Response to Limper et al. re: “Positional Changes in Arterial Oxygen Saturation and End-Tidal Carbon Dioxide at High Altitude: Medex 2015”

Dear Editor:

Thank you for the opportunity to respond to the letter by Limper et al. commenting on our recent publication (Kuenzel et al., 2020). The interesting data presented by Limper et al. (2020) in their letter and recent publication are a useful extension of our findings. There are of course multiple differences between our study and that of Limper et al. Our subjects were likely better acclimatized to altitude, having spent 2–3 weeks of gradual ascent to 5050 m. Second, it is unclear whether the 30° incline used in the Limper trial is enough to significantly alter SpO₂, although their data suggest that it is not. Finally, in our study we purposely asked subjects to remain awake throughout the data collection period to avoid confounding by altered control of breathing during sleep. Given this, there is no conflict in the data in our study and those of Limper et al. (2020).

Not surprisingly subjects in Limper's study showed much worse sleep quality at high altitude compared with sleep quality in their usual low-altitude sleep environment. Although hypoxia itself contributes to poor sleep at altitude (Khoo et al., 1996), other things besides oxygenation affect sleep quality, including the sleep environment and periodic breathing, which is a function of both “plant gain” and “controller gain” (Ainslie et al., 2013). This is also reflected in the 2018 update of the Lake Louise score that eliminated sleep as a questionnaire item (Roach et al., 2018). Positional saturation changes are real, but the response is highly variable among individuals, and it is unclear how consistent the response is from one time to the next in a given individual. Although we observed a mean positional SpO₂ difference of 2.5%, nearly 11% of our subjects had a positional SpO₂ change of >5%. At the same time, SpO₂ during sleep is among the lowest values experienced while at high altitude (Anholm et al., 1992; Ainslie et al., 2013) and it is unknown whether the positional changes we observed carry over to more profound nocturnal desaturation. Whatever the mechanism(s) behind impaired sleep at high altitude, changes in body position are likely to be less effective at ameliorating acute mountain sickness than acetazolamide (Fischer et al., 2004; Limper et al., 2020). We agree with the comments of Limper that findings in healthy young individuals cannot be extrapolated to patients with heart or lung disease when they travel to altitude.