Part 2 Horizon

At the cruising altitude of a jumbo jet,

8,500 metres above sea level,

a team of climbers are about to take the final steps to the top of the world.

Seeing Everest for the first time was incredible. It was almost like meeting a god.

Once you're above 8,000 metres, you're considered to be inside the Death Zone.

But this team know better than most just how dangerous the next few hours will be.

This is a team of doctors.

Oxygen saturation is 62%, figure 6, 2, over.

And they are here to rewrite the medical textbooks.

On the way to the summit, they will be tested as climbers, scientists,

and in the deadly environment of Everest, they will be tested as doctors as well.

In the last half hour, they've shown a complete disregard for human life.

With respect to your doctor, he will die.

There are guys on the mountain who are continuing to go up in bad shape.

I wouldn't give them a cat's chance of living.

This is the story of an expedition unlike any Everest has seen before.

It's the story of a team of men and women willing to risk everything

in the pursuit of knowledge.

I'm here to do a job and I want to get it done and go home.

I'm looking forward to getting home.

This is the story of doctors in the Death Zone.

'Mike, this is Denny, over.

'Mike, this is Denny. Do you copy, over?'

It's May 2007.

Green, blue, ten. Green, red...

Garden, hat, farmer...

For the past two months, this barren landscape has been home to an extraordinary experiment.

This is the largest medical research project ever to come to the Himalayas.

MAN GROANS

OK, swallow.

Expedition leader, Dr Mike Grocott, believes it will ultimately

transform our understanding of human physiology.

We have an opportunity here to write the textbooks for the next 20, 30 years.

This type of expedition with 200 volunteers coming to Base Camp, plus climbers going onto the mountain,

and science, hopefully, up to the summit, doesn't come along very often,

so it's important that when we're working and collecting data,

that we really focus and we're careful.

Make sure we calibrate everything, make sure we collect everything accurately.

I can't stress that enough.

Now, with five weeks of testing complete at Base Camp and Camp Two,

14 of these doctors are going to go to even greater extremes -

a bid for the summit.

It's harder than I imagined.

The point of going to the summit is to see

just how far the body can be pushed to the extremes of its physiology.

What is driving them to such lengths is the chance to study the one thing that gives life to us all...

Breathe, breathe.

..oxygen.

Only if they can reach the summit will they be able to conduct their ultimate experiment -

setting up a laboratory in the most extreme environment on Earth...

I really don't want to fall here.

..and taking blood from deep inside their own bodies.

This better get easier.

The reason lies a world away...

..in the life-and-death environment of intensive care.

Here, survival rarely hinges on the nature of an injury or illness.

So, two units on the ninth.

What kills Mike Grocott's patients is a lack of oxygen, also called hypoxia.

That's why he's convinced Everest is a unique natural laboratory.

At this extreme altitude, there are the lowest levels of oxygen on Earth.

As these doctors climb higher, they are effectively putting their own bodies into intensive care,

recreating in themselves the exact conditions they treat every day.

We're doing some fundamental research that couldn't possibly be done anywhere else.

It can't be done in a chamber.

It can't be done in a simulated group.

The only way of doing this sort of science is actually physically being here and doing it.

But only if they reach the summit, will they be able to discover the ultimate limits of human survival.

At 8,850 metres, Everest is not only the highest point on Earth,

it also marks the boundary between life and death.

The level of oxygen at the summit of Everest is close

to the absolute limit of what humans can survive.

It's the one place on Earth that can answer a fundamental question of human life.

MONITOR BEEPS

How little oxygen does the human body need

to stay alive?

The Caudwell Xtreme Everest Team have been climbing for four days.

They are a few metres away from Camp Three.

RADIO: 'Mike, good to see you, over.'

At an altitude of more than 7,000 metres,

they still have to climb over one-and-a-half kilometres

to reach the summit.

Hard going.

Here, there is only 40% of the oxygen there is at sea level.

We got up here. I mean, we weren't quick,

but we got up here...

..in an acceptable time. I feel good.

Climbing with Mike is anaesthetist Andre Vercueil and Belfast GP Nigel Hart.

The low oxygen is already affecting their ability to function.

I reckon I can feel the altitude here.

I just feel a little bit...

..not quite on the... I've been going over my children's birthdays.

I'm getting them reasonably quickly but...

