Effects of gender on the autonomic modulation of the cardiovascular responses to lower body negative pressure

Background: The cardiovascular responses to submaximal lower body negative pressure (LBNP) appear to differ between genders, but the underlying mechan isms are uncertain. Hypothesis: 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 o f 6 min each. Heart rate (HR), stroke volume (SV), BP, forearm blood flow a nd R-R interval data were acquired. Spectral analysis of the R-R interval d ata was used to assess autonomic modulation with the low frequency componen t (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 betwe en groups. LBNP of -50 mmHg evoked greater HR increases in the women than t he men (7.2 +/- 1.0 vs. 3.8 +/- 1.1 bpm; p < 0.05), while SV. cardiac outpu t and total peripheral conductance decreased more (-15 +/- 2 vs. -8 +/- 2 m l . beat(-1); -0.668 +/- 0.131 vs. -0.1778 +/- 0.124 L . 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 w hile the LF/HF ratio, an indicator of sympathetic neural modulation of HR v ariability, increased above rest at - 40 mmHg. These responses did not diff er significantly between groups. Conclusions: These results suggest that ge nder differences in the cardiovascular responses to LBNP are not due to gro ss differences in modulation of the autonomic nervous systems.