Optimal Time in the Menstrual Cycle For Spacewalks

Dear Sir:

With the building of the space station over the next 5 years, the requirement for frequent extra-vehicular activities (EVA'S) utilizing, at times, moderately heavy energy expenditures, raises concern regarding the potential for decompression sickness (DCS) (6). Some studies suggest an incidence of DCS four times greater in females than their male counterparts (II). DCS can precipitate endothelial injuries or dysfunction, for example of the pulmonary arteries (7), conducive to pulmonary hypertension, as well as endothelial insults to coronary and other systemic vessels (4), which could lead to ischemia, arrhythmias, necrosis and sudden death (6). These DCS-related vascular injuries would compound potential vascular insults as a result of brief missions (Apollo 15 for example), as well as interplanetary missions (8).

With spaceflights, estrogen's enhancement of magnesium (Mg) utilization and uptake by soft tissue and bone, provides a decided vascular advantage in young females over their male counterparts—in addition to their physiological loss of iron as a catalyst— since Mg is both an antioxidant and calcium blocker (8). Some protection may be provided against vascular complications of DCS during EVA'S, since estrogen has a direct effect on both smooth muscle and the endothelium, reducing the potential for vasospasm and clot formation (1,8).

Since with DCS, the pulmonary, coronary and peripheral vessels, for example, are all vulnerable to injury/dysfunction, which may be offset with adequate levels of estrogen, it seems reasonable to take advantage of the variability in estrogen's vascular effects, which are associated with the stages of the menstrual cycle.

It has been shown that females are more likely to develop DCS during menses, with the risk diminishing linearly, as the time increases since the last menstrual period (10). The mechanism for this time-associated correlation conceivably could be related to a significant increase in estradiol receptor concentrations, demonstrated in human uterine arteries, in the follicular phase (3). Animal studies have suggested that estrogen regulates estrogen receptor activation (1).

Endothelium-dependent vasodilitation (8) has been shown to be highest during the follicular phase of the menstrual cycle (2), and estrogen levels (2) and Mg ions (5) have been shown to be highest during the follicular phase as well.

These preliminary studies suggest that it might be appropriate to coordinate EVA'S with the menstrual cycle, in an attempt to reduce the potential for vascular insults of DCS, thereby possibly reducing or eliminating the discrepancy in the incidence of DCS related to gender. Finally, potential prevention of DCS by the use of 100% oxygen (6) is conducive to high oxidative stress with the potential for endothelial injuries (9), contributing to a two-edged sword; this provides further justification for coordinating EVA'S with the menstrual cycle.

William J. Rowe, M.D.

Former Assistant Clinical Professor Medicine Medical College of Ohio at Toledo
Swanton, OH


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Aviation, Space, and Environmental Medicine. Vol. 72, No. 8, pages 770-771, August 2001