I don't believe that they come as quickly as they probably should.

At this extreme altitude, the climbers are breathless, even at rest.

You feel slightly better than you expect to, I think,

but it's pretty a bleak and hostile environment out there.

It was here yesterday that the expedition faced its biggest test so far...

Get the oxygen down here, as well.

..an advance team were confronted with a climber from another expedition

in desperate need of medical attention.

He needs to be up here.

To save his life, the doctors wanted him to descend immediately, but his team refused to act.

We believe that if he stays the night here, he has a very high chance of dying.

In the last half hour, I've seen a complete disregard for human life.

But the climber was lucky.

Surrounded by some of the world's leading high-altitude doctors, they were able to keep him alive

and he returned to Base Camp this morning.

The experience has left the whole team critically aware of the dangers they face.

Things can very quickly deteriorate and...

you know, it was a warning to us all.

A wake-up call, not to be so bold to think that any of us

are beyond that risk. We're not. It goes with the territory.

To complete their science, they must push on.

Only five days of clear weather remain to attempt the summit.

But to reach Camp Four on the South Col,

they must first complete one of the most challenging parts of the climb.

In front of them is a near-vertical one kilometre wall of ice,

the Lhotse Face.

The Lhotse Face was both more frightening and more beautiful than I expected it to be.

And it's...

an utterly amazing environment -

better than any day you could have in the mountains anywhere else.

RADIO EXCHANGES: 'Camp Three, this is Camp Two, over.'

'Reading you loud and clear, Denny. Go ahead, over.'

'Have you got any idea what time they're leaving, over?'

'That's a good question. How many people can you see standing on Lhotse, over?'

'Camp Four, this is Camp Three.'

If you fell from the Lhotse Face, you would stand no chance.

When the ice is that hard, you wouldn't be able to stop.

You'd go all the way to the bottom.

It has taken the team a day of gruelling climbing

to get to Camp Four.

Well done. Well done. You deserve it.

An advance party has already reached this landmark.

It is led by the only member of the team

with previous experience of the mountain,

military doctor Sundeep Dhillon.

One of the questions I'm asked the most often is why am I coming back here to a place

that is inherently dangerous when I've already summited?

The answer to that really has to be that the project we're involved in is so unique,

but I really hope that the results it will bring will justify that.

Sundeep's team have brought with them all the contents of a medical laboratory.

Just getting this far has fulfilled a long-held dream for Dr Dan Martin.

I've looked forward to being on the South Col since I was at school,

many, many years, and I never thought I'd get here.

I've been looking forward to seeing this bicycle test for about a year,

18 months now, so it's difficult to say which is more exciting.

It gets all muddled up.

It's exciting to be here, but I think the research has sort of taken over now.

The lab stands at the threshold of the Death Zone,

where there is simply not enough oxygen to sustain human life.

Every moment spent here is so suffocating, the cells in their bodies are slowly dying.

Thumbs up if you are OK.

For the doctors, it is the ultimate location for their research.

But in this deadly environment, it's their medical skills that are needed once again.

RADIO: 'OK, Mike, the situation up here as far as we know it,

'I can see one, two, three, four, five,

'six people on the triangular face, over.

'A rescue party dealing with a Nepali woman.'

Can we revise that time? The guys have moved fast. We can see them over the ice bulge.

High on the mountain, a female Nepali climber, Usha Bista, has succumbed to the lack of oxygen.

She has been found by another team, delirious and alone at 8,500 metres.

We need someone to come in here who speaks... Someone who speaks

very good Nepali and good English.

Here was a person on their own getting into

that situation where they get sick,

no-one with them to realise they are becoming unwell,

becoming confused, and the outcome of all that is that

she ends up unconscious at 8,500 metres.

Not a great place to become unconscious with no-one around.

Mike knows if they don't act immediately, this could be fatal.

What did you drag her on?

Stretcher or no stretcher?

Does she know what day it is?

TRANSLATOR SPEAKS NEPALI

She says she is in a tent.

Whereabouts?

Summit. She went to summit.

The radio link is beginning to falter.

Mike uses the summit team's cameras to relay a message to Base Camp.

OK, so the key things are cerebral oedema, mild hypothermia, frostbite - ten digits.

She needs dexamethasone, high-flow oxygen,

passive re-warming, and get her to Base Camp as fast as possible.

