Comments on Point:Counterpoint: Hypobaric hypoxia induces/does not induce different responses from normobaric hypoxia
2012; American Physiological Society; Volume: 112; Issue: 10 Linguagem: Inglês
10.1152/japplphysiol.00356.2012
ISSN8750-7587
AutoresOlivier Girard, Michael S. Koehle, Martin J. MacInnis, Jordan A. Guenette, Samuel Vergès, Thomas Rupp, Marc Jubeau, Stéphane Perrey, Guillaume Y. Millet, Robert F. Chapman, Benjamin Levine, Johnny Conkin, James H. Wessel, Hugo Nespoulet, Bernard Wuyam, Renaud Tamisier, Patrick Lévy, Darren P. Casey, Bryan J. Taylor, Eric M. Snyder, Bruce D. Johnson, Abigail S. Laymon, Jonathon L. Stickford, Joshua C. Weavil, Jack A. Loeppky, Matiram Pun, Kai Schommer, Peter Bartsch, Mary Vagula, Charles F. Nelatury,
Tópico(s)Neuroscience of respiration and sleep
ResumoPoint:CounterpointComments on Point:Counterpoint: Hypobaric hypoxia induces/does not induce different responses from normobaric hypoxiaPublished Online:15 May 2012https://doi.org/10.1152/japplphysiol.00356.2012MoreSectionsPDF (79 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat REDUCED AIR RESISTANCE WITH TERRESTRIAL ALTITUDE ALTERS RUN SPRINT PERFORMANCEOlivier Girard.Author AffiliationsResearch Scientist ASPETAR - Qatar Orthopaedic and Sports Medicine Hospital.to the editor: Decades of research in exercise physiology have supported the common view that endurance performance suffers most greatly at altitude because oxidative energy production is limited (4). In contrast, Weyand et al. (5) reported that fit males are capable of running just as fast during “all-out” treadmill efforts of <1 min in hypoxic compared with normoxic conditions (13.00% and 20.93% oxygen, respectively), despite a reduction in the aerobic energy available for sprinting. As underlined by Mounier and Brugniaux (2) in their Counterpoint, it is noteworthy that most of our knowledge on this topic is derived from laboratory-based measurements. When comparing hypobaric and normobaric hypoxia, the effect of air resistance should not be neglected. Indeed, the decrease in air density upon ascent to terrestrial (natural) altitude reduces air resistance, which is likely to decrease the energy cost of running at high velocities, without the detrimental effect of reducing energy availability (3). This may explain—to a large extent—why sprinters generally achieve better performances with exposure to natural altitude. Anecdotally, multiple world records were set in the sprint (i.e. 100, 200, and 400 m) disciplines during the Mexico City Olympics held at an altitude of 2,240 m in 1968, whereas no records were set in the middle- or long-distance running disciplines. By using a mathematical supply-demand model, Arsac (1) calculated improvements in sprinting (i.e. 60, 100 m) times and changes in the components of the energy cost with changes in altitude from 0 to 4,000 m. It was concluded that as the cost of overcoming air resistance decreases with increasing altitude, better performances may be achieved attributable to more energy being available for acceleration. In summary, it should be underlined that the reduced air resistance with terrestrial altitude (hypobaric hypoxia) is likely to induce different responses than exposure to gas mixtures lowering the oxygen fraction (normobaric hypoxia) during running sprints. Of the few studies modeling the effects of natural altitude on the energetics of sprint performance, to date, no study has directly assessed the impact of a reduced air resistance per se, which warrants further research.REFERENCES1. Arsac L. Effects of altitude on the energetics of human best performances in 100 m running: a theoretical analysis. Eur J Appl Physiol 87: 78–84, 2002.Crossref | ISI | Google Scholar2. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google Scholar3. Peronnet F , Thibault G , Cousineau DL. A theoretical analysis of the effect of altitude on running performance. J Appl Physiol 70: 399–404, 1991.Link | ISI | Google Scholar4. West JB. High life: A history of high-altitude physiology and medicine. Oxford University Press, New York, 1998.Crossref | Google Scholar5. Weyand PG , Lee CS , Martinez-Ruiz R , Bundle MW , Bellizzi MJ , Wright S. High-speed running performance is largely unaffected by hypoxic reductions in aerobic power. J Appl Physiol 86: 2059–2064, 1999.Link | ISI | Google ScholarREFERENCES1. Arsac L. Effects of altitude on the energetics of human best performances in 100 m running: a theoretical analysis. Eur J Appl Physiol 87: 78–84, 2002.Crossref | ISI | Google Scholar2. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google Scholar3. Peronnet F , Thibault G , Cousineau DL. A theoretical analysis of the effect of altitude on running performance. J Appl Physiol 70: 399–404, 1991.Link | ISI | Google Scholar4. West JB. High life: A history of high-altitude physiology and medicine. Oxford University Press, New York, 1998.Crossref | Google Scholar5. Weyand PG , Lee CS , Martinez-Ruiz R , Bundle MW , Bellizzi MJ , Wright S. High-speed running performance is largely unaffected by hypoxic reductions in aerobic power. J Appl Physiol 86: 2059–2064, 1999.Link | ISI | Google ScholarCORRECT HYPOXIC DOSE AND EXHALED NO CLARIFICATIONMichael S. KoehleAssistant Professor and Martin J. MacInnisNormand Richard School of Kinesiology, University of British Columbia.to the editor: An important consideration for the interpretation of studies that compare normobaric hypoxia (NH) and hypobaric hypoxia (HH) is the correct calculation of hypoxic dose. In many studies, hypoxic doses are calculated based on the ambient Po2 instead of the PiO2, which negates the effect of PH2O (2). This omission creates a different PiO2 in each condition that can lead to spurious findings. Consider the calculations in Table 1: the ambient Po2 is the same, but as a result of the PH2O effect, the PiO2 is different for each condition.Table 1.ParameterHypobaricNormobaricPatm430 mmHg760 mmHgPH2O47 mmHg47 mmHgFiO20.2090.119Ambient Po2430 × 0.209 = 90 mmHg760 × 0.119 = 90 mmHgPiO2(430-47) × 0.209 = 80 mmHg(760-47) × 0.119 = 85 mmHgTo obtain a truly equivalent hypoxic dose across conditions, the ambient Po2 must differ. Thus it is essential to verify the proper calculation of the hypoxic dose when interpreting studies comparing NH and HH.Second, the rebuttal of Mounier and Brugniaux (6) is correct in stating that some researchers perceived the results of Hemmingsson and Linnarsson (4) as contentious; however, Mounier and Brugniaux (6) misinterpreted the source of the contention. That the partial pressure of exhaled nitric oxide (PENO) differs between NH and HH is actually well supported. For example, several studies have concluded that acute exposure to NH do not decrease the PENO (e.g. 3, 5); furthermore, a decreased PENO in response to HH is a common finding among subjects native to low altitude (e.g., 1, 3). The dispute over the work of Hemmingsson and Linnarsson (4) was largely centered on extrapolations of their data to previous studies of high-altitude natives that employed NO analyzers operating on different technological principles.REFERENCES1. Brown DE , Beall CM , Strohl KP , Mills PS. Exhaled nitric oxide decreases upon acute exposure to high-altitude hypoxia. Am J Hum Biol 18: 196–202, 2006.Crossref | PubMed | ISI | Google Scholar2. Conkin J. PH2O and simulated hypobaric hypoxia. Aviat Space Environ Med 82: 1157–1158, 2011.Crossref | Google Scholar3. Donnelly J , Cowan DC , Yeoman DJ , Lucas SJE , Herbison GP , Thomas KN , Ainslie PN , Taylor DR. Exhaled nitric oxide and pulmonary artery pressures during graded ascent to high altitude. Respir Physiol Neurobiol 177: 213–217, 2011.Crossref | PubMed | ISI | Google Scholar4. Hemmingsson T , Linnarsson D. Lower exhaled nitric oxide in hypobaric than in normobaric acute hypoxia. Respir Physiol Neurobiol 169: 74–77, 2009.Crossref | ISI | Google Scholar5. MacInnis MJ , Carter EA , Koehle MS , Rupert JL. Exhaled nitric oxide is associated with acute mountain sickness susceptibility during exposure to normobaric hypoxia. Respir Physiol Neurobiol 180: 40–44, 2012.Crossref | ISI | Google Scholar6. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different physiological responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google ScholarREFERENCES1. Brown DE , Beall CM , Strohl KP , Mills PS. Exhaled nitric oxide decreases upon acute exposure to high-altitude hypoxia. Am J Hum Biol 18: 196–202, 2006.Crossref | PubMed | ISI | Google Scholar2. Conkin J. PH2O and simulated hypobaric hypoxia. Aviat Space Environ Med 82: 1157–1158, 2011.Crossref | Google Scholar3. Donnelly J , Cowan DC , Yeoman DJ , Lucas SJE , Herbison GP , Thomas KN , Ainslie PN , Taylor DR. Exhaled nitric oxide and pulmonary artery pressures during graded ascent to high altitude. Respir Physiol Neurobiol 177: 213–217, 2011.Crossref | PubMed | ISI | Google Scholar4. Hemmingsson T , Linnarsson D. Lower exhaled nitric oxide in hypobaric than in normobaric acute hypoxia. Respir Physiol Neurobiol 169: 74–77, 2009.Crossref | ISI | Google Scholar5. MacInnis MJ , Carter EA , Koehle MS , Rupert JL. Exhaled nitric oxide is associated with acute mountain sickness susceptibility during exposure to normobaric hypoxia. Respir Physiol Neurobiol 180: 40–44, 2012.Crossref | ISI | Google Scholar6. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different physiological responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google ScholarTHE EFFECTS OF NORMOBARIC HYPOXIA AND HYPOBARIC HYPOXIA ON EXERCISE-INDUCED PULMONARY EDEMAJordan A. GuenettePostdoctoral Fellow and Michael S. KoehleRespiratory Investigation Unit, Queen's University School of Kinesiology, University of British Columbia.to the editor: There is unresolved controversy on whether high-intensity exercise is sufficient to elicit pulmonary edema in humans as detected via different imaging modalities (radiography, MRI, CT). Several recent studies postulated that hypoxic exercise may induce edema to a greater degree than normoxia because of the well known effects of hypoxia on pulmonary vasoconstriction (2–4). Despite a concerted effort and a sound physiological rationale, these studies were unable to demonstrate evidence of lung water accumulation in humans exercising in normobaric hypoxia, despite using sensitive and quantitative imaging techniques (CT and MRI). The present debate raises the question of whether there may be differences in pulmonary edema susceptibility following normobaric vs. hypobaric hypoxic exercise. An examination of previously published imaging studies using different forms of hypoxia suggests that there may be a difference. In contrast to the studies described above, it appears that athletes exercising in hypobaric hypoxia show compelling radiographic evidence of pulmonary edema (1). The physiological basis for this potential difference remains unclear but may be related, in part, to differences in fluid circulation and the trans alveolar-capillary membrane flux, which, as nicely pointed out by Millet et al. (5), may induce greater pulmonary vasoconstriction, thus modifying O2 diffusion via a decreased pressure gradient. Future lung imaging studies are required to compare pulmonary edema responses in the same subjects exercising under both forms of hypoxia in order fully elucidate this potential physiological difference.REFERENCES1. Anholm JD , Milne EN , Stark P , Bourne JC , Friedman P. Radiographic evidence of interstitial pulmonary edema after exercise at altitude. J Appl Physiol 86: 503–509, 1999.Link | ISI | Google Scholar2. Guenette JA , Sporer BC , MacNutt MJ , Coxson HO , Sheel AW , Mayo JR , McKenzie DC. Lung density is not altered following intense normobaric hypoxic interval training in competitive female cyclists. J Appl Physiol 103: 875–882, 2007.Link | ISI | Google Scholar3. Hodges AN , Sheel AW , Mayo JR , McKenzie DC. Human lung density is not altered following normoxic and hypoxic moderate-intensity exercise: implications for transient edema. J Appl Physiol 103: 111–118, 2007.Link | ISI | Google Scholar4. MacNutt MJ , Guenette JA , Witt JD , Yuan R , Mayo JR , McKenzie DC. Intense hypoxic cycle exercise does not alter lung density in competitive male cyclists. Eur J Appl Physiol 99: 623–631, 2007.Crossref | PubMed | ISI | Google Scholar5. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces different physiological responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.ISI | Google ScholarREFERENCES1. Anholm JD , Milne EN , Stark P , Bourne JC , Friedman P. Radiographic evidence of interstitial pulmonary edema after exercise at altitude. J Appl Physiol 86: 503–509, 1999.Link | ISI | Google Scholar2. Guenette JA , Sporer BC , MacNutt MJ , Coxson HO , Sheel AW , Mayo JR , McKenzie DC. Lung density is not altered following intense normobaric hypoxic interval training in competitive female cyclists. J Appl Physiol 103: 875–882, 2007.Link | ISI | Google Scholar3. Hodges AN , Sheel AW , Mayo JR , McKenzie DC. Human lung density is not altered following normoxic and hypoxic moderate-intensity exercise: implications for transient edema. J Appl Physiol 103: 111–118, 2007.Link | ISI | Google Scholar4. MacNutt MJ , Guenette JA , Witt JD , Yuan R , Mayo JR , McKenzie DC. Intense hypoxic cycle exercise does not alter lung density in competitive male cyclists. Eur J Appl Physiol 99: 623–631, 2007.Crossref | PubMed | ISI | Google Scholar5. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces different physiological responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.ISI | Google ScholarTHE NEUROMUSCULAR FUNCTION IN NORMOBARIC VERSUS HYPOBARIC HYPOXIASamuel VergesSenior Scientist, Thomas Rupp, Marc Jubeau, Stephane Perrey, and Guillaume Y. MilletHP2 Laboratory (U1042) Joseph Fourier University and INSERM Grenoble, France.to the editor: We would like to address the difference between normobaric (NH) and hypobaric (HH) hypoxia regarding the neuromuscular function, i.e. a critical factor determining exercise performance in hypoxia. Maximal voluntary contraction and muscle contractile properties are generally unchanged in both NH and HH (5). Only a few studies, however, have compared similar hypoxic exposure since NH studies mostly involved acute exposure whereas HH studies generally involved prolonged exposure with intermediate ascent periods to altitude. This potentially leads to acclimatization, fatigue and/or training. Additionally, other factors (e.g., hydration, stress, temperature, etc.) going beyond the strict definition of hypoxia from Mounier and Brugniaux (4) may then modulate the neuromuscular function. Maximal voluntary activation at rest is not modified under NH but remains to be investigated in HH. While increased or unchanged motor cortex excitability has been reported in acute NH, chronic HH induces hypoexcitability of both excitatory and inhibitory cortical circuits (3). Interestingly, this reduction in cortex excitability correlated well with acute mountain sickness (AMS) severity. Hence, more severe AMS in HH as underlined by Millet et al. (1) might be associated with cerebral perturbations (e.g., subedema) that may also affect the motor cortex. Whether such perturbations specific to HH may lead to greater central motor output impairment in HH compared with NH remains hypothetical. At last, NH enhanced both locomotor (2) and respiratory (6) muscle fatigue during dynamic contractions. Regarding the latter, however, reduced air density and work of breathing in HH may result in lesser fatigue at similar ventilatory level.REFERENCES1. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.ISI | Google Scholar2. Millet GY , Aubert D , Favier FB , Busso T , Benoit H. Effect of acute hypoxia on central fatigue during repeated isometric leg contractions. Scand J Med Sci Sports 19: 695–702, 2009.Crossref | PubMed | ISI | Google Scholar3. Miscio G , Milano E , Aguilar J , Savia G , Foffani G , Mauro A , Mordillo-Mateos L , Romero-Ganuza J , Oliviero A. Functional involvement of central nervous system at high altitude. Exp Brain Res 194: 157–162, 2009.Crossref | PubMed | ISI | Google Scholar4. