Assessment of the acute impact of normobaric hypoxia as a part of an intermittent hypoxic training on heart rate variability

AimTo assess the dynamics of ANS by means of heart rate variability (HRV) during and after acute exposure to normobaric hypoxia, representing a single session of an intermittent hypoxic training protocol.Material and methodsTwenty four healthy males aged 28.0 ± 7.2 (mean ± SD) breathed hypoxic air (FIO2 = 12.3 ± 1.5%) for one hour delivered via hypoxicator (AltiPro 8850 Summit+, Altitude Tech, Canada). Pulse oximetry and HRV were measured before, during and after the hypoxic exposure.ResultsAt the end of the hypoxic session all of the tested subjects had higher low frequency (lnLF) (6.9 ± 1.1 ms2 vs. 7.5 ± 1.1 ms2; p = 0.042), LF/HF (1.5 ± 0.8 vs. 3.3 ± 2.8; p = 0.007) and standard deviation 2 of the Poincaré plot (SD2) (92.8 ± 140.0 ms vs. 120.2 ± 54.2 ms; p = 0.005) as well as increase in the Total power (7.7 ± 1.1 ms2 vs. 8.1 ± 1.2 ms2; p = 0.032) and the Standard deviation of normal-to-normal interbeat intervals (SDNN) (57.3 ± 31.0 ms vs. 72.3 ± 41.1 ms; p = 0.024) but lower Sample entropy (SampEn) (1.6 ± 0.2 vs. 1.4 ± 0.2; p = 0.010). Immediately after the hypoxic exposure LF/HF lowered (3.3 ± 2.8 vs. 2.2 ± 1.8; p = 0.001) but lnHF significantly increased (6.6 ± 1.4 ms2 vs. 7.1 ± 1.3 ms2; p = 0.020).ConclusionAcute normobaric hypoxia as a part of a single session of an intermittent hypoxic training protocol leads to changes in the activity of the ANS. The sympathetic tone prevails during hypoxic exposure and parasympathetic tone increases immediately after the hypoxic factor is withdrawn.