# Moderate heart rate-matched hypoxic exercise: autonomic and cardiovascular responses to different degrees of hypoxic stress

## TL;DR

This study examines how different levels of hypoxia affect heart and autonomic responses during and after exercise, finding similar responses across hypoxic conditions.

## Contribution

The study reveals that HR-matched hypoxic exercise induces similar autonomic and cardiovascular responses regardless of hypoxia level.

## Key findings

- RMSSD decreased during hypoxic exercise and recovery, with lower increases after later exercise bouts.
- Post-exercise, systolic blood pressure and stroke volume decreased, while arterial pressure increased at 60 minutes.
- Cardiac output and baroreflex sensitivity remained unchanged despite varying hypoxia levels.

## Abstract

This study aims to assess the impact of HR-matched exercises under varying hypoxic stress levels on exercise and post-exercise autonomic and cardiovascular responses.

Twelve moderately aerobically trained healthy men (mean age: 23 ± 2 years, height: 179 ± 8 cm, weight: 71.2 ± 9.9 kg, BMI: 22.2 ± 2.2 kg/m2, VO2max: 53.1 ± 4.2 mL/min/kg) completed an interval exercise session at 75% of their normoxic maximum heart rate (75%HRmax) under three hypoxic conditions: FiO2 = 16.2% (2000 m a.s.l; H16), FiO2 = 14.3% (3000 m a.s.l; H14), and FiO2 = 12.6% (4000 m a.s.l; H12). Each session included 5 min of seated rest, a 5-min sub-maximal load warm-up, and five 5-min work intervals with 1-min passive recovery periods.

During hypoxic exercise, RMSSD decreased significantly following the first bout coinciding with an increase in heart rate. The RMSSD increase during 60-s recovery intervals was significantly lower after the 4th and 5th bouts compared to the 1st and 2nd bouts (p < 0.05). At 15 min post-exercise, mean RR, systolic blood pressure and stroke volume decreased. No changes were observed in cardiac output or baroreflex sensitivity. At 60 min post-exercise, SDNN, RMSSD, mean arterial pressure and diastolic blood pressure increased significantly compared to 15 min post-exercise. No condition or interaction differences were found.

Despite the decreased oxygen saturation with increased hypoxia levels, HR-matched interval exercise induced similar cardiac and autonomic responses across all hypoxic conditions. Baseline cardiac autonomic function and hemodynamics recovered within 60 min with no impact of hypoxia on baroreflex sensitivity.