title: Acute cardiovascular adaptation to 10 consecutive episodes of head-up tilt.
abstract: BACKGROUND
  The cardiovascular system is highly adaptable to sustained +Gz acceleration. Little is known as to whether the cardiovascular system can adapt to acute, repetitive +Gz exposures. This study tested the hypothesis that the cardiovascular system would adapt to repeated orthostatic challenges in a single session.
  
  
  METHODS
  Over a 70-min period, 14 subjects were exposed to 10 +75 degrees head-up tilts (HUT). Each tilt involved a 5-min supine period followed by a 2-min HUT. Heart rate (HR), systolic BP (SBP), diastolic BP (DBP), mean arterial pressure (MAP), total peripheral resistance (TPR), stroke volume (SV), and cardiac output (CO) were determined non-invasively. Cardiovascular responses to HUT10 for the final 30 s of the supine period and the first 30 s of the tilt period were compared with those of HUT1. Integrated cardiac baroreflex sensitivity (BRS) was assessed using the Valsalva maneuver.
  
  
  RESULTS
  MAP and DBP increased in both supine (MAP p = 0.009, DBP p = 0.002) and tilt periods (MAP p = 0.003, DBP p = 0.009) for HUT10 compared with HUT1. TPR increased during the tilt period only (p = 0.001) during HUT10 compared with HUT1. CO and SV were decreased during the supine period at HUT10 relative to HUT1; however, there were no differences in the tilt period at HUT10 for either CO or SV. There was no change in the response of BRS, HR, or SBP from HUT1 to HUT10.
  
  
  CONCLUSIONS
  This study indicates that 10 repetitive HUTs can elicit changes in the cardiovascular responses to orthostasis, reflected by an increased vascular resistance.
authors.affiliation: Aerospace Physiology Laboratory, RMIT University, School of Medical Sciences, P.O. Box 71, Bundoora, Victoria 3083, Australia. nberry@alphalink.com.au
authors.forename: Narelle M
authors.initials: NM
authors.lastname: Berry
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Rickards
authors.forename: David G
authors.initials: DG
authors.lastname: Newman
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D000337:Aerospace Medicine; D001794:Blood Pressure / Q000502:physiology*; D002302:Cardiac Output / Q000502:physiology*; D002307:Cardiography, Impedance; D003971:Diastole / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D013599:Systole / Q000502:physiology; D018667:Tilt-Table Test; D014636:Valsalva Maneuver