Abstract
Simulated altitude (normobaric hypoxia, NH) is used to study physiologic hypoxia responses of altitude. However, several publications show differences in physiological responses between NH and hypobaric conditions at altitude (hypobaric hypoxia, HH). The causality for these differences is controversially discussed. One theory is that the lower air density and environmental pressure in HH compared to NH lead to lower alveolar pressure and therefore lower oxygen diffusion in the lung. We hypothesized that, if this theory is correct, due to physical laws (Hagen-Poiseuille, Boyle), resistance respectively air compression (Boyle) at expiration should be lower, expiratory flow higher, and therefore peak flow and maximum expiratory flow (MEF) 75–50 increased in hypobaric hypoxia (HH) vs. normobaric hypoxia (NH). To prove the hypothesis of differences in respiratory flow as a result of lower alveolar pressure between HH and NH, we performed spirography in NH at different simulated altitudes and the corresponding altitudes in HH. In a cross over study, 6 healthy subjects (2 f/4 m, 28.3 ± 8.2 years, BMI: 23.2 ± 1.9) performed spirography as part of spiroergometry in a normobaric hypoxic room at a simulated altitude of 2800 m and after a seven-hour hike on a treadmill (average incline 14%, average walking speed 1.6 km/h) to the simulated summit of Mauna Kea at 4200 m. After a two-month washout, we repeated the spirometry in HH on the start and top of the Mauna Kea hiking trail, HI/USA. Comparison of NH (simulated 4200 m) and HH at 4200 m resulted in increased pulmonary ventilation during exercise (VE) (11.5%, p < 0.01), breathing-frequency (7.8%, p < 0.01), peak expiratory flow PEF (13.4%, p = 0.028), and MEF50 (15.9%, p = 0.028) in HH compared to NH, whereas VO2max decreased by 2%. At 2800 m, differences were only trendy, and at no altitude were differences in volume parameters. Spirography expresses higher mid expiratory flows and peak flows in HH vs. NH. This supports the theory of lower alveolar and small airway pressure due to a lower air density resulting in a lower resistance.
Highlights
The use of simulated altitude (Normobaric Hypoxia, normobaric hypoxia (NH)) has become increasingly popular in the past years due to a rising number of commercially available devices with increased nitrogen volume percentage and reduced oxygen volume percentage, claiming a similar effect to being exposed to terrestrial altitude or lowered environmental pressure in a hypobaric chamber, i.e., hypobaria (Hypobaric Hypoxia, hypobaric hypoxia (HH))
Expiratory flow volume seems to be higher in HH than in NH at that level of altitude, the differences in parameters of expiratory flow between NH and HH were not significant
Significant differences between NH and HH were found in peak expiratory flow (PEF), MEF75, and MEF50
Summary
The use of simulated altitude (Normobaric Hypoxia, NH) has become increasingly popular in the past years due to a rising number of commercially available devices (e.g., facemasks, tents, rooms) with increased nitrogen volume percentage and reduced oxygen volume percentage, claiming a similar effect to being exposed to terrestrial altitude or lowered environmental pressure in a hypobaric chamber, i.e., hypobaria (Hypobaric Hypoxia, HH). Basualto-Alarcon and colleagues (2012) found enhanced cardiorespiratory parameters as well as lower oxygen saturation levels in HH compared to NH during exposure to 3000 m and aerobic exercise performance [1]. Faiss and colleagues (2013) found only slightly lower minute ventilation and higher tidal volumes in HH compared to NH when performing submaximal exercise tests while being exposed to 3000 m for 24 h. They reported no changes in breathing frequency or oxygen saturations [5]
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