Camp Two may be the safest place overnight. Have you got that?

Most severe is a cerebral oedema, a swelling of the brain brought on by the high altitude.

Drugs and oxygen can help, but unless she is taken to a lower altitude she is likely to die.

She needs a doctor to go down with her.

We've just got here, but we're going to turn around and go back down to Camp Three with her.

It's the right thing to do, you know.

Andre volunteers to turn around and attempt the treacherous descent of the Lhotse Face.

Nick, this one here.

No hand-warmers. Not hand-warmers.

There's not enough people here that are in good enough shape to take her down on a stretcher.

To rescue somebody on a stretcher, ideally...

..you need 16 people.

We're going to try and do it with seven...

..because we think she's got a chance.

The crew are going to help you get her to the spur.

The rescue party are set to go when news breaks of further casualties.

Mike, there are four people in worse shape coming down.

Mike is faced with an agonising choice.

So it's a question of which people are most salvageable.

She is definitely salvageable.

Well, they're still up high.

-There's no telling.

-If they're up high, we ain't gonna get them out of here today.

There's nobody going back up to get them.

So, one, who's up there, and these are not nice decisions.

If one person is up there close to death and no-one's rescuing them, there's nothing we can do.

-What we can do is rescue her.

-When we get down to two, we'll talk to people.

We can save her life, so I'm going to go down with her.

Because we can save her life.

-Absolutely.

-And those guys may or may not die.

-These guys?

-Yes, these two guys.

It was a real reminder for us

who had yet still to go further up that this was indeed a difficult and dangerous place

and again, another warning of not to become complacent about the environment.

We have two guys.

Two guys for rescue.

Usha is to be evacuated down 1,500 metres to the Western Cwm,

where the expedition has a support team at Camp Two.

'I've said they can have anything they want of ours at two. Now, they may choose

'to go to their own camps...'

Mike radios his wife, Dr Denny Levett,

the team's Chief Medical Officer, to alert her to the incoming casualty.

We've offered to take care of the lady on her way down

and our doctors on the South Col have assessed her there.

At the moment, she is being transported down the Lhotse Face.

Usha is a stark reminder of just how little they understand

about the effects of high altitude on the human body...

..and the life-threatening symptoms it can produce.

It's been a very dramatic day, so we've split ourselves into a medical rescue team

and a scientific team here, such that we can do our work

and we can also rescue people safely from the mountain.

Off we go. 60rpm, please.

At this altitude, even the simple things don't come easily.

This neuropsychological test

is straightforward to complete at sea level but here, Nigel is struggling.

That doesn't look right.

The brain is so badly affected, many climbers return with permanent damage.

To counter these effects, all those conducting the tests used supplementary oxygen.

VJ, you can see, is on oxygen and the reason for that

is we want the person running the test to really know what they're doing.

We don't want somebody who is hypoxic and mad to control what is going on.

This test is the most ambitious and difficult experiment the team are attempting at Camp Four.

So Sundeep is the first person to do a maximal cycle exercise test at 8,000 metres.

SUNDEEP GROANS

-First person ever.

-First person ever, yeah.

THEY CHUCKLE

The doctors hope it will reveal why some people suffer so badly

from lower levels of oxygen, when others are unaffected.

Breathing just the thin air, Sundeep is pushing his body to its limit

as they analyse exactly how much oxygen he is using to do a precise amount of exercise.

The team have performed this test all the way from sea level,

but only here in Camp Four will they get their ultimate result.

It could explain why Sundeep is able to perform so well in this environment

when the levels of oxygen leave others on the edge of death.

And it could be that some people, with a given amount of oxygen delivered to the cell,

they use that oxygen in a much more efficient way

and we're really trying to measure that oxygen efficiency

in a lot of the experiments we're doing up here now.

What we would love to see

is that some people are much more efficient at using oxygen than others.

-How do you feel, mate?

-Good.

-There we are. Fantastic. Absolutely superb.

It's interesting, because, like you say, your heart rate goes up, your respiratory rate goes up

to 60 fairly quickly and you think you can maintain it. I don't know.

It's like most things. You think you can go for a few minutes longer.

I was...

starting to feel sick more than anything else.

Although all the data will be analysed over many months,

the potential of this experiment to save lives is enormous.