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google Scholar5. Perrey S , Rupp T. Altitude-induced changes in muscle contractile properties. High Alt Med Biol 10: 175–182, 2009.Crossref | PubMed | ISI | Google Scholar6. Verges S , Bachasson D , Wuyam B. Effect of acute hypoxia on respiratory muscle fatigue in healthy humans. Respir Res 11: 109, 2010.Crossref | PubMed | ISI | Google ScholarREFERENCES1. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.ISI | Google Scholar2. Millet GY , Aubert D , Favier FB , Busso T , Benoit H. Effect of acute hypoxia on central fatigue during repeated isometric leg contractions. Scand J Med Sci Sports 19: 695–702, 2009.Crossref | PubMed | ISI | Google Scholar3. Miscio G , Milano E , Aguilar J , Savia G , Foffani G , Mauro A , Mordillo-Mateos L , Romero-Ganuza J , Oliviero A. Functional involvement of central nervous system at high altitude. Exp Brain Res 194: 157–162, 2009.Crossref | PubMed | ISI | Google Scholar4. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google Scholar5. Perrey S , Rupp T. Altitude-induced changes in muscle contractile properties. High Alt Med Biol 10: 175–182, 2009.Crossref | PubMed | ISI | Google Scholar6. Verges S , Bachasson D , Wuyam B. Effect of acute hypoxia on respiratory muscle fatigue in healthy humans. Respir Res 11: 109, 2010.Crossref | PubMed | ISI | Google ScholarTHE ISSUE OF CONFINEMENT WITH CHRONIC NORMOBARIC HYPOXIARobert F. Chapman and Benjamin D. Levine.Author AffiliationsIndiana University Institute for Exercise and Environmental Medicine Presbyterian Hospital of Dallas.to the editor: Millet et al. (4) note differences in performance outcomes between chronic exposure to HH and chronic intermittent exposure to NH may largely depend on the “hypoxic dose” and training content. Our own data confirm that performance changes in endurance athletes after chronic altitude training are dependent on a combination of both erythropoietic and training responses (2, 3).With intermittent exposure to NH, the magnitude of the “normoxic dose” may be just as important as the “hypoxic dose.” Because logistics of NH delivery require a closed environment, the end result is most NH exposures are intermittent in nature. Upon return to a normoxic environment from altitude/hypoxia, circulating erythropoietin (EPO) levels typically fall below baseline levels (2, 3), potentially resulting in a selective hemolysis of the youngest circulating red blood cells—a phenomenon termed neocytolysis (1).Additionally, the confining nature of a hypoxic tent or room makes it difficult to achieve the same 24-h/full-time exposure to hypoxia as terrestrial altitude living. When the confinement is prolonged enough to deliver an adequate hypoxic dose, it may have unexpected consequences such as the uniform loss of plasma volume observed in both hypoxic and normoxic confined athletes in a recent placebo controlled trial (6). Therefore, if potential differences exist in performance outcomes between HH and NH (4, 5), they may not be due so much to differences in the manner in which hypoxia is achieved, but more from the fact that NH often involves physiological responses to confinement and intermittent exposure to normoxia.REFERENCES1. Alfrey CP , Rice L , Udden M , Driscol T. Neocytolysis: a physiologic downregulator of red blood cell mass. Lancet 349: 1389–1390, 1997.Crossref | ISI | Google Scholar2. Chapman RF , Stray-Gundersen J , Levine BD. Individual variation in response to altitude training. J Appl Physiol 85: 1446–1458, 1998.Link | ISI | Google Scholar3. Levine BD , Stray-Gundersen J. “Living high - training low”: effect of moderate-altitude acclimatization with low-altitude training on exercise performance. J Appl Physiol 83: 102–112, 1997.Link | ISI | Google Scholar4. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.ISI | Google Scholar5. