title: Cardiovascular regulation during body unweighting by lower body positive pressure.
abstract: BACKGROUND
  We hypothesized that human cardiovascular responses to standing in reduced gravity environments, as on the Moon or Mars, could be modeled using a lower body positive pressure (LBPP) chamber.
  
  
  METHODS
  Heart rate, blood pressure, body segment fluid shifts, ECG, indexes of sympathetic, parasympathetic balance, and baroreflex control of the heart and periphery plus echocardiographic measures of cardiac function were recorded from seven men and seven women supine and standing at 100% (Earth), 40% (-Mars), and 20% (-Moon) bodyweights (BW).
  
  
  RESULTS
  The fluid shifted from the chest was greater when standing at 100% BW than at 20% and 40% BW, while fluid pooled in the abdomen was similar at all BWs. Compared to moving from supine to standing at 100% BW, moving to 20% and 40% BW resulted in smaller decreases in stroke volume and pulse pressure, smaller increases in heart rate and smaller decreases in parasympathetic control of heart rate, baroreflex slope, numbers of blood pressure ramps, and much reduced indexes of sympathetic drive to the heart and periphery. However, peripheral vascular resistance, systolic pressure, and baroreflex effectiveness were elevated during 20% and 40% BW, compared to supine and standing at 100% BW.
  
  
  CONCLUSIONS
  Standing at reduced bodyweight suppressed indexes of sympathetic control of heart rate and peripheral vasomotion. Regulatory responses indicated a combination of arterial and cardiopulmonary baroreflex control: mean heart rate, vasomotion, and baroreflex sensitivity appeared to be more under cardiopulmonary control while baroreflex effectiveness appeared to be driven more by the arterial baroreflex.
authors.affiliation: Center for Biomedical Engineering, Room 1, University of Kentucky, Lexington, KY 40506-0070, USA. jevans1@uky.edu
authors.forename: Joyce M
authors.initials: JM
authors.lastname: Evans
authors.forename: Lindsay
authors.initials: L
authors.lastname: Mohney
authors.forename: Siqi
authors.initials: S
authors.lastname: Wang
authors.forename: Rachel K
authors.initials: RK
authors.lastname: Moore
authors.forename: Samy-Claude
authors.initials: SC
authors.lastname: Elayi
authors.forename: Michael B
authors.initials: MB
authors.lastname: Stenger
authors.forename: Fritz B
authors.initials: FB
authors.lastname: Moore
authors.forename: Charles F
authors.initials: CF
authors.lastname: Knapp
mesh_terms: D017704:Baroreflex / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology*; D002319:Cardiovascular System; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D018472:Hypogravity; D008297:Male; D013026:Space Flight; D013318:Stroke Volume / Q000502:physiology; D014655:Vascular Resistance / Q000502:physiology