It might, for the first time, reveal how the body becomes more efficient when faced with a lack of oxygen,

allowing a treatment to be developed that could recreate this life-saving effect in critically ill patients.

Even a minor improvement in oxygen efficiency could have a major impact

on the most oxygen-sensitive organ - the brain.

Chris Imray, a vascular surgeon, is examining the most dangerous

effect of high altitude, a lack of oxygen going to the brain.

This is phenomenal. I've never seen such a large artery.

It's, um... This is amazing.

The scan reveals an astonishing change inside Nigel's skull.

We're now at 8,000 metres. You can see this massive artery,

which is roughly twice the diameter that one would expect.

This, I guess, must be this sort of size because we're trying to get

a lot more oxygen there because there's so little oxygen around.

This is fantastic. This is really exciting stuff.

As long as it gets me to the summit of Everest, I'm excited.

At the moment, Nigel is OK.

Only when this process runs out of control

can it lead to a potentially fatal form of brain swelling.

It's called high altitude cerebral oedema, or HACE.

This is the exact condition that is affecting the Nepali climber, Usha.

OK, just come with me.

My name's Denny. Well done.

Just sit down here.

That's right, sweetheart. OK. Just get her into the middle.

Very tired-looking. All right.

How is she now? Any pain?

TRANSLATOR SPEAKS IN NEPALI

-No?

-No.

OK, just pop the head forwards.

-Different mask.

-110 over 70.

-Excellent.

The swelling of the brain has been arrested,

but it may be too late to save her from the frostbite.

Dehydration would make the injury from the frostbite worse,

so we're going to help things along by giving her some fluids into her vein.

I think tonight we need to keep her here to make sure the pain is under control.

She needs oxygen, one, to try and improve

the oxygen supply to the fingers and salvage any cells

that are just short of oxygen and not dead yet and, secondly,

because she's had the altitude sickness,

it's important that she stays on oxygen.

Despite her injuries, Usha is fortunate.

Others exposed to such low levels of oxygen do not survive the ordeal.

And it is this question, of who lives and who dies, that stands at the centre of the expedition.

In their largest experiment, the Caudwell Xtreme Everest Team

have recruited an army of guinea pigs to make their way to Base Camp.

Each of the 200 volunteers has given up

nearly a month of their lives to subject themselves to the effects of altitude.

Waiting for them at the end of the 60 kilometre journey is Dr Paul Gunning.

It's hard to think of a body system not affected by altitude.

We treat changes in altitude quite casually,

but the consequences to every system in the body are considerable

and we've seen that already on the trek with people becoming unwell

and feeling symptomatic from altitude.

Some will be affected more than others...

..and with this many volunteers being tested, the doctors believe they can find out why.

Today's a 600 metre gain,

so I'm pretty sure that some people

will get some splitting headaches and be suffering a bit.

The trekkers come from all walks of life,

ranging from 18 to 73 years old,

fit and unfit, men and women.

Just keep turning them, turning them, turning them.

Technically, I'm on my way to suffering a very large, large amount.

Top thing is the blood oxygenation, which usually is 98%

at around sea-level, which is why I'm breathing quite hard.

Keep that going.

Andy Walker is 34 years old and is already finding coping with the decreasing levels of oxygen hard.

ANDY COUGHS

Keep going, keep going, keep going. OK.

I am on vacation.

Taking leave from work, I sit back, a man of leisure(!)

Most intriguingly, what makes some of them do better than others

doesn't seem to depend on fitness or youth.

Yeah, I'm just feeling a bit ropey,

a bit nauseous when we first arrived. Not doing the walk.

I felt fine doing the walk. Maybe the adrenalin keeps you going,

like you have to get somewhere, but once we sat down,

not too good.

It seems that some people are simply born with a better ability to deal with low levels of oxygen.

What we're specifically trying to do here is not just look at

what the effects of altitude are, but who gets the changes in altitude, who doesn't,

and how they cope with changes in altitude and how they acclimatise to changes in altitude.

On the mountain, not adapting well to a lack of oxygen makes climbing difficult.

But in intensive care, it could be the difference between life and death.

It's a lottery the doctors are confronted with every day.

Somebody who appears to be very fit may become very seriously unwell

and, against all odds, they may die,

whereas a more frail patient may surprise us all and survive the most terrible of illnesses.

