Abstract
We appreciate the perceptive dialogue by Millet and Debevec (2020a) regarding the divergent findings regarding the prophylactic use of an expiratory resistance and dead-space mask, which proffered benefits in normobaric hypoxia (Patrician et al., 2019), but was detrimental in hypobaric hypoxia (Carr et al., 2020). We have followed with interest the continued debate surrounding normobaric and hypobaric hypoxia (Richalet, 2020; Millet and Debevec, 2020b), and given our discrepant findings, the influence of barometric pressure could indeed be considered. However, we feel that a primary contributor to the ergolytic effect of the mask in hypobaric hypoxia is mostly attributable to a subtle modification of mask design, which unbeknownst to the authors, was made before the start of the hypobaric field study. As mentioned in our study (Carr et al., 2020), the modifications inadvertently increased the expiratory resistance above that of the mask tested in normoxia. Thus, evaluation of the mask became extremely challenging because the majority of participants could not tolerate the mask and reported symptoms of dyspnea and difficulty falling asleep. The exact mechanism(s) for the diminished compliance are unclear, but ultimately are likely related to the heightened expiratory resistance. It is also noteworthy that the adverse responses to inordinately high resistance occurred even in the presence of the lower air density at high altitude.