Altered Venous Function during Long-Duration Spaceflights

Abstract

Aims: Venous adaptation to microgravity, associated with cardiovascular deconditioning, may contribute to orthostatic intolerance following spaceflight. The aim of this study was to analyze the main parameters of venous hemodynamics with long-duration spaceflight. Methods: Venous plethysmography was performed on 24 cosmonauts before, during, and after spaceflights aboard the International Space Station. Venous plethysmography assessed venous filling and emptying functions as well as microvascular filtration, in response to different levels of venous occlusion pressure. Calf volume was assessed using calf circumference measurements. Results: Calf volume decreased during spaceflight from 2.3 ± 0.3 to 1.7 ± 0.2 L (p < 0.001), and recovered after it (2.3 ± 0.3 L). Venous compliance, determined as the relationship between occlusion pressure and the change in venous volume, increased during spaceflight from 0.090 ± 0.005 to 0.120 ± 0.007 (p < 0.01) and recovered 8 days after landing (0.071 ± 0.005, arbitrary units). The index of venous emptying rate decreased during spaceflight from -0.004 ± 0.022 to -0.212 ± 0.033 (p < 0.001, arbitrary units). The index of vascular microfiltration increased during spaceflight from 6.1 ± 1.8 to 10.6 ± 7.9 (p < 0.05, arbitrary units). Conclusion: This study demonstrated that overall venous function is changed during spaceflight. In future, venous function should be considered when developing countermeasures to prevent cardiovascular deconditioning and orthostatic intolerance with long-duration spaceflight.

Keywords: blood volume; cardiovascular deconditioning; microgravity; venous plethysmography.

Figure 1

Example of the venous plethysmography curve. (A) A whole session that includes several venous occlusion steps at increasing pressure (20, 30, 40, 50, and 60 mmHg, from left to right). (B) Example of an occlusion step showing the points to determined plethysmography variables: a, b: start and end points for calf vein filling measurements; c, d: start and end points for calf vein emptying measurements.

Figure 2

Venous compliance. Venous compliance is assessed through the pressure/volume relationship. The diagram is drawn using the relative filling volume (ΔVmax-r, in percentage) and the venous occlusion pressure during two whole plethysmography sessions on the same cosmonaut. The first session occurred more than 2 months before space flight (B > 2) and the second one during the flight but before its third month (F < 3). A whole plethysmography session included five levels of venous occlusion (x-axis). Equations of the linear regressions are mentioned on the graph.

Figure 3

“Plateau” section of a venous plethysmography curve. This figure shows the drift of the upper plateau when the venous occlusion cuff is inflated (at 50 mmHg in this example). This drift is almost linear at the end of the plateau and this is due to microvascular filtration resulting in increased calf volume. The slope of the dashed line provides a quantification of this drift and is used to assess microvascular filtration.