title: G tolerance and the vasoconstrictor reserve.
abstract: OBJECTIVE
  Because leg arterial stiffness is higher in subjects with high G tolerance, we hypothesized that subjects with high G tolerance would have larger capacity for vasoconstriction.
  
  
  METHODS
  Sixteen subjects, eight with high and eight with low G tolerance (H and L group, respectively), were exposed to a cold pressor test (CPT) in supine and upright posture. Heart rate (HR), mean arterial pressure (MAP) and cardiac output (CO) were measured, and total peripheral resistance (TPR) and stroke volume (SV) were calculated.
  
  
  RESULTS
  In the supine position, CPT increased TPR more in the H group; 31 ± 18% than in the L group; 11 ± 7% (p < 0.05). The L group had larger increases in CO than the H group; 17 ± 16 vs. 3.4 ± 7% (p = 0.06). In the upright position, the H group had a larger MAP response to CPT than the L group; 26 ± 14 vs. 14 ± 7% (p = 0.06). The H group, but not the L group, had significant increases in TPR whereas the L group had significant increases in CO and SV.
  
  
  CONCLUSIONS
  In response to CPT, the high G tolerance group elevated MAP by increasing TPR, whereas the low G tolerance group showed a dependency on increased CO. The H group seemed to have a larger vasoconstrictor reserve. The results further suggest that vasoconstrictor reserve capacity could constitute the link between the recent finding that indicates a relationship between G tolerance and arterial distensibility in the legs.
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, School of Technology and Health, Royal Institute of Technology (KTH), Berzelius väg 13, 171 65, Stockholm, Sweden, patsu@kth.se.
authors.forename: Patrik
authors.initials: P
authors.lastname: Sundblad
authors.forename: Roger
authors.initials: R
authors.lastname: Kölegård
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology*; D002302:Cardiac Output / Q000502:physiology; D003080:Cold Temperature; D004576:Electromyography; D006112:Gravitation; D006339:Heart Rate / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology; D006801:Humans; D008297:Male; D011187:Posture / Q000502:physiology; D014655:Vascular Resistance / Q000502:physiology*; D014661:Vasoconstriction / Q000502:physiology*; D055815:Young Adult