Capacity to Compensate for Central Hypovolemia and Effects of Menstrual Cycle Phases.

Tolerance to central hypovolemia is dictated by exhaustion of the physiological capacity to compensate called the compensatory reserve. Such physiological compromise can have detrimental impact on performance in aerospace environments as well as survival from hemorrhage on the battlefield. We induced central hypovolemia using progressively stepwise lower body negative pressure (LBNP) in women during various phases of the menstrual cycle to test the hypothesis that similar tolerance across all menstrual cycle phases would be reflected by similar changes in compensatory reserve. Based on self-reporting of the last menstrual period, 40 healthy women, matched by demographics, were classified into 1 of 5 menstrual cycle phases: early follicular (EF, Days 1-7; = 10), late follicular and ovulatory (LF, Days 9-15, = 6), early luteal (EL, Days 16-18, = 6), midluteal (ML, Days 19-25, = 8), and late luteal (LL, Days 26-30, = 10). All subjects had a 28-30 d menstrual cycle and were not taking oral contraceptives. Tolerance to central hypovolemia was measured as time (seconds) from baseline to the onset of presyncopal symptoms induced by LBNP. Time to presyncope as well as hemodynamic and compensatory reserve responses were statistically indistinguishable across all menstrual cycle phases. Consistent with our hypothesis, compensatory reserve with associated hemodynamic responses and tolerance to central hypovolemia was not affected by menstrual cycle phases. Our findings indicate experimental comparisons of responses to central hypovolemia involving the participation of healthy women with normal menstrual cycles and not taking oral contraceptives can be conducted independent of menstrual cycle phase.