A team of mountaineering medics is launching an expedition to Mount Everest to learn how the human body copes with high altitudes. The results could lead to new treatments for critically ill patients, as BBC News medical producer Rachael Buchanan found out.
In May 1978, climbers Reinhold Messner and Peter Habeler became the first men to reach Everest's summit without bottled oxygen.
The team rehearsed on Mont Blanc in France
But as they staggered the final few metres, drawing a dozen breaths for every step, little did they suspect this experience could one day aid patients in intensive care units.
Messner described his time at the top of the world as being like "a single gasping lung floating over the mists and summit".
At this altitude, the air is so thin and there is so little oxygen present that, no matter how many breaths you take, your body is slowly dying.
Starved of O2, your lungs will fill with fluid, your brain will swell and if you don't descend, you will slip into a coma and die. No wonder they call it the "death zone".
This is where the parallels with critically ill patients arise. The low blood-oxygen levels endured by high altitude climbers are thought to be below those found in patients with severe heart and lung conditions on breathing machines, "blue babies" and cystic fibrosis sufferers.
So how can one group continue to function unaided while the other struggles to stay alive? If both are surviving at the extremes of life, can one help the other?
These are the questions a team of mountaineering medics from University College London's Centre for Aviation, Space and Extreme Environment Medicine (Case) hopes to answer in 2007 when they lead an expedition of their own to the world's most famous mountain.
As they climb, the team will measure the amount of oxygen in their own blood as well as the performance of their brains, lungs and metabolisms.
With jobs back home in anaesthesia, intensive care or remote medicine, they hope to draw a comparison between the human body pushed to its limits during critical illness and the changes experienced at extreme altitude.
But tackling Everest, let alone carrying out scientific studies on its summit, is not for novices.
Expedition leader Dr Mike Grocott says it was the combination of the group's hobbies and work life that first gave them the idea for such an unusual experiment.
"Several of the group are experienced high-altitude mountaineers who have visited the Himalayas to climb many times before. Having a team that can do the science and the climbing presents a great opportunity," he said.
What they want to study is something all high climbers suffer to some extent - altitude sickness. The condition shares some of the symptoms many of us go through on an average weekend - first you become light headed and euphoric as if you are drunk, then the hangover sets in.
You feel tired, you lose your balance, you can't sleep, you have a severe headache, loss of appetite, nausea - all due to an inevitable experience at high altitude - hypoxia - more commonly known as oxygen starvation.
In the air
Oxygen makes up 21% of the air and this fraction doesn't change wherever you are in the atmosphere. What does change is density and pressure, which drops as you rise. Everest is seriously high.
At a little under 9km (29,000ft), its peak noses into the same jet stream aeroplanes use, which means you are climbing up to the cruising height of a 747. At the summit, the atmospheric pressure is around one-third that at sea level; there is one-third the amount of oxygen in each litre of air. It has a big impact on your body.
If you aren't breathing enough oxygen, you run the risk of pulmonary oedema, where fluid on the lungs interferes with the body's essential oxygen uptake.
If allowed to progress to its conclusion, the victim will literally drown. To counter the lack of oxygen in the air, climbers breathe more deeply and rapidly.
The Ace gene may be involved in improved endurance
This increases the ratio of oxygen to carbon dioxide in the lungs, ensuring an increase in the number of red blood cells to capture that oxygen. But it also thickens the blood making it harder to pump, and stickier - more likely to clot - increasing the risk of thrombosis and stroke.
The Case team hopes to study this while experiencing it themselves, says Grocott.
"We asked ourselves what questions about the body's response to high altitudes remained unanswered and how could we learn from these to improve the care of patients with low oxygen levels on intensive care units. The result was the Xtreme Everest Expedition," he said.
No one has measured blood oxygen levels at the top of Everest before. The team plans to take arterial samples of blood - a simple procedure in the comfort of a nice warm hospital.
But under the challenging subzero conditions of Everest, the thick walls of the arteries and high pressure of the blood being pumped within them will make the task more difficult and dangerous - carrying the risk of thrombosis or bleeding.
The team will also use specialised probes to measure oxygen levels in body tissues and blood flow in different parts of the body.
There is a strong theory that at least some of the symptoms of acute mountain sickness are due to brain oedema - fluid on the brain - which causes it to swell.
Dr Montgomery believes Ace is also involved in survival at low oxygen levels
As your skull is a fixed space, any swelling puts pressure on your grey matter causing splitting headaches, loss of balance and eventually death.
To find out what happens to the brain at altitude, the team will measure this intracranial pressure and use infra-red spectrometry to gauge oxygen levels in their brains.
Their hope is that coupling this data with standard cognitive tests will determine the relationship between climbers' physical and mental reactions to altitude.
A concurrent genetics study will enable the team to explore why our bodies react in the way they do, and why some people can cope with low oxygen levels better than others.
In particular they will be looking at the Ace gene - the first gene linked to human fitness discovered by team member Hugh Montgomery in 1998. Genes come in pairs and there are two versions of this one.
The "I" version seems to be associated with improved endurance and Dr Montgomery believes having two copies of this gene aids survival at very low oxygen levels. He wants to investigate whether it is associated with a greater drive to breathe at altitude and more efficient burning of oxygen.
The team plans to analyse the DNA of as many of the thousand climbers who have succeeded in summiting Everest as possible. A further study will investigate the DNA of Tibetan populations dwelling at a range of altitudes.
The assumption is that those who have summited Everest successfully or belong to communities living at high altitudes will have the best genes for coping with low oxygen.
"We want to see what makes them so special. If we can find these 'survival genes' we will have identified new drug targets and ways of treating those who face low oxygen through illness," said Dr Montgomery.
Many try, and fail, to reach Everest's summit
Climbing Everest is not for the faint hearted. In recent years, much has been made of the commercialisation of the mountain which has allowed novices to be nursed up its slopes by guides and Sherpas.
But look in the sports section of any book shop and you will find numerous tales of sudden weather changes and poor decisions made in a hypoxic state of mind that have fatally marooned experienced guides and beginners alike.
Grocott agrees climbing Everest is a risky endeavour but insists they are planning for every eventuality. "We will consider the expedition a success, firstly, if everyone comes home safely, and only, secondarily, if we are effective in achieving all our scientific goals."