The lowest ever levels of oxygen in humans have been reported in climbers on the Caudwell Xtreme Everest expedition led by University College London doctors.
The world-first measurements of blood oxygen levels in climbers near the top of Mount Everest, published in this week's New England Journal of Medicine, could eventually help critical care doctors to re-evaluate treatment strategies in some long-term patients with similarly low levels of blood oxygen.
Taking part in the Caudwell Xtreme Everest expedition, ten doctors took blood from their own leg arteries close to the mountain's summit. With temperatures of minus 25C and winds above 20 knots, blood could not be collected at the top.
The blood was then carried down the mountain to the camp laboratory, set up at 6,400 metres, to be analysed.
The purpose of the study was to establish 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.
Blood oxygen is measured in kilopascals (kPa), and 12-14 kPa is considered normal. The climbing doctors found that at 8,400 metres they had an average blood oxygen level of just 3.28 kPa, and a lowest value of 2.55 kPa.
Based on calculations of the expected level of oxygen in the blood, the authors also speculate that accumulation of fluid in the lungs as a result of the high altitude might have contributed to the low oxygen levels.
Hospital patients with a level below 8 kPa are usually considered to be critically ill.