title: Effects of gender on the autonomic modulation of the cardiovascular responses to lower body negative pressure.
abstract: BACKGROUND
  The cardiovascular responses to submaximal lower body negative pressure (LBNP) appear to differ between genders, but the underlying mechanisms are uncertain.
  
  
  OBJECTIVE
  These differences are due to differences in the autonomic modulation of the cardiovascular system.
  
  
  METHODS
  There were 14 women and 13 men who underwent LBNP to -50 mmHg in 10 mmHg increments of 6 min each. Heart rate (HR), stroke volume (SV), BP, forearm blood flow and R-R interval data were acquired. Spectral analysis of the R-R interval data was used to assess autonomic modulation with the low frequency component (LF) set at 0.04 to 0.15 Hz and the high frequency component (HF) at 0.15 to 0.4 Hz.
  
  
  RESULTS
  The responses to LBNP to -40 mmHg did not differ between groups. LBNP of -50 mmHg evoked greater HR increases in the women than the men (7.2 +/- 1.0 vs. 3.8 +/- 1.1 bpm; p < 0.05), while SV, cardiac output and total peripheral conductance decreased more (-15 +/- 2 vs. -8 +/- 2 ml x beat(-1); -0.668 +/- 0.131 vs. -0.1778 +/- 0.124 L x min(-1); -0.009 +/- 0.002 vs. -0.004 +/- 0.001 units; p < 0.05). Normalized HF, an indicator of the vagal influence on HR variability, declined below rest at -40 mmHg while the LF/HF ratio, an indicator of sympathetic neural modulation of HR variability, increased above rest at -40 mmHg. These responses did not differ significantly between groups.
  
  
  CONCLUSIONS
  These results suggest that gender differences in the cardiovascular responses to LBNP are not due to gross differences in modulation of the autonomic nervous systems.
authors.affiliation: Department of Health and Human Performance, Iowa State University, Ames 50011, USA. wfranke@iastate.edu
authors.forename: W D
authors.initials: WD
authors.lastname: Franke
authors.forename: K
authors.initials: K
authors.lastname: Lee
authors.forename: S R
authors.initials: SR
authors.lastname: Graff
authors.forename: A B
authors.initials: AB
authors.lastname: Flatau
mesh_terms: D000328:Adult; D001341:Autonomic Nervous System / Q000502:physiology*; D001783:Blood Flow Velocity / Q000502:physiology; D002302:Cardiac Output / Q000502:physiology; D002320:Cardiovascular Physiological Phenomena; D005260:Female; D005542:Forearm / Q000098:blood supply; D006339:Heart Rate / Q000502:physiology; D006706:Homeostasis / Q000502:physiology; D006801:Humans; D008150:Lower Body Negative Pressure / Q000009:adverse effects*; D008297:Male; D012727:Sex Characteristics; D012815:Signal Processing, Computer-Assisted; D018561:Space Simulation / Q000009:adverse effects; D013318:Stroke Volume / Q000502:physiology; D014630:Vagus Nerve / Q000502:physiology