Abstract
The fractal scaling of the long-term heart rate variability (HRV) reflects the ‘intrinsic’ autonomic regulatory system. Herein, we examine how microgravity on the ISS affected the power-law scaling β (beta) of astronauts during a long-duration (about 6 months) spaceflight. Ambulatory electrocardiographic (ECG) monitoring was performed on seven healthy astronauts (5 men, 52.0±4.2 years of age) five times: before launch, 24±5 (F01) and 73±5 (F02) days after launch, 15±5 days before return (F03), and after return to Earth. The power-law scaling β was calculated as the slope of the regression line of the power density of the MEM spectrum versus frequency plotted on a log10–log10 scale in the range of 0.0001–0.01 Hz (corresponding to periods of 2.8 h to 1.6 min). β was less negative in space (−0.949±0.061) than on Earth (−1.163±0.075; P<0.025). The difference was more pronounced during the awake than during the rest/sleep span. The circadian amplitude and acrophase (phase of maximum) of β did not differ in space as compared with Earth. An effect of microgravity was detected within 1 month (F01) in space and continued throughout the spaceflight. The intrinsic autonomic regulatory system that protects life under serious environmental conditions on Earth is altered in the microgravity environment, with no change over the 6-month spaceflight. It is thus important to find a way to improve conditions in space and/or in terms of human physiology, not to compromise the intrinsic autonomic regulatory system now that plans are being made to inhabit another planet in the near future.
Highlights
Focus was placed on four specific frequency bands of long-term heart rate variability (HRV): the high-frequency (HF: 40.15 Hz), low frequency (LF: 0.04–0.15 Hz), very low frequency (VLF: 0.0033–0.04 Hz) and ultralow frequency (ULF: o 0.0033 Hz) components
Parameter tests indicate a we show that the fractal regulation of HRV follows a circadian difference in Midline-Estimating Statistic of Rhythm (MESOR), as discussed below, but no difference in the rhythm, some of its characteristics affected by microgravity circadian amplitude and acrophase, tested jointly or separately
The spectral region most affected by the space environment is the ULF spectral power, which is decreased in space, Table 1
Summary
The absence of gravitational stimuli during spaceflight induces a number of adaptive changes within the cardiovascular system that may affect crew health and safety.[1,2] Most imperative, cardiovascular modifications occurring in microgravity consist of altered blood volume distribution,[3,4] impaired myocardial properties,[5,6,7] and/or vascular remodeling.[8,9,10] In addition, the baroreflex in space is chronically unchallenged owing to removal of intravascular hydrostatic pressure gradients.[10,11,12,13] The interplay between baroreflex and hemodynamic and body fluid alterations is likely to affect neural mechanisms involved in dynamic cardiovascular regulation, but the way in which this occurs in astronauts in space is still poorly understood. Christensen et al.[15,16] noted an increase in platelet norepinephrine and epinephrine in 4 out of 5 cosmonauts studied shortly after exposure to microgravity, contrary to a marked decrease associated with head-down bedrest. The authors interpret their result as a likely increase in sympathoadrenal activity during microgravity, noting that the reason why sympathoadrenal activity does not decrease in space remains to be elucidated. We examine the time course of changes in the power-law scaling β of HRV before, during, and after long-duration (about 6 months) spaceflight, during which astronauts were exposed to the microgravity environment of the ISS. Focus was placed on four specific frequency bands of long-term (up to 24-hour) HRV: the high-frequency (HF: 40.15 Hz), low frequency (LF: 0.04–0.15 Hz), very low frequency (VLF: 0.0033–0.04 Hz) and ultralow frequency (ULF: o 0.0033 Hz) components
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