Re: “The Effect of an Expiratory Resistance Mask With Dead Space on Sleep,Acute Mountain Sickness,Cognition,and Ventilatory Acclimatization in Normobaric Hypoxia,” by Patrician et al. and “Global REACH 2018: The Effect of an Expiratory Resistance Mask with Dead Space on Sleep and Acute Mountain Sickness During Acute Exposure to Hypobaric Hypoxia” by Carr et al.

We read the two successive articles from the same extremely qualified research group, on the effects of expiratory resistance masks on hypoxia acclimatization indices (Patrician et al., 2019; Carr et al., 2020) recently published in High Altitude Medicine and Biology with great interest. By using the same methodology [i.e., an expiratory resistance mask with a small amount of dead space (ER/DS)], the authors showed that the benefits of ER/DS [i.e., reduction in apnea-hypopnea index (AHI), oxygen desaturation index (ODI), and headache severity] observed in normobaric hypoxia (NH) (Patrician et al., 2019) were not present when the experiments were replicated in hypobaric hypoxia (HH) (Carr et al., 2020). The authors should be commended for not translating directly the outcomes obtained in NH to HH and, particularly, for highlighting the need for terrestrial field testing under HH conditions.

A significant consideration regarding the contrasting results between the two studies, particularly regarding the ventilatory adaptation, which has been slightly neglected by the authors, might indeed pertain to the independent effects of barometric pressure (i.e., hypobaria). Principally, as the hypobaria-related reduction in air density might importantly modulate the alveolar dead space. Although the authors indicated that the ER/DS mask might have to be individually adapted to the ventilatory patterns, these might not only differ between participants but might, as has been shown before, also differ between NH and HH (Millet and Debevec, 2020) with a higher breathing frequency resulting in greater desaturation in the later. Importantly, HH is also known to provoke higher periodic breathing incidence with subsequently higher AHI and ODI (Heinzer et al., 2016).

One may, therefore, speculate that the specific characteristics of the ER/DS masks would need to be specifically tailored for each individual in each respective condition (i.e., HH might require a higher DS to compensate the larger hypocapnia induced by the higher hyperventilation and a lower ER to minimize the inspiratory strength and fatigue).

As mentioned, the reported comparative results (Patrician et al., 2019; Carr et al., 2020) lend further support to the growing body of data indicating that responses observed in NH may not be present, or might differ, under HH conditions. Similarly, Fulco et al. (2013) recently provided convincing evidence that preacclimatization in NH only resulted in effective ventilatory acclimatization when measured in NH and not when assessed in HH. Collectively these results highlight a specificity of the HH condition and, therefore, imply a direct and independent effect of barometric pressure (Millet and Debevec, 2020). However, further investigation employing different acclimatization enhancing strategies (e.g., expiratory resistance masks) are clearly needed with protocols also employing hypobaric normoxic condition (combining hypobaric chamber and hyperoxic gas) that can help to disentangle the hypobaric and the hypoxic effects in controlled environmental conditions. The question of the respective effects of hypobaria versus hypoxia on various physiological responses (performance; altitude illnesses; cardiorespiratory, cerebrovascular, and cognitive functions; oxidative stress and sleep quality) as well as human health and well-being is obviously of prime importance in high altitude/mountain medicine, space physiology, as well as in clinical settings.