J Appl Physiol 116: 1256, 2014; doi:10.1152/japplphysiol.00945.2013.
Letter to the Editor
Reply to Debevec and Millet Normand A. Richard,1 Inderjeet S. Sahota,4 Nadia Widmer,2 Sherri Ferguson,3,4 A. William Sheel,1 and Michael S. Koehle1,4,5 1
School of Kinesiology, University of British Columbia, Vancouver, British Columbia, Canada; 2Department of Medicine, University of British Columbia, Vancouver, British Columbia, Canada; 3Environmental Medicine and Physiology Unit, Faculty of Science, Simon Fraser University, Burnaby, British Columbia, Canada; 4Department of Biomedical Physiology and Kinesiology, Simon Fraser University, Burnaby, British Columbia, Canada; and 5Division of Sport Medicine, Department of Family Practice, Vancouver, British Columbia, Canada Submitted 14 August 2013; accepted in final form 14 August 2013 TO THE EDITOR: We agree with Drs. Debevec and Millet (1) that a study with either a longer duration (⬎6 h) or intermittent hypoxic exposures might detect differences in certain outcomes between hypobaric hypoxia (NH) and normobaric hypoxia (HH). Symptoms of acute mountain sickness (AMS) can develop within 1 hour (2) of continuous hypoxic exposure. Minute ventilation has been shown to achieve steady-state over 40 minutes (6) to a few hours (3), and changes in chemosensitivity are noticeable after a 3-hour hypoxic exposure (4). We selected to use a 6-h exposure to capture key, early changes in respiratory control and to cause AMS in approximately one-half of participants. One must also consider that when comparing different methods of inducing hypoxia (i.e., NH vs. HH) the influences of utilizing terrestrial altitude as opposed to a hypobaric chamber. Although terrestrial altitude can be more practical than chamber hypobaric hypoxia, it has the disadvantage of preventing blinding and being influenced by confounders such as weather patterns, travel stress, fatigue, and dehydration. There are myriad experimental variables in addition to duration that should be considered when comparing multiple studies. For example, the degree of hypoxia or altitude, amount of exercise training at altitude, cardiorespiratory fitness, participant age, sex, and ascent profile could be experimentally manipulated. The parameters we assessed (5) were restricted to control of breathing and AMS susceptibility, and we did not generalize our findings to exercise and other outcomes.
Address for reprint requests and other correspondence: M. S. Koehle, School of Kinesiology, Univ. of British Columbia, 210-6081 Univ. Blvd., Vancouver, BC, V6T 1Z1, Canada (e-mail:
[email protected]).
1256
As Drs. Debevec and Millet rightfully indicate, additional studies are necessary to comprehensively characterize the differences between hypobaric and normobaric hypoxia. We have shown, using a rigorous design that 6 h of continuous hypoxia is not sufficient to demonstrate cardiorespiratory or symptomatic differences between normobaric and hypobaric hypoxia at rest, yet we anticipate that this study will represent just one of many rigorously controlled studies comparing the two interventions to come. DISCLOSURES No conflicts of interest, financial or otherwise, are declared by the author(s). AUTHOR CONTRIBUTIONS Author contributions: N.A.R., A.W.S., and M.S.K. drafted manuscript; N.A.R., A.W.S., and M.S.K. edited and revised manuscript; N.A.R., I.S.S., N.W., S.F., A.W.S., and M.S.K. approved final version of manuscript. REFERENCES 1. Debevec T, Millet GP. Discerning normobaric and hypobaric hypoxia: significance of exposure duration. J Appl Physiol; doi:10.1152/japplphysiol.00873.2013. 2. Hackett PH, Roach RC. High-altitude illness. N Engl J Med 345: 107– 114, 2001. 3. Loeppky JA, Icenogle M, Scotto P, Robergs R, Hinghofer-Szalkay H, Roach RC. Ventilation during simulated altitude, normobaric hypoxia and normoxic hypobaria. Respir Physiol 107: 231–239, 1997. 4. Mahamed S, Cunningham DA, Duffin J. Changes in respiratory control after three hours of isocapnic hypoxia in humans. J Physiol 547: 271–281, 2003. 5. Richard NA, Sahota IS, Widmer N, Ferguson S, Sheel AW, Koehle MS. Acute mountain sickness, chemosensitivity and cardiorespiratory responses in humans exposed to hypobaric and normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00319.2013. 6. Savourey G, Launay JC, Besnard Y, Guinet A, Travers S. Normo- and hypobaric hypoxia: are there any physiological differences? Eur J Appl Physiol 89: 122–126, 2003.
8750-7587/14 Copyright © 2014 the American Physiological Society
http://www.jappl.org
