Gender differences in tolerance to lower body negative pressure.
BACKGROUND
Previous studies suggest that women have lower tolerance than men do to lower body negative pressure (LBNP). The mechanism(s) responsible has not been determined.


OBJECTIVE
Women would be less tolerant to presyncopal LBNP than men as determined by several indices of LBNP tolerance. Additionally, men and women, regardless of LBNP tolerance, would have similar cardiovascular responses to LBNP as presyncope was reached.


METHODS
The subjects were 18 men and 18 women (average age 25) of similar fitness levels who volunteered for the study. A step-wise LBNP protocol to presyncope was employed. HR, stroke volume (SV), cardiac output (Q), BP, and systemic vascular resistance (SVR) were measured before and throughout the LBNP stress. Data from women were compared with those from all men, and to men with similar and higher LBNP tolerance.


RESULTS
Women had significantly less LBNP tolerance than men regardless of index used: 30% less by duration of LBNP, 21% less by maximal LBNP tolerated, 44% less by a cumulative stress index, and 27% less by a linear tolerance index. Cardiovascular responses to LBNP were similar for women and men as presyncope was approached, whether the men were low-tolerant (LT) or high-tolerant (HT). In the 2 min pre-presyncope, HR increased by 80 +/- 6% in women, 72 +/- 7% in LT men and 96 +/- 14% in HT men; Q decreased by 47 +/- 3% in women, 52 +/- 6% in LT men and 55 +/- 2% in HT men. Similar comparisons occurred for the decline in BP and the rise in SVR.


CONCLUSIONS
Women have lower LBNP tolerance than do men, although there is considerable gender overlap in tolerances. The cardiovascular response to LBNP is similar regardless of gender or tolerance level as presyncope is approached. Understanding the gender differences in LBNP tolerance may lie in determining how the LBNP stress is translated into a "trigger" for cardiovascular decompensation.
Gotshall
R W
RW
Colorado State University, Department of Health and Exercise Science, Fort Collins 80523-1582, USA. gotshall@cahs.colostate.edu
D000328:Adult; D000704:Analysis of Variance; D001823:Body Composition; D001827:Body Height; D001835:Body Weight; D005260:Female; D006333:Heart Failure / Q000209:etiology; D006439:Hemodynamics / Q000502:physiology*; D006801:Humans; D008150:Lower Body Negative Pressure / Q000009:adverse effects*; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology; D010807:Physical Endurance / Q000502:physiology*; D012727:Sex Characteristics; D013312:Stress, Physiological / Q000503:physiopathology; D013575:Syncope / Q000209:etiology*; D013997:Time Factors