Year of Publication

2005

Document Type

Dissertation

College

Graduate School

Department

Biomedical Engineering

First Advisor

Charles F. Knapp

Abstract

Human cardiovascular adaptations to microgravity include decreased plasma volume, exercise capacity, baroreflex function as well as decreased orthostatic tolerance upon return to a gravity environment. Several countermeasures have been proposed and tested, although currently none have been developed to prevent post-spaceflight orthostatic intolerance (OI). Artificial gravity (AG) generated by short-radius centrifugation (SRC) has been proposed as a countermeasure to OI as well as other cardiovascular alterations. Methods: Fifteen men and fourteen women underwent three weeks of daily (5 days a week) exposure to intermittent (1.0 to 2.5 Gz) artificial gravity on a 1.9m human powered centrifuge (HPC) at the NASA Ames Research Center. Half the subjects exercised (active) to power the HPC while half rode passively (passive). A combination head-up tilt (HUT) and lower body negative pressure (LBNP) test was used to determine orthostatic tolerance before and after training. Oscillatory LBNP (OLBNP) was used at seven frequencies (0.01 to 0.15 Hz) for two minutes each to assess the dynamic responses of the cardiovascular system to orthostatic stress, before and after AG training. Results: Training improved overall tolerance in the group of subjects by 13% (pandlt;0.05); men were more tolerant than were women (pandlt;0.05); and active subjects were more improved than passive subjects (pandlt;0.05). Mechanisms of improvement appear to be through decreased total peripheral resistance (TPR) and increased stroke volume after training, and increased responsiveness of TPR to fluid shifts (faster changes in TPR to changes in calfcircumference [CC] and OLBNP after training). There was no change in spontaneous baroreflex sensitivity (BRS, calculated by sequence method) or number of sequences per number of heart beats (NNS), although BRS analysis did indicate that stimulation to the cardiac baroreceptors during 1.0 Gz and 2.5 Gz centrifugation was no different than supine control and 70?? HUT, respectively. Taken together, these results suggest that AG training improved tolerance through training of local mechanisms in the peripheral vasculature, or extrinsic control of peripheral vascular resistance, rather than through changes of autonomic control of heart rate.

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