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Measurement of inferior vena cava diameter for evaluation of venous return in subjects on day 10 of a bed-rest experiment

¹Department of Pediatrics, Kansai Medical University, Osaka 570-8506; ²Department of Developmental Medical Sciences, University of Tokyo, Tokyo 113-0033; ³Department of Healthcare and Economics and Quality Management, School of Public Health, Kyoto University, Kyoto 606-8501; ⁴Nakano Children's Hospital, Osaka 535-0022; and ⁵Department of Education, University of Utsunomiya, Tochigi 321-8505, Japan

Submitted 10 October 2003 ; accepted in final form 23 January 2004

We evaluated the usefulness of measurements of the inferior vena cava (IVC) diameters on abdominal echograms as an indicator of changes of venous return in subjects with orthostatic intolerance (OI) induced by simulated microgravity. We performed a standing test and recorded the IVC diameters on abdominal echograms in 12 subjects placed on a 20-day 6° head-down-tilt bed-rest experiment. We found that different patterns of changes in IVC diameter occurred in the standing test on day 10 of the experiment; in five subjects with a marginal decrease in pulse pressure, IVC diameters in the upright position were markedly decreased compared with those in the supine position. In five subjects with feelings of discomfort, the IVC diameters in the upright position distended or did not decrease from those in the supine position. These results suggested that the changes in IVC diameter on the standing test indicated the presence of various types of hemodynamic responses of OI caused by simulated microgravity. In this study, we also evaluated changes in body-water compartments by conducting multifrequency bioelectrical impedance analysis. Longitudinal data analysis showed that the total body-water-to-fat-free mass and extracellular fluid-to-fat-free mass ratios decreased during the experimental period and recovered thereafter, and that the ratio of intracellular fluid to fat-free mass decreased during the experiment. No significant difference in changes in body-water compartments was seen among subjects with different patterns of changes in IVC diameters. Measurement of IVC diameter was useful to estimate hemodynamic changes in subjects with OI.

head-down-tilt; orthostatic intolerance; abdominal echograms; body-water compartments

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