It has long been suspected that high-altitude climbers have low oxygen levels in their blood. Now a study has confirmed it, claiming the lowest levels ever found in humans, significantly surpassing levels in critically ill patients. The measurements were made at an altitude of 27,559 feet (8,400 meters) by a team of scientists at University College London who led the Caudwell Xtreme Everest expedition and collected blood samples from climbers' leg arteries right after they reached the peak. The climbers' blood oxygen levels were close to the human limit of tolerance, which at sea-level would only been seen in patients near death. The research could inform better treatments for the critically ill. The team climbed with oxygen tanks to get well acclimated to the high altitude and then removed their masks 20 minutes prior to testing to equilibrate their lungs with the low-oxygen atmosphere. Blood collected from four supine team members was then analyzed within two hours at a science laboratory set up at the team's camp at 21,000 feet (6,400 meters) on Everest. At that altitude, the barometric pressure was about 35 percent of what it is at sea level. The average blood oxygen levels of the four climbers was 24.6 millimeters of mercury (3.28 kiloPascals), with the lowest value being 19.1 millimeters of mercury (2.55 kiloPascals); the normal value in humans is between 90 and 105 millimeters of mercury (12 to 14 kiloPascals). Patients' whose blood oxygen falls below 60 millimeters (8 kiloPascals) typically are considered critically ill. The climbers also had accumulated fluid in their lungs as a result of the high altitude, which might have contributed to their extremely low oxygen levels, said Mike Grocott, Caudwell Xtreme Everest team expedition leader and a senior lecturer in Critical Care Medicine at University College London. "By observing healthy individuals at high altitude where oxygen is scarce, we can learn about physiological changes that can improve critical care at the hospital bedside, because low oxygen levels are an almost universal problem in critical care," Grocott said. Many medical interventions that are aimed at restoring oxygen in patients' blood cells have thus far proven ineffective or even detrimental. Hospitalized patients with a standard atmospheric pressure level below 0.078 are considered critically ill, which is 0.046 atm higher than the average level of the four climbers. The results, detailed in Jan. 8 issue of The New England Journal of Medicine, may inspire doctors to re-evaluate their treatment strategies when looking after patients in long-term critical care such as patients with acute respiratory distress syndrome (ARDS), cystic fibrosis, emphysema, septic shock, "blue baby" syndrome and other critical illnesses.
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