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* To whom correspondence should be addressed. E-mail: minson{at}uoregon.edu.
Women have decreased orthostatic tolerance compared with men, and anecdotal evidence suggests women are most susceptible to orthostatic intolerance in warm environments. As estrogen and progesterone affect numerous physiological variables that may alter orthostatic tolerance, the purpose of our study was to compare orthostatic tolerance across the menstrual cycle in women during combined orthostatic and heat stress, and to compare these data to men. Eight normally menstruating women (22 ± 4.0 years) and 8 males (23 ± 3.5 years) completed the protocol. Female subjects were studied during their early follicular (EF), ovulatory (OV), and mid-luteal (ML) phases. Male subjects were studied twice within 2-4 weeks. Heart rate, cardiac output, blood pressure, core temperature (Tc), and cutaneous vascular conductance (CVC) were measured during three head-up tilt tests, consisting of two tilts in the thermoneutral condition and one tilt after a 0.5°C rise in Tc. There was no difference in orthostatic tolerance across the menstrual cycle, despite higher CVC in the ML phase after heating (EF: 42.3 ± 4.8, OV: 40.1 ± 3.7, ML: 57.5 ± 4.5; P<0.05). Orthostatic tolerance in the heat was greater in men verses women (P<0.05). These data suggest that although many physiological variables associated with blood pressure regulation fluctuate during the menstrual cycle, orthostatic tolerance in the heat remains unchanged. Additionally, our data supports a clear sex difference in orthostatic tolerance and extends upon previous data to show that the sex difference in the heat is not attributable to fluctuating hormone profiles during the menstrual cycle.
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