Abstract
Purpose: This study aimed to investigate the differences between normobaric (NH) and hypobaric hypoxia (HH) on supine heart rate variability (HRV) during a 24-h exposure. We hypothesized a greater decrease in parasympathetic-related parameters in HH than in NH.Methods: A pooling of original data from forty-one healthy lowland trained men was analyzed. They were exposed to altitude either in NH (FIO2 = 15.7 ± 2.0%; PB = 698 ± 25 mmHg) or HH (FIO2 = 20.9%; PB = 534 ± 42 mmHg) in a randomized order. Pulse oximeter oxygen saturation (SpO2), heart rate (HR), and supine HRV were measured during a 7-min rest period three times: before (in normobaric normoxia, NN), after 12 (H12), and 24 h (H24) of either NH or HH exposure. HRV parameters were analyzed for time- and frequency-domains.Results: SpO2 was lower in both hypoxic conditions than in NN and was higher in NH than HH at H24. Subjects showed similarly higher HR during both hypoxic conditions than in NN. No difference in HRV parameters was found between NH and HH at any time. The natural logarithm of root mean square of the successive differences (LnRMSSD) and the high frequency spectral power (HF), which reflect parasympathetic activity, decreased similarly in NH and HH when compared to NN.Conclusion: Despite SpO2 differences, changes in supine HRV parameters during 24-h exposure were similar between NH and HH conditions indicating a similar decrease in parasympathetic activity. Therefore, HRV can be analyzed similarly in NH and HH conditions.
Highlights
Altitude training is an effective method commonly used by elite athletes (Millet and Brocherie, 2020)
The two main methods used for hypoxic exposure are hypobaric hypoxia (HH; FIO2 = 20.9%; PB
The following differences were already reported for a matched inspired oxygen pressure (PIO2) in HH vs. NH, respectively: (1) lower pulse oximeter oxygen saturation (SpO2) (Saugy et al, 2016c; Aebi et al, 2020; Debevec et al, 2020); (2) greater performance impairment (Beidleman et al, 2014; Saugy et al, 2016a), both recently confirmed in a study with perfectly controlled environmental factors (Takezawa et al, 2021); (3) higher oxidative stress (Faiss et al, 2013; Ribon et al, 2016); (4) impaired nitric oxide bioavailability (Faiss et al, 2013); (5) greater fluid retention (Loeppky et al, 2005; Conkin and Wessel, 2008), and (6) sleep structure perturbation (Heinzer et al, 2016; Saugy et al, 2016b)
Summary
Altitude training is an effective method commonly used by elite athletes (Millet and Brocherie, 2020). Prolonged altitude exposure increases erythropoiesis, which is explained by increased plasma erythropoietin concentration (Stray-Gundersen et al, 2001) and leads to an apparent increase in peripheral tissues oxygen delivery. This altitude-related increase in red blood cell mass increases. Hypocapnia and blood alkalosis are greater in HH than NH (Savourey et al, 2003; Coppel et al, 2015) Despite all these differences, which suggest that exposure in terrestrial altitude (HH) may induce more pronounced stress with larger physiological responses than simulated altitudes (NH), there is still an ongoing debate about their clinical significance. All protagonists agreed that further studies with a rigorous methodological approach and new perspectives would allow a more comprehensive understanding of the role of barometric pressure per se in different hypoxic conditions
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