Enhanced Cardiac Vagal Activity and Mood After Low-Dose Hypoxic Gas Inhalation in Healthy Young Adults

Abstract

Purpose Developing mindfulness-based strategies to efficiently improve cardiac vagal activity (CVA) is crucial for enhancing mood and managing stress. Recent studies have suggested that inhaling hypoxic gas could enhance CVA. However, the dynamics of CVA in response to acute hypoxia remain unelucidated, indicating that the proper hypoxic conditions expected to trigger the hormetic stress effect on CVA are unknown. Therefore, we aimed to achieve a comprehensive understanding of the hypoxic conditions required to improve CVA and mood following hypoxia. Methods Twenty-one healthy adults were assigned to participate in both hypoxic (NH) and normoxic (NN) conditions. Heart rate variability, saturation of percutaneous oxygen (SpO₂), and mood were monitored across the following sessions: Pre (5 min), Hypoxia^1–2 (10 min; NH, fraction of inspiratory oxygen (FIO₂): 13.5% or NN, FIO₂: 20.9%), and Post^1–4 (20 min). The Baevsky stress index (SI) was incorporated into the square root. For time domain analysis of CVA, both the standard deviation of NN intervals (SDNN) and the root mean square of successive differences (RMSSD) were utilized. Results In the NH condition, SpO₂ decreased to 88.1 ± 0.6 during hypoxia, accompanied by reductions in log transformed (ln) SDNN and lnRMSSD. After hypoxia, both indicators rebounded, exhibiting a supercompensation phenomenon. Pleasure levels declined during hypoxia but rapidly rebounded afterward, which was linked to fluctuations in lnRMSSD and SI. Conclusion We discovered that acute short-term inhalation of low-dose hypoxic gas with an FIO₂ of 13.5% enhances both CVA and mood following hypoxia. This strategy could provide a practical resilience-building method.