Minimal Influence of Hypobaria on Heart Rate Variability in Hypoxia and Normoxia

Abstract

Introduction: The present study evaluated the putative effect of hypobaria on resting HRV in normoxia and hypoxia. Methods: Fifteen young pilot trainees were exposed to five different conditions in a randomized order: normobaric normoxia (NN, P B = 726 ± 5 mmHg, F I O 2 = 20.9%), hypobaric normoxia (HN, P B = 380 ± 6 mmHg, F I O 2 ∼ =40%), normobaric hypoxia (NH, P B = 725 ± 4 mmHg, F I O 2 ∼ =11%); and hypobaric hypoxia (HH at 3000 and 5500 m, HH3000 and HH5500, P B = 525 ± 6 and 380 ± 8 mmHg, respectively, F I O 2 = 20.9%). HRV and pulse arterial oxygen saturation (SpO 2) were measured at rest seated during a 6 min period in each condition. HRV parameters were analyzed (Kubios HVR Standard, V 3.0) for time (RMSSD) and frequency (LF, HF, LF/HF ratio, and total power). Gas exchanges were collected at rest for 10 min following HRV recording. Results: SpO 2 decreased in HH3000 (95 ± 3) and HH5500 (81 ± 5), when compared to NN (99 ± 0). SpO 2 was higher in NH (86 ± 4) than HH5500 but similar between HN (98 ± 2) and NN. Participants showed lower RMSSD and total power values in NH and HH5500 when compared to NN. In hypoxia, LF/HF ratio was greater in HH5500 than NH, whereas in normoxia, LF/HF ratio was lower in HN than NN. Minute ventilation was higher in HH5500 than in all other conditions. Discussion: The present study reports a slight hypobaric effect either in normoxia or in hypoxia on some HRV parameters. In hypoxia, with a more prominent sympathetic activation, the hypobaric effect is likely due to the greater ventilation stimulus and larger desaturation. In normoxia, the HRV differences may come from the hyperoxic breathing and slight breathing pattern change due to hypobaria in HN.

Similar Supine Heart Rate Variability Changes During 24-h Exposure to Normobaric vs. Hypobaric Hypoxia

“…Our study confirmed a higher oxygen saturation in NH vs. HH after 24 h of hypoxic exposure. This is already well described for shorter exposition times ( Aebi et al, 2020 ; Debevec et al, 2020 ) and logically was also already reported in the three support experiments of the present study. However, the latter was still debated for exposures longer than 8 h ( Coppel et al, 2015 ).…”

“…Therefore, HRV could be analyzed similarly in NH and HH conditions. Furthermore, together with previous results on acute exposures ( Aebi et al, 2020 ), it forms a reliable basis for a similar interpretation of HRV in NH and HH during both acute and long hypoxic exposures.…”

“…As expected, HR was higher in hypoxic conditions compared to normoxia in response to the increased sympathetic activity and vagal withdrawal. Several studies have further found lower HR in NH vs. HH ( Savourey et al, 2003 ; DiPasquale et al, 2015b ; Heinzer et al, 2016 ; Ross et al, 2019 ; Aebi et al, 2020 ) but only during short exposures. In addition, studies examining longer exposures (>8 h) found no difference in HR ( Faiss et al, 2013 ; Richard et al, 2014 ).…”

“…The effects of NH vs. HH on HRV were studied only during acute exposures (≤ 30 min) ( Basualto-Alarcón et al, 2012 ; Aebi et al, 2020 ). The LF/HF ratio was slightly higher in HH, but the evidence for a hypobaric effect per se on HRV in hypoxia during acute exposures was not strong enough ( Aebi et al, 2020 ). Therefore, the present study aimed to compare the influence of a 24-h exposure in NH vs. HH on supine HRV.…”

“…However, recent research suggested that NH and HH are not interchangeable (Fulco et al, 2011;Millet et al, 2012;Saugy et al, 2014Saugy et al, , 2016aCoppel et al, 2015;DiPasquale et al, 2015a;Conkin, 2016) although the differences between HH and NH are still debated (Millet and Debevec, 2020;Richalet, 2020). The following differences were already reported for a matched inspired oxygen pressure (P I O 2 ) in HH vs. NH, respectively: (1) lower pulse oximeter oxygen saturation (S p O 2 ) (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). In addition, breathing patterns are affected as well: lower tidal volume, lower minute ventilation, higher physiological dead space, and higher respiratory frequency during HH exposure compared to NH (Savourey et al, 2003;Conkin and Wessel, 2008;Saugy et al, 2014).…”