caudwell team plan another trek in 2009
The lessons learned by medics from a trip to Everest could help the treatment of critically ill patients.
The team that braved the Himalayan summit to study the body's responses to extreme adversity has recorded the lowest ever human blood oxygen level.
The results could see treatment plans for some patients with similarly low blood oxygen levels re-evaluated.
The "Caudwell Xtreme Everest" work has been published in the New England Journal of Medicine.
The blood readings established what has long been suspected - that high-altitude climbers have incredibly low levels of oxygen in their blood, which at sea-level would only be seen in patients close to death.
The average arterial oxygen level was 3.28 kilopascals or kPa - the lowest was 2.55 kPa.
The normal value in humans is 12-14 kPa and patients with a level below 8 kPa are considered critically ill.
Expedition leader Dr Mike Grocott said: "We rarely see below 6 kPa in our patients.
"Yet our readings were well below this and we were walking and talking and functioning normally.
"This gives us some perspective about what levels of oxygen deprivation people can tolerate."
He said some critically ill patients may have adapted to the low oxygen levels and may not need the aggressive interventions, such as ventilation, that are currently given to get blood oxygen levels closer to normal ranges.
"All these interventions carry a risk of harm and you have to weigh up the benefits versus potential damage to organs like the lungs.
"Maybe we could be less aggressive in treating some of these patients."
He said other intensive care researchers were planning to investigate it.
Acting as guinea pigs themselves, the London team of medics - five anaesthetists, two GPs and a vascular surgeon - took the first readings of human blood oxygen 8,400m above sea level.
The team climbed with oxygen tanks, then removed their masks 20 minutes prior to testing to give time for their lungs to get used to the low-oxygen atmosphere and to avoid any skewing of the readings.
The team were unable to make the measurement on the summit of Everest as conditions were too severe, with temperatures at -25C and winds above 20 knots.
Having descended a short distance from the summit, the doctors removed their gloves, unzipped their down suits and drew blood from the femoral artery in the groin.
The samples were then carried by Sherpas back down the mountain and analysed within two hours at a science lab set up at the team's camp at 6,400m.
Based on calculations of the expected level of oxygen in the blood, the Caudwell Xtreme team speculate that accumulation of fluid in the lungs as a result of the high altitude might have contributed to the low oxygen levels.
They hope ongoing research will eventually lead to better treatments for patients with acute respiratory distress syndrome, cystic fibrosis, emphysema, septic shock, "blue baby" syndrome and other critical illnesses.
Dr Andy Tomlinson, an anaesthetist at the City General Hospital in Stoke-on-Trent and a keen climber, said: "The figures they have got are quite incredible. They are phenomenally low oxygen levels.
"There is obviously a difference between critically ill patients and fit and healthy climbers.
"Never the less, there are lessons to be learned for critical care."
Dr Peter Nightingale of the Royal College of Anaesthetists said: "This may well make doctors re-evaluate their current provision of oxygen and the researchers may well be right that patients can run on lower oxygen levels.
"But we do not know and we need more research."