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Another aspect of living above 3,000m, and something which came as a bit of a shock when we started our research, is a phenomenon known as periodic breathing (Cheyne-Stokes respiration) during sleep. This is pretty much "what it says on the tin" - a cessation of breathing for a short period - and is caused by what seems to a non-medically trained person to be a counter-intuitive physical response to the low oxygen levels. Anyone not trained in this area would expect the body to "cry out for more oxygen" when there is not enough of the stuff in the air but if we understand the literature correctly periodic breathing occurs because our physiology doesn't work quite like that. The lack of oxygen at altitude results in significantly increased breathing, as mentioned here, which in turn results in reduced levels of carbon dioxide in the blood, presumably because the increased breathing means the body simply exhales more of the gas. And because the level of carbon dioxide is used by the body to "tell it to breathe", a reduced level in the blood means the body's signals to breathe are reduced and it simply doesn't bother taking any more breaths. After a while another response cuts in and the body breathes again. When you are awake this does not seem to be problematic as you can sort of force yourself to breathe but during sleep an odd pattern of breathing can develop. According to the literature the result is that the sufferer moves from normal breathing to holding their breath or, effectively, not breathing for what probably looks and feels like "forever" before other signals cut in and normal breathing starts again. This can cause the sufferer to wake up suddenly with a feeling of suffocation, causing, understandably, significant anxiety both for the sufferer and anyone in the tent with them who may have woken up to find that their partner had stopped breathing. The message to both sufferer and companion appears to be to stay calm and wait for normal breathing to resume after a short period - probably easier said than done! Periodic breathing is also explained in detail in the International Society for Mountain Medicine and www.altitude.org articles and a number of other sources and is worth reading about. This was not one of the affects of altitude either of us was looking forward to and in the event it is difficult to say whether or not either of us suffered from it. Barbara has no recollection of being affected while Gary does recollect coming to on some occasions with a strange breathing pattern but not enough to cause concern or really wake him from his sleep. Therefore if this was periodic breathing it must have been a mild dose.
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