title: Simulations of gravitational stress on normovolemic and hypovolemic men and women.
abstract: BACKGROUND
  Earth-based simulations of physiologic responses to space mission activities are needed to develop prospective countermeasures. To determine whether upright lower body positive pressure (LBPP) provides a suitable space mission simulation, we investigated the cardiovascular responses of normovolemic and hypovolemic men and women to supine and orthostatic stress induced by head-up tilt (HUT) and upright LBPP, representing standing in lunar, Martian, and Earth gravities.
  
  
  METHODS
  Six men and six women were tested in normovolemic and hypovolemic (furosemide, intravenous, 0.5 mg x kg(-1)) conditions. Continuous electrocardiogram, blood pressure, segmental bioimpedance, and stroke volume (echocardiography) were recorded supine and at lunar, Martian, and Earth gravities (10 degrees, 20 degrees, and 80 degrees HUT vs. 20%, 40%, and 100% bodyweight upright LBPP), respectively. Cardiovascular responses were assessed from mean values, spectral powers, and spontaneous baroreflex parameters.
  
  
  RESULTS
  Hypovolemia reduced plasma volume by approximately 10% and stroke volume by approximately 25% at supine, and increasing orthostatic stress resulted in further reductions. Upright LBPP induced more plasma volume losses at simulated lunar and Martian gravities compared with HUT, while both techniques induced comparable central hypovolemia at each stress. Cardiovascular responses to orthostatic stress were comparable between HUT and upright LBPP in both normovolemic and hypovolemic conditions; however, hypovolemic blood pressure was greater during standing at 100% bodyweight compared to 80 degree HUT due to a greater increase of total peripheral resistance.
  
  
  CONCLUSIONS
  The comparable cardiovascular response to HUT and upright LBPP support the use of upright LBPP as a potential model to simulate activity in lunar and Martian gravities.
authors.forename: Qingguang
authors.initials: Q
authors.lastname: Zhang
authors.forename: Charles F
authors.initials: CF
authors.lastname: Knapp
authors.forename: Michael B
authors.initials: MB
authors.lastname: Stenger
authors.forename: Abhijit R
authors.initials: AR
authors.lastname: Patwardhan
authors.forename: Samy C
authors.initials: SC
authors.lastname: Elayi
authors.forename: Siqi
authors.initials: S
authors.lastname: Wang
authors.forename: Vladimir I
authors.initials: VI
authors.lastname: Kostas
authors.forename: Joyce M
authors.initials: JM
authors.lastname: Evans
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D002320:Cardiovascular Physiological Phenomena; D016022:Case-Control Studies; D018534:Earth, Planet; D004562:Electrocardiography; D005260:Female; D006112:Gravitation; D018470:Gravity, Altered / Q000009:adverse effects*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D020896:Hypovolemia / Q000503:physiopathology*; D008297:Male; D018535:Mars; D016081:Moon; D011187:Posture / Q000502:physiology; D013026:Space Flight; D018561:Space Simulation; D013312:Stress, Physiological / Q000502:physiology*; D013318:Stroke Volume / Q000502:physiology; D014655:Vascular Resistance / Q000502:physiology; D055815:Young Adult
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