And it may be that the things that predict

how people do at high altitude will be the same things

that predict whether they would survive a critical illness.

The team are convinced the answer to this mystery lies in the data they are collecting from the trekkers.

Just pop your chin on the block and look down there for us.

With this many subjects, they hope to hunt down the genes

that not only make people natural-born climbers, but also natural-born survivors.

I think in our wildest dreams, what we would love to see is that some people

have certain genes which allow them to use oxygen more efficiently than others

and so, what we're really, really hoping from this trip

is that we could target treatment to poor oxygen users.

They already know one of the genes that controls

how well someone responds to low oxygen. It's called the ACE gene.

This is one of the reasons why, for this study, we have a huge sample size

and this is so that we can see, overall, who are the people that perform well

and then we can relate that to underlying genes.

If they can pinpoint the other genes involved, they might be able

to develop a drug that could turn poor oxygen users into strong ones.

The ideal scenario would be that we could switch yourselves into a more efficient mode of functioning

so that whilst you're exposed to these low levels of oxygen, you use the oxygen most efficiently

and should hopefully survive and, for me,

that would be the best possible outcome of Xtreme Everest.

But the team are hoping one test will have a more immediate effect.

High on Everest, the doctors at Camp Four have a single goal.

They want to find out how little oxygen the human body requires to survive.

It's the extreme physiology

which we're interested in here.

How much oxygen is there in someone standing on the top of Mount Everest

who is able to function in a reasonably normal way?

That will be useful to us in intensive care, just to know

how low an oxygen level people can tolerate and still survive.

In the next 24 hours, they will attempt to take a sample of blood

from deep within an artery on the summit of Everest.

Mac, Mike, over.

RADIO: 'Have an updated weather forecast for you. Everything remains stable.'

Copy that, over.

'There is a but.

'The wind is moderate to strong and increases more and more.

'At the summit, the wind speeds range between 30 and 80 kilometres per hour.

'Received so far, over?'

Copy that, over.

The smallest of windows in the weather brings the summit in reach.

This evening, everybody in this tent will be

hopefully climbing to the summit of Everest, which is 8,850 metres.

Ascending the 900 metres from Camp Four to the summit

will take the team over some of Everest's most infamous landmarks.

First, they must overcome the steep ice slope that leads to the plateau called the Balcony.

From here, a perilously narrow ridge leads to the summit.

To avoid the approaching gales, the team has to reach the top by midday tomorrow.

It means leaving the South Col in the dead of night.

Further down the mountain at Camp Two,

Denny will face an agonising wait.

Mike is my husband and my other half.

I've thought about it quite a lot, it's not going to be an easy time

and it will be a nerve-racking time, waiting for him to come back down.

'I think it's the fear of the unknown and the knowledge

'that people have come a cropper in the past and not made it up and down,

'so it's a combination of excitement and feeling on the edge of your seat

'and also really wanting to hear Mike's voice.'

The departure time is nearing for the summit team.

But there's a troubling development.

We're a bit worried - the weather forecast changed

and it's a bit windier than we hoped.

And it's been windy this afternoon on the South Col, so I think the guys are a bit nervous setting out.

WIND HOWLS

Risking the weather could be a life or death decision,

but at this altitude, the climbers are in no fit state to make it.

The air that they are breathing is so thin

that it's affecting their ability to think clearly and may even be making them delirious.

So the team has agreed that any decision made high on the mountain

has to be referred to those further down.

At Base Camp, this responsibility

falls to expedition manager, Mac Mackenny.

We're going to give them a call in one hour.

So that will be 22.30.

And then just get a check in, asking them a series of questions,

so we can basically establish what state they are in, how they're doing.

That's critical.

Absolutely critical.

Mac has to repeat the checklist every hour to ensure each climber is

mentally and physically fit enough to endure the rigours of Everest.

Denny Levett, Denny Levett, this is Mac at Base Camp, over.

Heh heh heh heh.

If not, he has the authority to order them off the mountain.

It's a heavy responsibility, as for every 15 people who summit, one person dies trying.

OK, standing by.

Everyone OK? Michael, how are you doing?

Despite the volatile conditions, the team get the go-ahead.

Denny, this is Mike. Do you read?

Reading you loud and clear, hun, go ahead.

OK, we're heading off in it.

Very best of luck from all of us and be careful.

