Experiments carried out on Mount Everest by a team of doctors have recorded the lowest ever blood oxygen levels, newspapers reported today. The Daily Telegraph said the research is being carried out to learn more about the body in extreme conditions, with the hope of finding new treatments for patients in intensive care.
It said that the doctors believe that patients may cope with low oxygen by becoming ‘acclimatised’ in a similar way to mountaineers, meaning that current “potentially hazardous” methods to raise their oxygen levels could be avoided. It quoted one of the doctors as saying, “If replicated in patients, the findings could have the potential to save lives”, but they would need “careful evaluation before they can be translated into clinical practice”.
As the authors say, these measurements give some idea of how humans adapt to high altitudes and what the limits are. The study is unique in having recorded the lowest ever documented blood oxygen levels, but the findings do have restricted applications. Mountaineers and critically ill people are not directly comparable and, as the researchers acknowledge, further research is needed.
Where did the story come from?
The research was carried out by Drs Michael Grocott, Daniel Martin and colleagues from the Centre for Altitude, Space and Extreme Environmental Medicine at the University College London Institute of Human Health and Performance. The work was funded by numerous associations and foundations. The study was published in the peer-reviewed medical journal New England Journal of Medicine .
What kind of scientific study was this?
This was a physiological study involving 10 experienced adult climbers (nine men, one woman) aged between 22 and 48 years, who were climbing the southeast ridge of Mount Everest as part of the Caudwell Xtreme Everest research expedition. All climbers had previously climbed, without incident, to a height of 7,950m (26,083ft). The height of Mount Everest at its summit is 8,848m (29,029ft). At this altitude, oxygen pressures are believed to be the lowest that humans can tolerate while still maintaining normal body function.
The researchers say that only 4% of climbers currently attempt to climb to the summit without the use of supplementary oxygen. This study involved taking direct measurements of arterial oxygen content (CaO2) and arterial oxygen pressure (PaO2) at these extreme altitudes while the climbers breathed ambient air (natural atmospheric air). This was done to see how blood oxygen levels would compare to those measured at lower altitudes and at sea level.
Arterial blood samples were initially taken from the climbers in London (altitude 75m; 246ft). They were then taken at Everest base camp (altitude 5,300m; 17,388ft), at Camp 2 (altitude 6,400m; 20,997ft), at Camp 3 (altitude 7,100m; 23,294ft), and during the descent at a location known as the ‘Balcony’ (altitude 8,400m; 27,559ft), which is just below the summit. Measurements at the summit could not be taken due to adverse weather conditions.
The London and base camp samples were taken from the radial artery in the forearm and analysed immediately. The blood samples obtained during the expedition were taken from the femoral artery in the upper thigh, and stored in the airtight syringe before being placed in a plastic bag and surrounded by ice water in a vacuum flask. A Sherpa then transported the samples back to a laboratory that had been set up at Camp 2. The blood samples were tested within two hours of being taken. Barometric pressure was taken at the altitude where the arterial blood samples were taken.
The climbers could use supplemental oxygen at or above Camp 3, but the blood samples were taken after the climber had been breathing ambient air for an adequate length of time (20 minutes) to act as a ‘washout’ period. In addition to measuring oxygen pressure, the doctors also measured carbon dioxide pressure, pH, haemoglobin and lactate levels, and calculated arterial blood oxygen saturation.
What were the results of the study?
The climbers reached the summit on May 23 2007, having spent 60 days at an altitude above 2,500m (8,202ft) to acclimatise. Although blood samples were taken from all 10 climbers in London, only nine were taken at base camp and Camp 2. Six were taken at Camp 3, and only four at the Balcony. Reasons for incomplete samples included some of the climbers feeling unwell or being absent when the Sherpa was ready to descend with the samples, or not reaching the necessary altitude.
Although arterial oxygen pressure declined with increasing altitude, oxygen saturation remained relatively stable. Up to the altitude of 7,100m (23,294ft), haemoglobin concentration increased sufficiently to maintain arterial oxygen content. At the Balcony (8,400m), atmospheric pressure was 272mmHg (36.3kPa) and the average arterial oxygen pressure in the four climbers with blood samples was 24.6mmHg (3.28kPa). But the oxygen content was 145.8ml/l, which was 26% lower than it had been at 7,100m.
Oxygen saturation was 54% at this level, and carbon dioxide arterial concentration was 13.3mmhg (1.77kPa; compared to sea level values of 36.6mmHg or 4.88kPa). The average difference between oxygen pressure in the artery and alveolar oxygen pressure in the lungs was 5.4mmHg (0.72kPa decrease in oxygen pressure from the lung to the artery).
What interpretations did the researchers draw from these results?
The researchers say that the decreases in arterial oxygen pressure observed with increasing altitude are representative of the fall in atmospheric pressure. However, arterial oxygen saturation seemed to remain stable. Haemoglobin (oxygen carrying molecules) in the blood were found to increase with increasing altitude, which allowed the blood oxygen content to remain at a similar level to that seen at lower altitude.
The researchers discuss possible physiological reasons for the increased alveolar-arterial oxygen difference observed at high altitude (i.e. the impaired oxygen transfer between the lungs and the blood).
What does the NHS Knowledge Service make of this study?
As the authors say, these arterial blood gas and haemoglobin measurements provide some idea of the human body's limits, and how it adapts to high altitude. The study is unique in being the first published research to have recorded blood oxygen levels and blood pressure at 8,400m above sea level.
The study does have some limitations, one of which is the small number of climbers (four) that could be analysed at high altitude. Additionally, the fact that the climbers were acclimatised at this level with no deterioration in cognition or function suggests that they may not be typical of many people, or that they may have been benefiting from prior use of supplemental oxygen. However, the effects of sudden removal of oxygen apparatus at high altitude are unknown. Therefore, it could be that those who had used supplemental oxygen were less acclimatised and hence had lower arterial oxygen pressure when breathing ambient air compared with someone who had been breathing ambient air throughout the climb.
Additionally, a small increase in blood oxygen pressure would have occurred during the two hours that the blood was stored and transported to the laboratory. This must be considered.
This research does give some insight into how the body may adapt when subjected to low oxygen levels. It has extended enquiry into how critically ill people may also adapt to low arterial oxygen and tissue perfusion. However, the two situations are not directly comparable, and specific research into the physiological adaptations of critically ill people is needed.
Sir Muir Gray adds...
Now that’s the type of research I would like to do, important findings in a great study.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Independent, 8 January 2009
The Daily Telegraph, 8 January 2009
BBC News, 8 January 2009
Links to the science
NEJM 2009, 360:140-149