title: Head down tilt at -6 degrees to -24 degrees can neutralize the cardiovascular effects of LBNP at -15 or -35 mmHg.
abstract: BACKGROUND
  The purpose of this study was to identify the combinations of head down tilt (HDT) and lower body negative pressure (LBNP) that would counterbalance each other's effects on cardiovascular and endocrine variables to produce a "neutral point" (NP).
  
  
  METHODS
  We conducted 8 30-min experiments in 14 normotensive subjects (2 male, 12 female). Conditions included four levels of HDT (-6 degrees to -24 degrees) and two of LBNP (-15 and -35 mmHg). We determined blood plasma mass density, hematocrit, plasma aldosterone concentration, and plasma renin activity (PRA) before and at the end of stimulation. The effect of stimulus duration was tested using continuous measurements of heart rate (HR), blood pressure (BP), and thoracic electrical impedance (Z0).
  
  
  RESULTS
  NPs were found for all variables except BP, which remained unchanged. NPs were similar for all variables. The 15 mmHg LBNP was compensated by 20 degrees HDT and 35 mmHg LBNP by 27.5 degrees HDT. Longer stimulus duration required increasing HDT angles to balance HR at 35 mmHg LBNP but had no influence on Z0 NP.
  
  
  CONCLUSIONS
  Antiorthostatic positioning can compensate cardiovascular effects of LBNP in a similar fashion for all variables that are significantly influenced by LBNP and body angle, commensurate with stimulus magnitude. Arterial BP remained stable with stimulation and seems to be the primarily defended variable. Why stimulus duration seems to influence the NP for HR remains to be elucidated.
authors.affiliation: Institute for Adaptive and Spaceflight Physiology, Graz, Austria. helmut.hinghofer@meduni-graz.at
authors.forename: Helmut
authors.initials: H
authors.lastname: Hinghofer-Szalkay
authors.forename: Bernd
authors.initials: B
authors.lastname: Haditsch
authors.forename: Ingrid
authors.initials: I
authors.lastname: Loder
authors.forename: Karl
authors.initials: K
authors.lastname: Pilz
authors.forename: Andreas
authors.initials: A
authors.lastname: Rössler
authors.forename: Daniela
authors.initials: D
authors.lastname: Jezova
mesh_terms: D000328:Adult; D000450:Aldosterone / Q000097:blood; D002307:Cardiography, Impedance; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006400:Hematocrit; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D010953:Plasma Volume / Q000502:physiology; D011187:Posture / Q000502:physiology*; D012083:Renin / Q000097:blood
chemical_list: D000450:Aldosterone; D012083:Renin