Headaches, nausea, weakness and dizziness, combined with a feeling that you just can’t get enough oxygen with each breath are just some of the signs of altitude sickness. Researchers have now found a link between the abnormal breathing patterns experienced while sleeping at altitude and the symptoms of altitude sickness, particularly headaches. The finding suggests that finding a way to breath normally while sleeping could provide a way to combat such symptoms.

Breathing patterns are affected by the fall in the partial pressure of oxygen at altitudes higher than around 2,500 m (8,200 ft). While awake, climbers are able to regulate their breathing consciously, but when asleep a cycle known as Cheyne–Stokes breathing is experienced. This involves an alternation between rapid, deep (hyperventilation) and slow shallow (hypoventilation) breathing, with periods where breathing stops all-together, known as apnoea. This pattern is the body’s response to varying concentrations of carbon dioxide (CO2) and oxygen in the blood and is an attempt to regulate these levels.

"The lack of oxygen at high altitude causes the climbers to hyperventilate, which leads to a decline of CO2 in the blood," explains Peter Stein, who is in the department of anesthesiology, intensive care medicine and pain therapy at University Hospital Frankfurt. "The decline of CO2 leads to episodes of hypoventilation or even apnoea when the conscious breathing control subsides during sleep. As a consequence the oxygen level drops, causing an arousal and subsequent hyperventilation.”

To investigate breathing patterns at altitude, a group of medical researchers from Germany followed six mountain climbers up Mount Kilimanjaro in Tanzania, hauling a bunch of gear up the mountainside to the base camp, at around 4,000 m (13,100 ft) elevation. To monitor concentrations of oxygen and CO2 in the blood going to the brain, the researchers attached NIR (near infrared) electrodes to the climbers’ foreheads while they slept. NIR spectroscopy is a technique that identifies molecules based on their absorption and reflection of light at near infrared wavelengths.

"The most challenging part was to transport not only the NIR spectroscope into basecamp but also all the equipment necessary to provide electricity," Stein said. "Therefore we bought a lightweight generator and enough fuel to provide power throughout all the nights.”

It was found that the Cheyne–Stokes breathing pattern caused changes in the total haemoglobin and oxygenated haemoglobin but not in the concentration of deoxygenated haemoglobin. From this they concluded that, while the abnormal breathing pattern did alter the flow of blood to the brain, it did not reduce the amount of oxygen reaching the brain tissue.

Another interesting correlation was that climbers most prone to headaches were those who experienced Cheyne–Stokes breathing patterns at night. From this they concluded that possible measures for the prevention of symptoms of altitude sickness such as headaches might be developed by understanding and treating these abnormal breathing patterns.

"Our experiments reveal a pathomechanism contributing to the aetiology of the most common symptom of altitude sickness: headache," says Stein. "I hope that based on our findings it will be possible to develop new therapeutic approaches that help to increase comfort and safety for climbers in the future."

Findings of the research were published in the Journal of Near Infrared Spectroscopy.