title: Long-term exposure to space's microgravity alters the time structure of heart rate variability of astronauts.
abstract: BACKGROUND
  Spaceflight alters human cardiovascular dynamics. The less negative slope of the fractal scaling of heart rate variability (HRV) of astronauts exposed long-term to microgravity reflects cardiovascular deconditioning. We here focus on specific frequency regions of HRV.
  
  
  METHODS
  Ten healthy astronauts (8 men, 49.1 ± 4.2 years) provided five 24-hour electrocardiographic (ECG) records: before launch, 20.8 ± 2.9 (ISS01), 72.5 ± 3.9 (ISS02) and 152.8 ± 16.1 (ISS03) days after launch, and after return to Earth. HRV endpoints, determined from normal-to-normal (NN) intervals in 180-min intervals progressively displaced by 5 min, were compared in space versus Earth. They were fitted with a model including 4 major anticipated components with periods of 24 (circadian), 12 (circasemidian), 8 (circaoctohoran), and 1.5 (Basic Rest-Activity Cycle; BRAC) hours.
  
  
  RESULTS
  The 24-, 12-, and 8-hour components of HRV persisted during long-term spaceflight. The 90-min amplitude became about three times larger in space (ISS03) than on Earth, notably in a subgroup of 7 astronauts who presented with a different HRV profile before flight. The total spectral power (TF; p < 0.05) and that in the ultra-low frequency range (ULF, 0.0001-0.003 Hz; p < 0.01) increased from 154.9 ± 105.0 and 117.9 ± 57.5 msec2(before flight) to 532.7 ± 301.3 and 442.4 ± 202.9 msec2(ISS03), respectively. The power-law fractal scaling β was altered in space, changing from -1.087 ± 0.130 (before flight) to -0.977 ± 0.098 (ISS01), -0.910 ± 0.130 (ISS02), and -0.924 ± 0.095 (ISS03) (invariably p < 0.05).
  
  
  CONCLUSIONS
  Most HRV changes observed in space relate to a frequency window centered around one cycle in about 90 min. Since the BRAC component is amplified in space for only specific HRV endpoints, it is likely to represent a physiologic response rather than an artifact from the International Space Station (ISS) orbit. If so, it may offer a way to help adaptation to microgravity during long-duration spaceflight.
authors.affiliation: Executive Medical Center, Totsuka Royal Clinic, Tokyo Women's Medical University, Tokyo, Japan; Halberg Chronobiology Center, University of Minnesota, Minneapolis, MN, USA.
authors.forename: Kuniaki
authors.initials: K
authors.lastname: Otsuka
authors.affiliation: Halberg Chronobiology Center, University of Minnesota, Minneapolis, MN, USA.
authors.forename: Germaine
authors.initials: G
authors.lastname: Cornelissen
authors.affiliation: Space Biomedical Research Group, Japan Aerospace Exploration Agency, Tokyo, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Furukawa
authors.affiliation: Department of Medicine, Tokyo Women's Medical University, Medical Center East, Tokyo, Japan.
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Kubo
authors.affiliation: Department of Medicine, Tokyo Women's Medical University, Medical Center East, Tokyo, Japan.
authors.forename: Mitsutoshi
authors.initials: M
authors.lastname: Hayashi
authors.affiliation: Department of Medicine, Tokyo Women's Medical University, Medical Center East, Tokyo, Japan.
authors.forename: Koichi
authors.initials: K
authors.lastname: Shibata
authors.affiliation: Space Biomedical Research Group, Japan Aerospace Exploration Agency, Tokyo, Japan; Faculty of Child and Family Studies, Tohoku Fukushi University, Miyagi, Japan.
authors.forename: Koh
authors.initials: K
authors.lastname: Mizuno
authors.affiliation: Space Biomedical Research Group, Japan Aerospace Exploration Agency, Tokyo, Japan; Ministry of Education, Culture, Sports, Science and Technology, Tokyo, Japan.
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Aiba
authors.affiliation: Space Biomedical Research Group, Japan Aerospace Exploration Agency, Tokyo, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ohshima
authors.affiliation: Space Biomedical Research Group, Japan Aerospace Exploration Agency, Tokyo, Japan.
authors.forename: Chiaki
authors.initials: C
authors.lastname: Mukai
keywords: Cardiology; Health Sciences; Medicine