Postspaceflight orthostatic hypotension occurs mostly in women and is predicted by low vascular resistance

Abstract

About 20% of astronauts suffer postspaceflight presyncope. We studied pre- to postflight (5- to 16-day missions) cardiovascular responses to standing in 35 astronauts to determine differences between 1) men and women and 2) presyncopal and nonpresyncopal groups. The groups were presyncopal women, presyncopal men, and nonpresyncopal men based on their ability to stand for 10 min postflight. Preflight, women and presyncopal men had low vascular resistance, with the women having the lowest. Postflight, women experienced higher rates of presyncope (100 vs. 20%; P = 0.001) and greater losses of plasma volume (20 vs. 7%; P < 0.05) than men. Also, presyncopal subjects had lower standing mean arterial pressure (P < or = 0.001) and vascular resistance (P < 0.05), smaller increases in norepinephrine (P < or = 0.058) and greater increases in epinephrine (P < or = 0.058) than nonpresyncopal subjects. Presyncopal subjects had a strong dependence on plasma volume to maintain standing stroke volume. These findings suggest that postflight presyncope is greatest in women, and this can be ascribed to a combination of inherently low-resistance responses, a strong dependence on volume status, and relative hypoadrenergic responses. Conversely, high vascular resistance and postflight hyperadrenergic responses prevent presyncope.

Understanding post-spaceflight orthostatic intolerance - a simulation study

“…The results of Figure 4 indicate that if the volume status is not compromised, relatively large changes in any of the other parameters need to be hypothesized in order to account for the clinical picture seen post-flight. Reductions in total blood volume following spaceflight are highly variable; recent reports suggest a mean overall reduction of approximately 6% in male pre-syncopal astronauts [6]. Our simulations give credence to the view that hypovolemia is one of the principal contributors, yet at a level of 6% it might not be soly responsible for the clinical picture of the syndrome.…”

Simulated Stand Tests and Centrifuge Training to Prevent Orthostatic Intolerance on Earth, Moon, and Mars

“…18 It is reasonable to expect that if POI occurs in environments with lower gravitational acceleration, such as the moon or Mars, that the time to presyncope might be much longer. Such a long time delay is potentially problematic because the astronaut may be otherwise asymptomatic and performing a number of different tasks when presyncope ensues.…”

“…8 On the other hand, women (all non-finishing) were shown to have the ability to increase their TPR from supine to standing by margins even larger than most male finishers, but had lower pretest TPR and blood pressure, as well as a higher arterial pressure threshold at which presyncope occurred. 18 This comparison suggests that while an increase in TPR may be necessary to maintain arterial pressure and prevent presyncope, it may be insufficient if pretest arterial pressure is too near to its presyncopal threshold.…”

Estrogen receptors: Their roles in regulation of vasopressin release for maintenance of fluid and electrolyte homeostasis

“…It is well established that the osmotic threshold for vasopressin (VP) secretion is reset during pregnancy and during the luteal phase of the menstrual cycle [13,15,80,81,94]. Furthermore, women astronauts are more susceptible to orthostatic hypotension following extended weightlessness [99]. Since VP (also known as antidiuretic hormone) acts on the kidneys to regulate water excretion and is a potent vasoconstrictor agent important for preventing decreases in blood pressure, it was natural to hypothesize that gonadal steroids might influence VP secretion.…”