Autonomic cardiovascular response to acute hypoxia and passive head-up tilting in humans.

Acute hypoxia may alter autonomic cardiovascular reflexes during orthostasis. Heart rate variability (HRV), arterial blood pressure (MAP), and respiratory sinus arrhythmia (RSA) were recorded during supine (SUP) and passive head up tilt (HUT) in eight healthy humans, spontaneously breathing either room air or 10% O₂ in N₂. In the time domain, heart rate increased and variability decreased with HUT in both trials, with no difference between trials. In the frequency domain, normalized low frequency HRV increased, and normalized high frequency HRV decreased with HUT in both trials, with no difference between trials. MAP was 74.9 (8.6) and 77.5 (11.7) mmHg when SUP in the room air and hypoxia trials, respectively. A significant increase in MAP occurred with HUT in the room air trial but not in the hypoxia trial. In both trials, end tidal CO₂ decreased with HUT, with no difference between trials. In the room air trial, end tidal O₂ increased with HUT, whereas during the hypoxia trial, end tidal O₂ decreased with HUT. The distribution of heart beats relative to the phase of ventilation (%HBIN and %HBOUT) was similar in both trials: the %HBIN was 43.5 (3.3) % and %HBOUT was 56.5 (4.2) % breathing room air when SUP, and 45.5 (3.0) and 54.5 (3.2) when hypoxic and SUP. For both trials, this distribution did not change with HUT. As both HRV and RSA showed similar responses to HUT when spontaneously breathing either room air or 10% O₂ in N₂, we suggest that autonomic cardiovascular reflexes are preserved during acute hypoxia.