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.
The South Col (Camp 4), at 7950m above sea level, is the most inhospitable place I have ever been to.
It lies as a saddle between the peaks of Lhotse and Everest. Wind funnels through this gap in the mountains continuously, scouring snow from the ground and projecting it horizontally at high speed. A number of tents are normally to be found huddled together at the bottom of the route which leads up to the summit of Everest.
Traditionally climbers arrive at the South Col, from camp 3, in the early afternoon. They rest and brew up drinks until early evening when they leave for the summit. On their return the camp is dismantled and the climbers descend as rapidly as possible. The aim is to spend as little time as possible on this litter strewn wasteland. Our aim was to set up a physiology laboratory on the Col, and spend a number of days trying to study as many of our investigators as possible.
Once the Sherpas had erected a suitably sized tent on the Col we set about trying to construct the world’s highest laboratory. The first hurdle was a reliable source of electricity from which to run the various pieces of equipment we intended to use. A bank of 12 volt batteries fed by solar panels mounted on the outside of the tent was the answer to this. Despite the howling gales one useful natural phenomenon at the South Col is sunlight; glaring rays beam down from dawn until sunset.
Then we needed to build the exercise bicycle which forms a vital component of the cardiopulmonary exercise (CPX) testing system. The bike had to be brought to the South Col in small pieces as the whole bike would be far too heavy and cumbersome for any Sherpa to carry up the steep Lhotse Face. Wearing oxygen masks and following simple instructions with plenty of photos we slowly pieced together the bicycle without damaging any of its delicate internal components.
Next the breath-by-breath analysis system needed to be calibrated to the thin cold air which is to be found 8000m. The missing link that co-ordinates the bicycle and the breath analysis system is a laptop computer. For those of you who have attempted research at altitude you will know that computer hard drives begin to fail after about 4500m. To get around this we had to make special hard drives from the memory cards more commonly found in cameras. To our amazement the laptops worked at this extreme altitude; the highest a computer has ever been successfully used on Earth, as far as we know! Numerous other pieces of research equipment were unpacked and warmed up ready for the subjects to be investigated.
After multiple glitches we eventually managed to perform the highest CPX tests ever. Subjects cycled to exhaustion whilst we measured systemic oxygen consumption. Needless to say, the maximum work load achievable by subjects was considerably less than they achieved at lower altitudes. Other tests included transcranial Doppler measurement of middle cerebral artery blood velocity (blood flow speed to the brain), near infra-red spectroscopy to assess brain oxygenation, neurocognitive assessments (higher brain function), sidestream-darkfield imaging of the sublingual microcirculation (blood flow under the tongue) and spirometry (lung function measurements). Considering the appalling conditions outside the tent we managed to complete a fairly extensive series of experiments in the limited time we could stay at the Col .
There remained one more experiment to perform, an arterial blood gas sample from the summit of Everest. We left the South Col at around nine thirty on the night of the 22nd May and began the long, cold, slow climb up towards the summit. We climbed through the night and as the sun began to rise we came face to face with the famous Hilary Step. Having conquered this final hurdle we arrived at the summit of Mount Everest (8848m) at around six thirty in the morning of the 23rd May.
It was cold and windy but worth every step of the arduous journey we had taken to get there. We decided that taking an arterial blood sample on the summit itself was too dangerous and that attempting to erect our light weight shelter would no doubt end in disaster. After about forty minutes on the summit we descended as far as ‘The Balcony’ (8475m) where conditions were more suitable for taking blood.
Within the shelter of our simple tent we were able to remove our big mittens and expose the femoral artery of each subject in order to take a small sample of blood. Taking arterial blood at such an altitude is no easy task and we were glad of the practice we’d had on other large mountains. The samples were carried by one of our nimblest Sherpas (Pasang Tenzing Sherpa) to the Camp 2 laboratory some 2000m below in a staggering two hours.
Successful analysis in the blood gas machine at Camp 2 revealed shockingly low levels of oxygen in the blood of the climbers studied. These results are the highest arterial blood gases ever obtained and go a long way to show just how adaptable the human body can be to extreme hypoxia.
So there ended our endeavours to explore the physiology of the human body when pushed to extremes. We safely descended the mountain and returned to base camp a number of days later. By that time nearly all 200 of the trekkers had passed through the laboratories and completed their testing. All that remains now is for the investigators to test one another for one last time at base camp before trekking back to Lukla for the somewhat hair-raising flight to Kathmandu.