And come back safely. I love you.

I love you too and we'll be very careful, over.

There is that anticipation.

This is it, this is really it.

We're going to be able to go after all.

I just remember butterflies in my stomach,

wondering how difficult it would be and would we make it,

but then, finally, you're off.

Already, the safeguards are looking fragile.

-Mike, this is Denny, over.

-STATIC

We've got some pretty bad communications at the moment.

We're getting comms with Camp Two,

but unfortunately we have no communication at all with Camp Four.

I don't know if they can hear us, but we can't hear them.

Mike, this is Denny, over.

FUZZY RESPONSE

Mike, you're breaking up.

FUZZY RESPONSE

Sorry, could you repeat, please, Mike.

CRACKLING

I just have to sit here and wait for the next 12 hours, I guess.

-Scary, though.

-Yeah, scary. It's not a nice feeling, is it?

And it's his birthday today. He's 41 today.

Anyway, a good thing to do on your 41st birthday, get to the summit of Everest, I guess.

The team have to move fast.

If they have not reached the Balcony by daybreak, they could be too late to reach the summit.

But there's a more pressing reason to keep to the schedule.

The climbers have entered the Death Zone.

The air they are breathing is simply too thin to sustain life.

Even just a few hours' exposure to these low levels of oxygen

can result in irreparable damage to their body tissue.

-Mike, this is Denny.

-Hello, this is Mike, over.

Another hour passes and Mac is still out of range.

Are you all still together?

Are you all still together?

Excellent, that sounds brilliant.

So frustrating that we can't hear ourselves.

-Mac, Base Camp, this is Denny, over.

-Denny, this is Mac at Base Camp.

Quite keen, if possible, when they get the chance,

to try another frequency with you being the only one who can hear them, over.

Well, to try and change radio stations means removing gloves etc

and I don't think that's necessarily wise since we have got a signal, over.

OK, I'm starting to lose you as well now, Denny.

But I understand you say to wait until light,

so Mac, standing by.

-She's got a point.

-Yep, absolutely.

It's all right when it's all going well, but if there's trouble...

I don't like working off luck. I'd sooner know.

Despite losing half of their lifeline, the summit team clamber further into the night.

We climbed and climbed and climbed and for the first,

it must be about three hours, it was really, really hard work

and I think quite a lot of us were thinking,

perhaps we'd like to turn around and go back to the South Col.

At that stage, it starts to feel a lot more difficult.

The rhythm, not just coming so well

and your feet were quite cold.

Wondering, you know, is it frostbite?

And if it's frostbite, should we move on? Will I have to turn back?

-Mike, this is Denny, over.

-CRACKLES

Mike, Mike, this is Denny, over.

That's everything, hun. Will you call again in an hour?

OK, thinking of you then, lots of love.

Standing by.

Mac at Base Camp, this is Denny, over.

Yes, this is Mac, go ahead.

I've just had contact from the climbers.

Location, just between Balcony and South Summit.

One of the most exciting things about climbing through the night

so high on Everest is seeing dawn, which just takes for ever.

It also brings with it the promise of warmth,

which makes it seem that forever is taking even longer.

Dawn, really, when you look back at what you've just climbed

and you think, oh my goodness, how am I ever going to get down there?

It looked really frightening to look back and see what you'd done.

The team have been climbing for eight hours.

The summit is tantalisingly close.

But they are yet to face one of Everest's greatest perils,

the Ridge.

You are going into an area which is littered with bodies,

so many people have died on that ridge.

At just half a metre wide, there is no room for error.

You are wondering why you're doing it,

but you're pushing yourself forward, because you know this is it.

You're only going to get one chance at this mountain, despite the danger.

You have to be lucky on Everest.

You have to be lucky for every step that you take,

because a lot of the things that happen up there are outwith your control.

As the climbers go further, the odds against them all surviving get shorter.

More than 30 people have lost their lives climbing this exposed track.

But high on Everest, the Ridge is not the biggest threat,

it is the phenomena these doctors have come here to try and understand,

the desperate shortage of oxygen called hypoxia.

Climbing high up on the mountain when there is so little oxygen is almost a dreamlike state.

You feel like you're drunk, you feel soporific.

You just want to sit down in the snow and lie down and do nothing

and every footstep is an effort of will and physically pushing the body.

