The equivalent air altitude model, often employed in altitude medicine research and interventional settings, stipulates that different combinations of barometric pressure (PB) and O2 fraction that produce the same partial pressure of O2 (PO2) result in similar physiological responses (Conkin & Wessel, 2008). One could thus assume that normobaric hypoxia (NH) at the same PO2 might serve as a surrogate for hypobaric hypoxia (HH). However, an ever-growing body of evidence challenges this assumption by indicating numerous differences in physiological responses between NH and HH, which thereby suggests an independent PB effect. Early research work identified key differences in ventilatory (Loeppky et al. 1997; Savourey et al. 2003), fluid balance (Levine et al. 1988) and NO metabolism (Hemmingsson & Linnarsson, 2009) responses to NH vs. HH. In addition, greater acute mountain sickness severity has been implicated in HH vs. NH. (Roach et al. 1996). The evidence from initial work comparing the physiological effects of NH vs. HH has previously been summarized (Millet et al. 2012).
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