Heart rate variability and short duration spaceflight: relationship to post-flight orthostatic intolerance

Background: Upon return from space many astronauts experience symptoms of orthostatic intolerance. Research has implicated altered autonomic cardiovascular regulation due to spaceflight with further evidence to suggest that there might be pre-flight autonomic indicators of post-flight orthostatic intolerance. We used heart rate variability (HRV) to determine whether autonomic regulation of the heart in astronauts who did or did not experience post-flight orthostatic intolerance was different pre-flight and/or was differentially affected by short duration (8 – 16 days) spaceflight. HRV data from ten-minute stand tests collected from the 29 astronauts 10 days pre-flight, on landing day and three days post-flight were analysed using coarse graining spectral analysis. From the total power (PTOT), the harmonic component was extracted and divided into high (^PHI:>0.15 Hz) and low (P_LO: = 0.15 Hz) frequency power regions. Given the distribution of autonomic nervous system activity with frequency at the sinus node, P_HI/P_TOT was used as an indicator of parasympathetic activity; P_LO/P_TOT as an indicator of sympathetic activity; and, P_LO/P_HI as an estimate of sympathovagal balance.