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google Scholar6. Siebenmann C , Robach P , Jacobs RA , Rasmussen P , Nordsborg N , Diaz V , Christ A , Olsen NV , Maggiorini M , Lundby C. “Live high-train low” using normobaric hypoxia: a double-blinded, placebo-controlled study. J Appl Physiol 112: 106–117, 2012.Link | ISI | Google ScholarREFERENCES1. Alfrey CP , Rice L , Udden M , Driscol T. Neocytolysis: a physiologic downregulator of red blood cell mass. Lancet 349: 1389–1390, 1997.Crossref | ISI | Google Scholar2. Chapman RF , Stray-Gundersen J , Levine BD. Individual variation in response to altitude training. J Appl Physiol 85: 1446–1458, 1998.Link | ISI | Google Scholar3. Levine BD , Stray-Gundersen J. “Living high - training low”: effect of moderate-altitude acclimatization with low-altitude training on exercise performance. J Appl Physiol 83: 102–112, 1997.Link | ISI | Google Scholar4. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.ISI | Google Scholar5. Mounier R , Brugniaux JV. Counterpoint: Hypobaric hypoxia does not induce different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012a.ISI | Google Scholar6. Siebenmann C , Robach P , Jacobs RA , Rasmussen P , Nordsborg N , Diaz V , Christ A , Olsen NV , Maggiorini M , Lundby C. “Live high-train low” using normobaric hypoxia: a double-blinded, placebo-controlled study. J Appl Physiol 112: 106–117, 2012.Link | ISI | Google ScholarPRO COMMENTARY POSITION ON HYPOXIA DEBATEJohnny ConkinEnvironmental Physiologist and James H. Wessel, IIIUniversities Space Research Association.to the editor: Ever since the derivation of the alveolar gas equation (2, Fig. 4) was published there has been a physiological foundation to expect different outcomes under normobaric hypoxia (NH) and hypobaric hypoxia (HH) given the same hypoxic PiO2, termed the N2 dilution or RER effect (6). A central theme in a seminal publication (5, Fig. 25) was the inability to define equivalent air altitude (EAA) given only equivalent hypoxic PiO2. This complicated issue was never resolved in the 1950s; therefore, the simple but flawed EAA concept became ingrained in textbooks and publications. The flaw was rediscovered and confirmed empirically (our conclusion) when multiple investigators consistently noticed differences in their NH and HH results given the same hypoxic PiO2 (1). All the evidence in (4) suggests that a complex interplay of physics and physiology determines hypoxic dose. An important publication (3), missed in the current debate, showed that higher PiO2 was needed under hypobaric conditions to equate the physiological responses in a sheep model with NH responses. It seems clear that hypoxic PiO2 is only an approximate measure of ultimate hypoxic dose; it is too far removed from a critical point of action in the brain. We hypothesize that two hypoxic exposures separated by a large difference in PB will produce similar responses, for example, the onset, intensity, and incidence of acute mountain sickness, only if O2 and CO2 tensions and pH in the cerebrospinal fluid are similar for the two exposures, a condition not directly linked to equivalent PiO2s.REFERENCES1. Conkin J , Wessel JH. Critique of the equivalent air altitude model. Aviat Space Environ Med 79: 975–982, 2008.Crossref | PubMed | Google Scholar2. Fenn WO , Rahn H , Otis AB. A theoretical study of the composition of the alveolar air at altitude. Am J Physiol 146: 637–653, 1946.Link | ISI | Google Scholar3. Hirai K , Kobayashi T , Kubo K , Shibamoto T. Effects of hypobaria on lung fluid balance in awake sheep. J Appl Physiol 64: 243–248, 1988.Link | ISI | Google Scholar4. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces induce different physiological responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.Google Scholar5. Rahn H , Fenn WO. A Graphical Analysis of the Respiratory Gas Exchange: The O2 - CO2 Diagram, 2nd ed. Washington, DC. The American Physiological Society, 1956, p. 25–26.Google Scholar6. Rahn H , Otis AB. Survival differences breathing air and oxygen at equivalent altitudes. Proc Soc Exp Biol Med 70: 185–186, 1949.Crossref | ISI | Google ScholarREFERENCES1. Conkin J , Wessel JH. Critique of the equivalent air altitude model. Aviat Space Environ Med 79: 975–982, 2008.Crossref | PubMed | Google Scholar2. Fenn WO , Rahn H , Otis AB. A theoretical study of the composition of the alveolar air at altitude. Am J Physiol 146: 637–653, 1946.Link | ISI | Google Scholar3. Hirai K , Kobayashi T , Kubo K , Shibamoto T. Effects of hypobaria on lung fluid balance in awake sheep. J Appl Physiol 64: 243–248, 1988.Link | ISI | Google Scholar4. