to the editor: We thank the authors of the comments (see Ref. 1) who contributed their thoughts to this debate (3, 5). Many additional interesting arguments were provided, and most of the comments are in line with our view that hypobaric hypoxia induces different physiological responses from normobaric hypoxia but also that the underlying mechanisms and clinical/practical relevance have not yet been investigated enough.
Regarding the mechanisms, we concur with Verges et al. (see Ref. 1) that motor cortex excitability might be affected differently between HH and NH (4). We also agree with Laymon et al. (see Ref. 1) and Verges et al. (see Ref. 1) that the cost of breathing is likely different between HH and NH. Several authors (Conkin; Guenette and Koehle; Loeppky; Taylor et al.; Vagula and Nelatury; see Ref. 1) supported the view that pressure change associated with hypobaria plays a role in pulmonary fluid flux differences between HH and NH (2). However, because “there is no direct comparison…in HH vs NH…on AMS severity” as noted in our Point (3), the question of the clinical relevance for AMS remains unanswered (Loeppky vs. Schommer and Bartsch; see Ref. 1).
Finally, the comments of Conkin and Wessel (see Ref. 1) or Koehle et al. (see Ref. 1) on the inaccuracy of defining correct hypoxic levels, those of Chapman and Levine (see Ref. 1) on the differences in intermittent exposure between NH and HH or those of Girard (see Ref. 1) on the impact on air resistance during repeated sprints support that, from a practical point of view, training in NH is different by nature from training in HH. To conclude, we would like to borrow the statement of Pun (see Ref. 1): Hypobaric hypoxia has complex and multitude of stimuli compared with normobaric hypoxia.
No conflicts of interest, financial or otherwise, are declared by the authors.
Author contributions: G.P.M. drafted manuscript; G.P.M. and V.P. edited and revised manuscript; G.P.M., R.F., and V.P. approved final version of manuscript.
- Copyright © 2012 the American Physiological Society