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Microgravity-induced changes in aortic stiffness and their role in orthostatic intolerance

Departments of ¹Biomedical Engineering and of ²Anesthesiology and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland; and ³Human Adaptation and Countermeasures Office, Space and Life Sciences Directorate, National Aeronautics and Space Administration Johnson Space Center, Houston, Texas

Submitted 28 August 2006 ; accepted in final form 29 October 2006

Microgravity (µG)-induced orthostatic intolerance (OI) in astronauts is characterized by a marked decrease in cardiac output (CO) in response to an orthostatic stress. Since CO is highly dependent on venous return, alterations in the resistance to venous return (RVR) may be important in contributing to OI. The RVR is directly dependent on arterial compliance (C_a), where aortic compliance (C_ao) contributes up to 60% of C_a. We tested the hypothesis that µG-induced changes in C_a may represent a protective mechanism against OI. A retrospective analysis on hemodynamic data collected from astronauts after 5- to 18-day spaceflight missions revealed that orthostatically tolerant (OT) astronauts showed a significant decrease in C_a after spaceflight, while OI astronauts showed a slight increase in C_a. A ground-based animal model simulating µG, hindlimb-unweighted rats, was used to explore this phenomenon. Two independent assessments of C_ao, in vivo pulse wave velocity (PWV) of the thoracic aorta and in vitro pressure-diameter squared relationship (PDSR) measurements of the excised thoracic aorta, were determined. PWV showed a significant increase in aortic stiffness compared with control, despite unchanged blood pressures. This increase in aortic stiffness was confirmed by the PDSR analysis. Thus both actual µG in humans and simulated µG in rats induces changes in C_ao. The difference in C_a in OT and OI astronaut suggests that the µG-induced decrease in C_a is a protective adaptation to spaceflight that reduces the RVR and allows for the maintenance of adequate CO in response to an orthostatic stress.

microgravity; arterial compliance; hindlimb unweighting; resistance to venous return

This article has been cited by other articles:

				E. C. Tuday and D. E. Berkowitz Microgravity and cardiac atrophy: no sex discrimination J Appl Physiol, July 1, 2007; 103(1): 1 - 2. [Full Text] [PDF]

				M. D. Delp Arterial adaptations in microgravity contribute to orthostatic tolerance J Appl Physiol, March 1, 2007; 102(3): 836 - 836. [Full Text] [PDF]