Without additional oxygen, the climbers could be overcome by hypoxia,

their body systems shutting down one by one,

leading to a loss of consciousness and, eventually, death.

Using the bottled gas only increases the amount of oxygen they breathe by two per cent,

but this tiny fraction is enough to make a vital difference.

It is not uncommon to have to take 15 huge breaths

between each pace and despite that,

every 10 to 15 steps, you just want to collapse down into the snow.

Eight hours in the Death Zone is taking the climbers to the brink of their physical ability.

At sea level, they would be taking ten breaths every minute.

But here, in the critically thin air, their breathing rate has soared to 80 breaths a minute.

And their oxygen-starved minds will struggle to make sense of the world around them.

Essentially, you're climbing by yourself.

Once you've got your oxygen mask on, the person who's four yards in front of you could be on another planet,

and you hear your own breathing, you hear blood rushing in your ears,

you hear your own feet on the snow and that's about it.

The summit is just 50 metres away, but they will be some of the hardest to overcome.

Ahead of the team stands a sheer face of rock

named after Everest's first summiteer, Sir Edmund Hillary.

The Hillary Step is the most technical part of the climb.

And however well you've done up to that point, there's always a certain amount of fear

that you might not be able to negotiate it.

You can sort of climb up on to the top of the step, almost,

and then round another little block and you're right on the edge there

on a tiny little step and as you look down, it's just miles down into Tibet

and you're quite careful about your footing at that point as you edge your way round.

At that altitude, just hauling yourself up a rock face is hard work.

You're really out of breath, gasping as if someone's strangling you.

Once you've got to the Hillary Step and you've managed to get over it,

that's it, the climb's done and you just walk to the summit.

We knew that no-one was going to stop us getting to the summit then, I think.

It went on for what seemed like an eternity,

because, you know it's not that far, you know it's not that further up,

but it seemed to go on and on and on and you can't actually initially see the summit.

You really just come round a corner and suddenly the summit is there.

The mountaineering challenge may be over, but the science is yet to begin.

The team have subjected themselves to this ordeal

to quantify what the extreme limits of human survival are.

Once they reach the summit, they will take blood from deep within their arteries

and measure just how little oxygen there is keeping them alive.

Pasang, Pasang, is that you, over?

At last, the radio silence is broken.

Pasang, Pasang, is that you, over?

Yes, it is Pasang, over.

This is Denny at Camp Two. Are you at the summit, over?

Yeah, we have arrived at the summit, over.

Congratulations!

Who else is with you?

Ah, we are all of us in the summit, over.

Fantastic news.

That's excellent.

Mac at Base Camp, this is Denny.

Denny, this is Mac at Base Camp. Go ahead, over.

I'm pleased to report that Mike, Dan...

Uh, Sundeep, Nigel, Chris, Dave and all the Sherpas are on the summit.

WHOOPING

Five years of planning, preparation and, ultimately,

sheer determination, finally get their reward.

It's a great moment when you get to the top.

The views are...

everything that you would imagine they would be,

and we had a pretty clear day.

The scale on Everest is almost incomprehensible.

The way I try and explain it to people who've never been

is that next time you fly, have a look out the window and imagine trying to walk up to that altitude.

Just to stand on the top and look from the top

is an amazing sense of achievement,

but there's an odd feeling you can't explain

when you're standing on the top of the mountain looking down on everything else.

And then there's nothing really to compare to it.

You instantly forget about the horror of getting to the top,

and for a while you forget about the long journey down.

And you're just sort of held in time,

looking from the summit. You don't want to leave.

You just want to stand there and make the most of it, having spent weeks getting there.

-Mike, over.

-Mike, I love you.

I am very, very proud of you, honey.

Thanks, angel.

Standing by.

Standing by.

Really good news.

At Base Camp, word of the team's success has spread.

But the climbers are already off the summit.

The team's ultimate ambition has hit a problem.

The fierce winds have driven them 400 metres down the mountain

to set up their final laboratory.

Mike was obviously thinking clearly,

because I think it was the right decision,

and before we'd even realised it,

he was saying, "Right, it's time to go."

VJ, Sundeep, over.

With their oxygen masks removed, the climbers are breathing some of the thinnest air in the world.

Oxygen saturation for Chris Imray, is 62 per cent. Figure 6 - 2, over.

