Higher Muscle Tissue Oxygenation When Exposed to Hypobaric Hypoxia Than Normobaric Hypoxia

Abstract

There has been recent debate on the potential difference in physiological response between exposure to simulated altitude (normobaric hypoxia) and terrestrial altitude (hypobaric hypoxia). Purpose: To determine the difference in the physiological response to normobaric and hypobaric hypoxia during exercise. Methods: Eight recreationally active subjects (27 ¡ 5 years old, 73.1 ¡ 7.4 kg body weight, 170.6 ¡ 6.7 cm height, and 19.3 ¡ 9.2% body fat) completed incremental cycling exercise to volitional fatigue in three separate environments: normobaric normoxia (NN; 350 m), normobaric hypoxia (NH; simulated 3094 m), and hypobaric hypoxia (HH; 3094 m). Heart rate, blood oxygen saturation, and muscle tissue oxygenation were measured at rest and continuously throughout the exercise trials. Results: Blood oxygen saturation (SpO2) was ,10% higher in NN compared to the two hypoxic conditions (p , 0.001) at rest and all exercise stages, with no difference between NH and HH (p . 0.05). Heart rate was higher at rest in HH (98 ¡ 13 bpm) compared to NN (83 ¡ 15 bpm, p 5 0.011) and NH (84 ¡ 14 bpm, p 5 0.001) which persisted until 165 watts at which point no difference was observed (p . 0.05). Muscle tissue oxygenation was 17% higher in HH compared to NN and 19% higher than NH throughout exposure (p , 0.05). Conclusion: These data indicate that the hypoxic stresses resulting from normobaric and hypobaric hypoxia are not the same and that hypobaric hypoxia may not result in hypoxia at the level of the tissue.

Independent effects of acute normobaric hypoxia and hypobaric hypoxia on human physiology

“…Indeed, there is considerable debate on the physiological interchangeability of normobaric NH and HH with barometric pressure touted as a primary parameter of interest 14 – 17 . Exercise performance trials 5 , 10 , nitric oxide exhalation 6 , ventilatory responses 6 , 8 , 12 , oxidative stress markers 6 , gene expression markers 9 and tissue oxygenation using near infrared spectroscopy (NIRS) 3 have all been used to examine the differences between NH and HH. Disparate observations between NH and HH indeed complicate the formation of a conclusive stance on environmental interchangeability 14 – 17 .…”

“…Work is needed to elucidate the independent effects FiO 2 and barometric pressure decreases on tissue oxygenation when PiO 2 is matched. For example, during an acute hypoxic exposure/exercise time course (~ 6–12 min), a 17% and 19% higher muscle oxygenation (tissue saturation index derived) during HH as compared to NH and NN has been shown 3 . However, logistical limitations associated with the timing of subject travel from low to high elevation precluded equivalent exposure times between NH and HH despite matched PiO 2 3 .…”

“…For example, during an acute hypoxic exposure/exercise time course (~ 6–12 min), a 17% and 19% higher muscle oxygenation (tissue saturation index derived) during HH as compared to NH and NN has been shown 3 . However, logistical limitations associated with the timing of subject travel from low to high elevation precluded equivalent exposure times between NH and HH despite matched PiO 2 3 . This is a notable limitation because physiological changes during high altitude sojourns are thought to differ as time as exposure lengthens 29 .…”

“…Rationale for the presented work is thereby contextualized by this apparent muscle oxygenation maintenance during acute hypoxic exposures 3 , 30 , 31 . Subtle inconsistencies in regional tissue oxygenation, cardiovascular response, and body fluid observations warrant further comparison of NH and HH.…”

“…The current investigation utilized a novel field-based site for its unique geographical proximity of sea level and high elevation (4200 m). This novel research site and careful attention to the study design overcame previous exposure duration limitations 3 to better isolate FiO 2 and barometric pressure as independent variables. Therefore, the primary purpose of this investigation was to examine how acute exposure to NH and HH during rest and exercise effect regional skeletal muscle and brain tissue oxygenation, cardiovascular variables, and body fluid.…”