The relationship between acute exposure to hypoxia and heart rate variability in unacclimatized individuals

Acute exposure to hypoxia results in a stress response categorized by sympathetic dominance, including hyperventilation to prevent arterial desaturation. As an estimation of overall stress, heart rate variability (HRV) reflects the balance between sympathetic and parasympathetic autonomic regulation. PURPOSE: The purpose of this study was to determine whether resting sea level (SL) HRV is correlated with arterial desaturation and respiratory responses to acute normobaric hypoxia exposure (equivalent 3500 m). METHODS: Resting HRV, %SpO2, and respiratory rate (RR) were measured in 19 subjects (male n=9, female n=10) at SL for 15 minutes. HRV was measured (Firstbeat Bodyguard2) including RMSSD, HF, and LF components. VO2 peak was estimated using a standard graded exercise test (GXT) on a bicycle ergometer. Subjects later returned and resting HRV, %SpO2, and RR were measured in a normobaric hypoxic chamber (Colorado Mountain Systems, Inc.) set at 3500 m (treatment) or sea level (control). Subjects exercised on a bicycle ergometer at 65% capacity where HR, %SpO2 and RPE were recorded. RESULTS: There was a significant correlation observed in treatment subjects between RR (PIII – PII) and LFHF ratio (PIII - PII) (p\u3c0.01), RR (PIII – PII) and PII LFHF ratio (p\u3c0.05), and VO2 peak and exercise-induced change in %SpO2 (PIII) (p\u3c0.05). No significant correlation was observed between HRV in the time domain (RMSSD) and %SpO2 at rest, %SpO2 during exercise, or respiratory rate during rest (p\u3e0.05) when exposed to hypoxia. CONCLUSION: HRV as a measure of overall stress and sympathovagal balance may be predictive of the change in RR, but not in the change in %SpO2. Subsequent studies may require a controlling for key variables (caffeine use, time of day, etc.) or paired study design