Lukla airport is perched precariously on a high mountainside in northeast Nepal: each morning tiny planes skim the ridge on the far side of the valley and land on a short, sloping runway that juts into the abyss and ends alarmingly with a high stone wall, depositing climbers who are on their way to Everest.

The airport staff were puzzled, then, to see the 8.30am Yeti Airlines flight from Kathmandu disgorge nine rather high-spirited British children.

The Sherpas who had arrived to meet the flight watched in bemusement as the children spilled out of the gates, playing and fighting and falling over and grazing knees. Little did they realise that they were witnessing the start of a medical experiment.

Accompanied by their parents and a grandmother, and encouraged, dragged and occasionally carried by the Sherpas, the children would ascend to above 11,000ft (3,350m), while being monitored by a team of child physiologists. They were guinea-pigs in an experiment to test the effects of altitude on young children.

The Smiths Medical Young Everest Study had begun as a plan for a family holiday. Monty Mythen, Smiths Medical chairman of anaesthesia and critical care at University College London (UCL), was to set up one of a series of laboratories on the route to Everest Base Camp, which would test the effects of altitude on 208 adult volunteers as part of the Caudwell Xtreme Everest project.

The effects of climbing to high altitude, where the group would be breathing in far less oxygen, replicate the effects of critical illness. About 25 per cent of people adapt quickly, 50 per cent adapt more slowly. A remaining 25 per cent fail to adapt at all: in intensive care they die, and in the mountains they may die unless they descend quickly.

“If we can find the physiological switches that help some people adapt to low oxygen levels, we will save lives,” Professor Mythen said. The only problem was that the results could not be applied safely to young children. The professor had a solution: he hoped that his family would visit him at his laboratory. He suggested running a similar set of altitude tests on his own children, and on four of his nieces and nephews. Dr Samatha Sonnappa, a respiratory paediatrician who was to accompany the expedition, offered her own son as a ninth participant. Smiths Medical, which has spent £4 million on research at UCL and produces respiratory devices, agreed to sponsor the trip.

The proposed study caused controversy when it was reported in The Timeslast month. “Friends thought we were absolutely mad,” Professor Mythen’s wife, Kate, said.

The expedition began inauspiciously. The group had not even left Lukla when Harriet Themans, 11, felt sick. While she was being treated, Alice Mythen, 9, fell and twisted her ankle. She could hardly walk: it seemed unlikely that she would manage a day of trekking on a rocky path in the Himalayas. Tom Mythen, 6, the youngest participant, fell over on his nose.

Janet Stocks, Professor of Respiratory Physiology, who was in charge of the medical team, tried to keep calm. “I just thought, ‘What have we let ourselves in for?’,” she said.

Half an hour later, with Harriet feeling a little better and Tom showing almost no sign of his encounter with a paving stone, the children set off. Alice joined them, limping and supported by her mother and a Sherpa. As well as the medical team and the parents, there were 20 Sherpas and porters, carrying clothes and several boxes of medical equipment, and two Sherpanis to look after the smaller children.

If the scientists of the Victorian era took family holidays, they must have looked like this.

Kalsang Sherpa, head of the Walking and Climbing Company, was in charge of logistics. “I have done many horrible things in my life,” he said. He once fell 140ft into a crevice on the Western Cwm, the glacier that fills the southwest side of Everest, ripping the skin off one side of his head. He has never worked with children. “British children are quite spoilt,” he said. “I want to teach them what things are like here.”

The children had arrived equipped with six handheld computer consoles but were forced to leave them behind in Kathmandu. “I never thought I’d say this,” said Charlotte Mythen, 11, after trekking for 17 minutes. “But I really miss my Game Boy.”

The older girls coped with the loss by making up rude songs; the younger boys by treating the walk as a military operation. “This is a sniper’s rifle,” said William Ogden, 10, holding up his trekking pole. “I also have a Gatling gun and some grenades.”

Professor Stocks was worried: at the rest stops it seemed impossible to make the children rest. William’s mother, Caroline Ogden, told The Times: “I had thought the altitude might slow him down. It does not seem to have had any effect.”

She sounded very slightly dispirited. At Lukla the air pressure is about 25 per cent less than at sea level, and oxygen levels in the children’s blood fell from about 98 per cent saturation to 92-94 per cent. All the children, that is, apart from one, who appeared relatively unaffected. As they climbed higher she appeared to display the characteristics of a high-altitude mountaineer, until they approached 10,000ft, whereupon her oxygen levels fell back and those of the other children recovered.

The medical team also tested the heart rate of the children, how fast they were breathing, their short-term memory, their lung capacity and the blood flow to the brain. On steps beside a Buddhist stupa, they carried out exercise tests. There were sleep tests too, when the children went to bed in “life vests” that monitored heart and respiratory rates and their blood oxygen levels.

On the second day of trekking the path cut along a deepening gorge, occasionally swinging out over a foaming green river on high-wire bridges. Katrina, 7, is scared of heights, and crossed them while holding hands with her grandmother, singing Twinkle Twinkle Little Star. Later she began to feel dizzy and had to be carried by a Sherpa named Nima.

“Are we there yet?” the children asked. “Very near,” the Sherpas replied. They were passing into a landscape of increasing grandeur: jagged peaks piled with snow rose on all sides. The children sometimes seemed immune to it all: “Ate some dodgy porridge, seen some yaks, met a monk and walked up all these hills,” Harriet said that evening.

The final day of the trek was the most gruelling: the path tumbled down between two cliffs and then rose steadily and continuously. A horse arrived to pick up Grandma, who rode among the trekkers, like a general among an army of marching child soldiers. The air grew thinner, the children struggled harder to breathe. Professor Stocks said: “This is how it feels for children with leukemia.”

As they climbed the final 2,000ft to Namche Bazaar, where Professor Mythen was waiting in his laboratory, the trek turned into a straightforward race. William had complained that the adults were slowing him down, he led the way with Patrick Mythen, 13, the oldest of the children.

Alice said: “I was so relieved to get here. I nearly cried when I saw my dad. I have never really cried when I have been happy before.”

The medical tests continued. The researchers hope that analysis of the results will reveal why different children adapted at different rates to the low oxygen levels, and in turn that this will open the way for new treatments for children with chronic lung disease, cystic fibrosis, sickle cell disease and sleep disorders.

Professor Stocks said: “The range of responses we have seen will also help us develop treatment for children in intensive care tailored to their needs.”

This week the children climbed higher, to 12,700ft, where the air pressure is 64 per cent lower than at sea level. Most will turn back there (including William, despite his own modest plans to summit Everest), but Patrick plans to push on to Everest Base Camp, at 17,600ft, where the air pressure is 50 per cent lower than at sea level, and climbers, breathing in 50 per cent less oxygen, have to fight for every breath. “I hope I make it,” he said.

Dizzy heights

— Some people suffer mild symptoms of acute mountain sickness (AMS) at 2,000m. Above that altitude, climbers try to stick to “the 300m rule”, not ascending more than 300m a day

— Above 3,000m 75 per cent of people suffer mild symptoms. These include lethargy, headaches, mild anorexia, nausea, a feeling of breathlessness and disrupted sleep patterns

— Severe forms of AMS include high-altitude cerebral oedema, (HACE), and high-altitude pulmonary oedema, (HAPE). HACE involves fluid leaking into the brain; HAPE occurs when fluid builds up in the lungs. Both can be fatal if the victim does not descend as fast as possible

— At Everest Base Camp, at 5,300m, many climbers also report vivid dreams, sometimes horrifying, sometimes pleasant. About 40 per cent of climbers also suffer retinal haemorrhages

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