Respiratory gas exchange disorders may predict serious hypoxemia and even acute mountain sickness in individuals who rapidly ascend to high altitude. Thus, monitoring arterial oxygen saturation (SaO 2 ) can be used to predict HAPE. Studies have shown that 80-100% of patients with serious hypoxemia develop HAPE. Accordingly, Roach et al. [16] speculated that noninvasive measurement of SaO 2 was a simple and convenient way to predict the onset of HAPE. Similarly, Burtscher et al. [17] suggested that measurement of SaO 2 could be used to predict the onset of acute mountain sickness. In a prospective cohort study, Shen et al. [18] found that there were significant individual differences in the changes in oxygen saturation with increases in altitude. They found that when the SaO 2 decreased by >30% at low altitude, the susceptibility to HAPE increased markedly and about 62% of the participants developed HAPE. Thus, they speculated that a decrease in SaO 2 > 30% offers a biomarker to predict susceptibility to HAPE.