I've just had one of the best days of my life.

At sea level, this is a routine procedure, but at this altitude,

it requires all the doctor's skills.

The result is equally unusual.

The colour is blue.

It is very dark, as if it was in an artery.

It is flowing into the syringe of its own accord.

Arterial blood coming from the heart is usually bright red.

The uncharacteristic colour can only mean one thing.

Severe oxygen depletion.

That is an amazing piece of science.

No one has ever done blood gases above 8,000 metres.

What altitude are we? 86 or something?

It is outrageous science.

One of the team's Sherpas, Pasang, takes these blood samples to Camp Two

for an exact measurement of the oxygen content.

For him, it's a familiar journey.

For the doctors, it's fraught with peril.

Whenever you summit a mountain of any size,

your celebration of reaching the summit

shouldn't really begin until you're back down safely.

It's almost universally true that most climbing accidents happen on the way down,

when you may have taken your focus off why you are there.

You've got to the summit, you're now coming down, you're on ground that you've already been over.

It's now daylight, rather than dark, so you might feel more comfortable with where you are,

but both physically and mentally, you're far more exhausted.

The team have made it to the relative safety of the South Col.

At under 8,000 metres, it is just out of the Death Zone.

Congratulations.

OK, Mac, we have all climbers and all Sherpas back in camp.

Everybody fully intact and feeling well, over.

I think that's it.

Well done.

Carried by the Sherpa Pasang,

the precious blood samples arrive at Camp Two in record time.

Pasang!

Very well done.

Give me a hug.

Well done. Very fast. Two hours.

Two hours from the summit.

Amazing.

-He summited...

-10.30.

10.30. Two hours exactly.

Very quick.

The team already believe that the blood samples will be low in oxygen,

but now Denny hopes to back it up with some solid figures.

We knew from preliminary work that the numbers

that we were going to get are those

which most of our colleagues would say are not compatible with life.

Thank you very much. This is very precious.

And we wanted to demonstrate that not only are they compatible with life,

but actually you can make very rational thoughts up there and effectively,

you can function to pretty near sea-level norms.

It was vital that we got this data as high up the mountain as we could,

because the higher up we were,

the more outrageous the results would seem.

X08. So this is Sundeep's arterial.

PC02, 1.77. Woah.

P02, 4.9. I have never seen a carbon dioxide that low.

The preliminary results are amazing.

Ordinarily, the P02 number, meaning oxygen content, would be around 13.

Anything less than eight would place someone in Intensive Care.

In a clinical situation, you would never see an alive patient with gases anything like that.

Very, very low.

Somehow, Dan has managed to summit Everest

with the lowest blood oxygen level ever measured in a living human.

P02, 3.50.

Dan should definitely be dead.

In a stroke, they have re-written our understanding of the limits of survival.

It's incredibly satisfying to have climbed Everest

and to know that I don't really have to go back again is good.

From all sorts of perspectives, we achieved what we wanted to do.

CHEERS

I'm really happy.

They all look so well, don't they?

They all look so well.

Well done. I am so glad you're back!

I don't have a huge drive to go back now, but I guess

I've only been home a couple of months, so we'll see how things go.

It's a good feeling to be home. Everybody else is home.

The nervousness about impending disaster doesn't really go away

until you're back in Kathmandu or even back in London.

As we said earlier, the haemoglobin's gradually been coming down. Any signs of infection?

Mike is living proof that the human body can survive the condition

that kills many of his patients in Intensive Care.

And now the team can find out why.

Guys, all right?

First, they will have to analyse the tens of millions of pieces of data.

Once complete, it will be the most comprehensive study of hypoxia in the human body.

And that could unlock the secret of life in the most critically ill.

I think it's a fantastic achievement.

This is the most ambitious project that's ever been carried out at altitude, certainly.

Over 90 per cent of what we planned to do, we've done,

and that is a fantastic achievement and I think that data

will be very powerful in terms of giving us information

about adaptation to hypoxia that will hopefully help our patients in the future.

By taking themselves to the highest point on Earth

and risking their own lives, the doctors may one day save many more.

There wasn't much time whilst we were away

to actually take pleasure in what we'd achieved.

And it's actually quite nice at the moment, when you come back and see your friends

you haven't seen for a while and they say, "How was it?"

And you think, "That was really good, actually."

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