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces induce different physiological responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.Google Scholar5. Rahn H , Fenn WO. A Graphical Analysis of the Respiratory Gas Exchange: The O2 - CO2 Diagram, 2nd ed. Washington, DC. The American Physiological Society, 1956, p. 25–26.Google Scholar6. Rahn H , Otis AB. Survival differences breathing air and oxygen at equivalent altitudes. Proc Soc Exp Biol Med 70: 185–186, 1949.Crossref | ISI | Google ScholarSLEEP IN NORMOBARIC VERSUS HYPOBARIC HYPOXIAHugo Nespoulet, Bernard Wuyam, Renaud Tamisier, Samuel Verges, and Patrick Levy.Author AffiliationsHP2 Laboratory (U1042) Joseph Fourier University and INSERM Grenoble, France.to the editor: Breathing changes occurring during sleep are useful to better understand differences between normobaric (NH) or hypobaric (HH) hypoxia. Continuous NH (1), intermittent NH (6), and HH (2) significantly reduce oxygen transport during sleep, i.e. lower oxygen saturation measured by pulse oximetry (SpO2) compared with daytime. These exposures are associated with periodic breathing (PB) and central apneas. Mounier et al. (4) emphasizes the role of O2 chemosensitivity for hypoxia/altitude adaptation. We recently showed that O2 and CO2 chemosensitivities are closely related to ventilation during sleep in NH (5). High chemosensitivity increases mean nocturnal SpO2 and thus appears as a major factor for altitude tolerance despite this possibly leading to frequent PB (5); Millet et al. (3) notes that ventilation and arterial CO2 are lower and hypoxemia more pronounced in HH compared with NH. This could be due to larger dead space ventilation in HH. We believe that mechanisms leading to differences in diurnal ventilation and blood gases between NH and HH also affect nocturnal respiratory events and blood gases. Particularly in HH, more apneas may arise from arterial CO2 reduction. Whether the occurrence of more PB and central apneas would be beneficial as shown in our study (5) or deleterious with further SpO2 reduction during sleep must be elucidated. We suggest that blood gases during sleep are critical regarding the relationship between acute mountain sickness and HH or NH. It is therefore important to further compare HH and NH effects on ventilatory control and blood gases during sleep.REFERENCES1. Berssenbrugge A , Dempsey J , Iber C , Skatrud J , Wilson P. Mechanisms of hypoxia-induced periodic breathing during sleep in humans. J Physiol 343: 507–524, 1983.Crossref | PubMed | ISI | Google Scholar2. Bloch KE , Latshang TD , Turk AJ , Hess T , Hefti U , Merz TM , Bosch MM , Barthelmes D , Hefti JP , Maggiorini M , Schoch OD. Nocturnal periodic breathing during acclimatization at very high altitude at Mount Muztagh Ata (7,546 m). 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Berssenbrugge A , Dempsey J , Iber C , Skatrud J , Wilson P. Mechanisms of hypoxia-induced periodic breathing during sleep in humans. J Physiol 343: 507–524, 1983.Crossref | PubMed | ISI | Google Scholar2. Bloch KE , Latshang TD , Turk AJ , Hess T , Hefti U , Merz TM , Bosch MM , Barthelmes D , Hefti JP , Maggiorini M , Schoch OD. Nocturnal periodic breathing during acclimatization at very high altitude at Mount Muztagh Ata (7,546 m). Am J Respir Crit Care Med 182: 562–568, 2010.Crossref | ISI | Google Scholar3. Millet GP , Faiss R , Pialoux V. Point: Hypobaric hypoxia induces different responses from normobaric hypoxia. J Appl Physiol; doi:10.1152/japplphysiol.00067.2012.ISI | Google Scholar4. Mounier R , Brugniaux JV. Counterpoint: H
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