title: Fluid loading effects on temporal profiles of cardiovascular responses to head-down tilt.
abstract: BACKGROUND
  Susceptibility of healthy astronauts to orthostatic hypotension and presyncope is exacerbated upon return from spaceflight. Up to 64% of astronauts experience postflight orthostatic intolerance. A promising countermeasure for postflight orthostatic intolerance is fluid loading by giving crew salt tablets and water prior to re-entry. The primary goal of the current study was to determine the optimal time window prior to re-entry when individual crew-persons would initiate fluid loading.
  
  
  METHODS
  There were 16 subjects who were given two 6-h exposures, with and without fluid loading (conditions), to head-down tilt (HDT) to simulate the effects of microgravity. Pre- and post-HDT stand tests of orthostatic tolerance were given. Physiological measurements recorded included heart rate, blood pressure, peripheral blood volume, total peripheral resistance, and impedance cardiography. Echocardiography measures of stroke volume and cardiac output were also recorded.
  
  
  RESULTS
  Data were analyzed with three-way repeated measures ANOVA (gender × condition × time). Only the condition × time interaction was significant for mean arterial pressure. Post hoc multiple comparison tests revealed significant increases in mean arterial pressure occurred between hours 1 and 3 of HDT after fluid loading (10 mmHg higher than no fluid).
  
  
  CONCLUSIONS
  These findings indicate that the optimal time for crew to begin fluid loading is within 1 to 3 h prior to re-entry. Nonsignificant trends of multiple cardiovascular responses showed similar time profiles. The large amount of individual variability suggests that fluid loading alone may be an inadequate countermeasure for all crewmembers. Further research is needed on possible adjunct methods of tailoring countermeasures for individuals.
authors.forename: Patricia
authors.initials: P
authors.lastname: Cowings
authors.forename: William
authors.initials: W
authors.lastname: Toscano
authors.forename: Dionisios
authors.initials: D
authors.lastname: Kanis
authors.forename: Theparat
authors.initials: T
authors.lastname: Saicheur
authors.forename: Anusha
authors.initials: A
authors.lastname: Ravikumar
authors.forename: Fiyore
authors.initials: F
authors.lastname: Gebreyesus
mesh_terms: D000328:Adult; D000337:Aerospace Medicine; D001826:Body Fluids / Q000502:physiology*; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology*; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic; D012965:Sodium Chloride / Q000008:administration & dosage; D055815:Young Adult
chemical_list: D012965:Sodium Chloride