title: Heart rate and blood pressure responses to +Gz following varied-duration -Gz.
abstract: BACKGROUND
  The push-pull effect has been defined previously as decreased +Gz tolerance caused by previous baseline zero or -Gz exposure. Earlier work indicates that the delay in BP (BP) recovery during +Gz is a function of time at -G7, and is due to the lengthened time-course of sympathetically mediated peripheral vasoconstriction.
  
  
  OBJECTIVE
  The purpose of this study was to retrospectively determine whether heart rate (HR) varies with BP as duration at preceding -Gz increased.
  
  
  METHODS
  Continuous ECG R-R interval data from 15 s of +2.25Gz after preceding 2, 5, 10, or 15 s at 2Gz obtained from previous experiments were analyzed and compared with the previously reported BP data. Repeated measures ANOVA and regression analyses were used to compare +2.25Gz HR responses after the four -Gz conditions and one control +2.25Gz condition.
  
  
  RESULTS
  An initial rapid rise in HR was observed for all conditions with a consistent steady-state plateau achieved after the first 7 s of +2.25Gz. However, there were significant differences in mean HR attained during the +2.25Gz plateau for preceding 15 s -2.0 Gz vs. the control, 2, 5, and 10s -Gz conditions (109+/-1.1 vs. 102+/-1.8, 100+/-2.0, 97+/-1.1 and 101+/-1.1, bpm, respectively; p<0.05).
  
  
  CONCLUSIONS
  HR, unlike BP, increases briskly across all preceding -Gz time conditions, adapting within the initial baroreflex-compensatory time frame typically expected for +Gz exposures. These results suggest there may be a threshold effect for HR response. Consequently, vasoconstrictor response is a critical adaptive mechanism during +Gz when preceded by long (>10 s) -Gz exposures.
authors.affiliation: Defence and Civil Institute of Environmental Medicine, Aerospace Life Support Section, Toronto, Ontario, Canada. len.goodman@dciem.dnd.ca
authors.forename: L S
authors.initials: LS
authors.lastname: Goodman
authors.forename: R D
authors.initials: RD
authors.lastname: Banks
authors.forename: J D
authors.initials: JD
authors.lastname: Grissett
authors.forename: P L
authors.initials: PL
authors.lastname: Saunders
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000337:Aerospace Medicine; D000704:Analysis of Variance; D001794:Blood Pressure / Q000502:physiology*; D001919:Bradycardia / Q000175:diagnosis / Q000209:etiology* / Q000503:physiopathology; D004562:Electrocardiography; D006339:Heart Rate / Q000502:physiology*; D006706:Homeostasis; D006801:Humans; D018471:Hypergravity / Q000009:adverse effects*; D018472:Hypogravity / Q000009:adverse effects*; D008991:Monitoring, Physiologic; D012044:Regression Analysis; D012189:Retrospective Studies; D013997:Time Factors; D014661:Vasoconstriction / Q000502:physiology*