The Impact of Gender on Orthostatic Tolerance and on the Renal, Cardiovascular and Cardio-Endocrine Responses to Simulated Microgravity
- Paper number
IAC-05-A1.P.19
- Author
Dr. Marlene Grenon, McGill University, Canada
- Coauthor
Dr. Richard J. Cohen, Massachussets Institute of Technology (MIT), United States
- Coauthor
Dr. Shelley Hurwitz, United States
- Coauthor
Ms. Christine Kim, United States
- Coauthor
Dr. Gordon H. Williams, United States
- Year
2005
- Abstract
Introduction: Actual and simulated microgravity exposure induces cardiovascular deconditioning, which is manifested by a decreased tolerance to orthostatic stress. Orthostatic intolerance (OI) has been reported to occur more frequently among women than among men. We assessed the renal, cardio-endocrine, and cardiovascular responses to simulated microgravity in women and men in order to examine the potential impact of gender on cardiovascular deconditioning, and hypothesized that 1) women would have a poorer tolerance to orthostatic stress and that, 2) renal, cardio-endocrine and cardiovascular factors would be implicated in this phenomenon.
Methods: Fifteen healthy female and fourteen healthy male subjects were equilibrated on a constant 200 mEq sodium, 100 mEq potassium diet for 3-5 days, after which they underwent tilt-stand test (pre-TST). After the equilibration period, male subjects immediately started 14-16 days of head-down tilt-bed rest (HDTB), after which tilt-stand test was repeated (post-TST). Female subjects were discharged and came back on their next menstrual cycle to undergo 14-16 days of HDTB and the post-TST, in order for pre-TST and post-TST to occur on the same day of their menstrual cycle (which corresponded to two consecutive cycles). During various phases the following were performed: daily 24-hour urine collections and hormonal measurements, plethysmography, and cardiovascular system identification (CSI), a non-invasive method to assess autonomic function and separately quantify parasympathetic and sympathetic responsiveness.
Results: All of the female subjects experienced pre-syncope during the pre-TST. This is significantly different than the male subjects, in which 50% experienced OI during pre-TST (p=0.02). After HDTB, all female subjects also experienced OI compared to 64% of males who experienced pre-syncope (p=0.004). The time to pre-syncope within the female group was shorter after HDTB by a median of 6.5 minutes (p=0.02). The time to pre-syncope was significantly different between genders before (p=0.005) and after HDTB (0.001). At baseline, women demonstrated a higher serum aldosterone (p=0.02) and a higher urinary excretion of aldosterone (p=0.03) than men. With HDTB, both genders experienced a significant increase in plasma renin activity, an initial loss of urinary sodium and an increase in potassium excretion, although no significant difference could be detected between the genders in the hormonal or renal responses to HDTB. Related to the autonomic function, women demonstrated a higher parasympathetic responsiveness (p=0.01) and a lower sympathetic responsiveness (p=0.05) at baseline than men. No significant change in autonomic function was detected in women after HDTB, and gender did not have an impact on the responses of autonomic function to HDTB. Lastly, women also had a lower venous compliance than men before (p=0.02) and after HDTB (p=0.0002).
Conclusions: Although gender does not seem to influence the renal, cardio-endocrine and cardiovascular responses to a simulated microgravity environment, it seems to play an important role on orthostatic tolerance before and after HDTB, with a greater incidence of OI developing among women than among men. Factors likely implicated include autonomic function and the renin-angiotensin aldosterone system. This knowledge will be important in the development of appropriate countermeasures for exploratory missions.
- Abstract document