Members of the Caudwell Xtreme Everest expedition, testing human adaptation to hypoxia on the roof of the world, write a diary blog for Nature from 30 March, 2007.
A number of the Caudwell Xtreme Everest (CXE) Investigators have just safely returned from a week of working at camp 2 on Mount Everest, in the Western Cwm – 6,400m above sea level. Whilst there we managed to perform a large number of studies under very difficult conditions. The temperature fluctuations experienced at base camp were magnified and now we had to worry about added dangers of heavy snowfall and avalanche.
We took venous and arterial blood to look at haemoglobin, haematocrit and gas partial pressures. We were genuinely shocked by the partial pressure of oxygen in our arterial blood at this altitude; values lower than I’d ever seen during my years working with critically ill patients on intensive care units. Yet we were still able to function normally and carry out complex tasks, including stripping and rebuilding the blood gas machine on site!
Many studies were completed at Camp 2, such as the highest ever breath-by-breath cardiopulmonary exercise (CPX) tests, neuropsychological tests, body composition and nutritional studies, brain Doppler, sidestream darkfield examination of the sublingual microcirculation, spirometry and a comparison of different supplemental oxygen circuits during exercise. By the end of the week the entire team was exhausted and ready to come back to base camp for a well earned rest. Unfortunately to get down to base camp meant once again picking our way through the Khumbu icefall. In the week that we had been up at Camp 2 huge sections of the icefall had collapsed which meant the Sherpas had to seek a new route through. A sobering reminder that everything beneath us is slowly moving downhill and that at any moment a big movement could either engulf or crush us as we attempt to scale this giant mountain.
Last time I explained our primary hypothesis which is centred on a change in oxygen utilisation, or efficiency, as a result of exposure to hypoxia. There are many other aspects to the research on this expedition and hopefully I can give you a taster of these now.
Although concentrating on oxygen utilisation, there are still components of the process of oxygen delivery which interest us. We are looking at the way in which systemic oxygen delivery changes during exercise at altitude using a pulse contour analysis device (LiDCO). Simultaneously we can look at regional blood flow, in particular that of the gut by using the somewhat unpopular technique of gastric tonometry. Our willing volunteers (only CXE investigators for this study!) have a special naso-gastric tube inserted which measures gastric perfusion as they perform exercise on a CPX bicycle. As subjects exercise on the CPX bicycle we will also use near infra-red spectroscopy (NIRS) to measure the amount of oxygen in their muscles as the exercise load increases. To complete our investigation of oxygen delivery at altitude we move from systemic, through regional down to microvascular blood flow. Using a technology based on orthogonal polarization spectral (OPS) imaging we are able to easily visualise the sublingual microcirculation at altitude. Using the a side-stream darkfield (SDF) camera we can illuminate erythrocytes as they pass through the microcirculation and record their passage to analyse flow characteristics once back at sea level. The microcirculation forms a vital link between the systemic circulation and cellular tissue. Without a properly functioning microcirculation the delivery of oxygen from the erythrocytes to cells may be impeded.
We have a large program to investigate the performance of the neurological system at altitude. In both the investigator team and trekking group we shall be using a number of techniques to look at changes in cerebral function as altitude increases. This may help us to understand the physiology of the hypoxic brain following severe traumatic head injury, a sadly common presentation to intensive care units across the globe. Each subject will undergo a number of specially selected neuro-cognitive tests. These consist of almost an hour of written, oral and dexterity tests designed to tease out subtle changes in cognition associated with hypoxia. We shall also be using techniques to study pupil reaction time and scanning eye movements known as saccades. In a similar manner to the study of muscle oxygenation we will use NIRS to look at brain oxygen levels as subjects exercise. Transcranial Doppler is also being used to look at blood flow within the brain. In a small sub-group of subjects, potential changes in brain structure are being explored using magnetic resonance imaging.
Maintenance of body mass at altitude is well known to any of you who have ventured to these heights; people lose weight in astonishing amounts. This phenomenon is also seen in critically ill patients on intensive care. We are looking into why such incredible weight loss occurs at altitude by studying body composition and dietary absorption and excretion.
There are other studies looking at changes in taste and smell at altitude, how to relieve the distressing problem of breathing related sleep disturbance at altitude, changes in electrocardiogram tracings and the development of a novel supplemental oxygen re-breathing circuit. The circuit will be tested by climbers on the mountain and is based on re-breathing technology used by fire-fighters and deep sea divers in their breathing circuits. If the re-breathing circuit proves to be beneficial to climbers deprived of oxygen high on Everest its benefits may well be transferable to those individuals dependent on long term oxygen therapy due to lung disease back at home.
Hopefully you now have some idea of the vast scale of the Caudwell Xtreme Everest project and the enormous task that it has been to coordinate so many studies in such an inhospitable environment. Our plan for the next week is to relax at base camp before preparing to once again ascend Everest to carry on the scientific program right to the summit of the mountain.