Purpose: Orthostatic intolerance is more prevalent in women. The purpose of this investigation was to evaluate the physiological responses of orthostatic tolerant and intolerant females to progressive lower body negative pressure (LBNP) and to identify predictors of orthostatic tolerance.
Methods: Following baseline measurements, eleven healthy, moderately active women (mean age 24 ± 3 yr) underwent an orthostatic challenge involving four 12-minute stages of progressive LBNP at -15, -30, -45 and -60 mmHg. Traditional haemodynamic characteristics, as well as baroreceptor sensitivity, were analyzed across all stages.
Results: Five women became presyncopal during the test and were classified as low tolerant (LT) while the remaining six were classified as high tolerant (HT). LBNP by group (tolerance) interactions were significantly different for stroke volume (P=0.008) and the rate of decline (slope) of stroke volume (P=0.03). During the early stages of LBNP, the LT group displayed a higher stroke volume than the HT group (76.4 ± 8.6 vs. 60.0 ± 13.3 mL/beat; P=0.02) yet by the final stage, stroke volumes were similar (22.5 ± 11.9 vs. 22.7 ± 4.5 mL/beat, P = 0.99). Baroreceptor sensitivity, heart rate variability and blood pressure variability were not significantly different between the groups.
Conclusions: The results of this investigation suggest that orthostatic intolerance in women can be identified during the initial stages of an LBNP challenge, as evidenced by a more rapid decline in stroke volume.