title: The arterial baroreflex and inherent G tolerance.
abstract: OBJECTIVE
  High G tolerance is based on the capacity to maintain a sufficient level of arterial pressure (AP) during G load; therefore, we hypothesized that subjects with high G tolerance (H group) would have stronger arterial baroreflex responses compared to subjects with low G tolerance (L group). The carotid baroreflex was evaluated using the neck pressure method (NP), which assesses open-loop responses.
  
  
  METHODS
  The carotid baroreflex was tested in 16 subjects, n = 8 in the H and L group, respectively, in the supine and upright posture. Heart rate and AP were measured.
  
  
  RESULTS
  There were no differences between groups in the maximum slopes of the carotid baroreflex curves. However, the H group had a larger systolic and mean AP (SAP, MAP) increase to the initial hypotensive stimuli of the NP sequence in the upright position compared to the L group, 7.5 ± 6.6 vs 2.0 ± 2.4 and 4.1 ± 3.4 vs 1.1 ± 1.1 mmHg for SAP and MAP, respectively. Furthermore, the L group exhibited an increased latency between stimuli and response in AP in the upright compared to supine position, 4.1 ± 1.0 vs 3.1 ± 0.9 and 4.7 ± 1.1 vs 3.6 ± 0.9 s, for SAP and MAP. No differences in chronotropic responses were observed between the groups.
  
  
  CONCLUSIONS
  It is concluded that the capacity for reflexive vasoconstriction and maintained speed of the vascular baroreflex during orthostatic stress are coupled to a higher relaxed GOR tolerance.
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, School of Technology and Health, Royal Institute of Technology, Berzelius väg 13, 171 65, Solna, Sweden. patsu@kth.se.
authors.forename: Patrik
authors.initials: P
authors.lastname: Sundblad
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, School of Technology and Health, Royal Institute of Technology, Berzelius väg 13, 171 65, Solna, Sweden.
authors.forename: Roger
authors.initials: R
authors.lastname: Kölegård
authors.affiliation: Department of Cardiology, Erasmus Hospital, Faculty of Medicine, Université libre de Bruxelles, Brussels, Belgium.
authors.forename: Pierre-Francois
authors.initials: PF
authors.lastname: Migeotte
authors.affiliation: Department of Cardiology, Erasmus Hospital, Faculty of Medicine, Université libre de Bruxelles, Brussels, Belgium.
authors.forename: Quentin
authors.initials: Q
authors.lastname: Delière
authors.affiliation: Department of Environmental Physiology, Swedish Aerospace Physiology Centre, School of Technology and Health, Royal Institute of Technology, Berzelius väg 13, 171 65, Solna, Sweden.
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D062186:Arterial Pressure / Q000502:physiology*; D017704:Baroreflex / Q000502:physiology*; D002346:Carotid Sinus / Q000502:physiology*; D006801:Humans; D018471:Hypergravity; D008297:Male; D054971:Orthostatic Intolerance / Q000503:physiopathology; D011187:Posture / Q000502:physiology*
keywords: Acceleration; Arterial pressure; Baroreflex; G tolerance; Posture