Dynamics of Autonomic Cardiac Control during Acute Normobaric Hypoxia Modeling in Volunteers with Varying Baseline Hypoxic Resistance

The study investigates the dynamics of autonomic cardiac control in healthy volunteers with different baseline hypoxic resistance during acute normobaric hypoxia. The primary focus was on changes in the autonomic nervous system, assessed through the heart rate variability (HRV) indices and the pre-ejection period (PEP). The study involved 62 volunteers (24 men and 36 women, mean age 18 18; 19 years) retrospectively divided into two groups: hypoxia-resistant (n = 16) and hypoxia-sensitive (n = 46). A hypoxic test (HT) was performed on the ReOxy device, assessing the dynamics of HRV, PEP, blood pressure (BP), and heart rate (HR) before, during, and after testing. The overall response to hypoxia was manifested as a significant decrease in PEP and the HRV indices RMSSD and pNN50, indicating sympathetic activation and reduced vagal tone. The recovery period showed an increase in these indices and a decrease in BP. Resistant volunteers initially had higher baseline values of RMSSD, pNN50, and high-frequency component power (HF). During HT, they exhibited a lesser degree of desaturation (ΔSpO2 = –14.9 –16.7; –12.6) compared to the sensitive group (ΔSpO2 = –19.2 –19.6; –18.4), with a lesser increase in HR, though a more pronounced decrease in PEP. During recovery, the resistant group demonstrated higher indices of parasympathetic activity alongside a decrease in HR. The study confirms that high hypoxic resistance is associated with an initially pronounced parasympathetic tone, an adequate sympathetic activation in response to a stimulus, and a vagal rebound during recovery. Sensitive individuals exhibited vagal depression and prolonged sympathetic activation, which correlates with desaturation. The obtained data justify the importance of considering autonomic sensitivity when applying hypoxia adaptation techniques.