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Role of individual predisposition in orthostatic intolerance before and after simulated microgravity

¹Division of Endocrinology, Hypertension, and Diabetes, Brigham and Women's Hospital, Boston 02115; ²NASA Center for Quantitative Cardiovascular Physiology, Modeling and Data Analysis, Harvard-Massachusetts Institute of Technology Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; and ³Department of Cardio-Thoracic Surgery, McGill University, Montreal, Quebec, Canada H3G 1A4

Submitted 1 December 2003 ; accepted in final form 12 January 2004

Orthostatic intolerance (OI) is a major problem after spaceflight. Its etiology remains uncertain, but reports have pointed toward an individual susceptibility to OI. We hypothesized that individual predisposition plays an important role in post-bed rest OI. Twenty-four healthy male subjects were equilibrated on a constant diet, after which they underwent tilt-stand test (pre-TST). They then completed 14-16 days of head-down-tilt bed rest, and 14 of the subjects underwent repeat tilt-stand test (post-TST). During various phases, the following were performed: 24-h urine collections and hormonal measurements, plethysmography, and cardiovascular system identification (a noninvasive method to assess autonomic function and separately quantify parasympathetic and sympathetic responsiveness). Development of presyncope or syncope defined OI. During pre-TST, 11 subjects were intolerant and 13 were tolerant. At baseline, intolerant subjects had lower serum aldosterone (P < 0.01), higher excretion of potassium (P = 0.01), lower leg venous compliance (P = 0.03), higher supine parasympathetic responsiveness (P = 0.02), and lower standing sympathetic responsiveness (P = 0.048). Of the 14 subjects who completed post-TST, 9 were intolerant and 5 were tolerant. Intolerant subjects had lower baseline serum cortisol (P = 0.03) and a higher sodium level (P = 0.02) compared with tolerant subjects. Thus several physiological characteristics were associated with increased susceptibility to OI. We propose a new model for OI, whereby individuals with greater leg venous compliance recruit compensatory mechanisms (activation of the renin-angiotensin-aldosterone system and sympathetic nervous system, and withdrawal of the parasympathetic nervous system) in the face of daily postural challenges, which places them at an advantage to face orthostatic stress. With head-down-tilt bed rest, the stimulus to recruit compensatory mechanisms disappears, and differences between the two subgroups attenuate.

cardiovascular deconditioning; renin-angiotensin-aldosterone system; leg venous compliance; autonomic function; head-down-tilt bed rest

This article has been cited by other articles:

				S. M. Grenon, S. Hurwitz, N. Sheynberg, X. Xiao, B. Judson, C. D. Ramsdell, C. Kim, R. J. Cohen, and G. H. Williams Sleep restriction does not affect orthostatic tolerance in the simulated microgravity environment J Appl Physiol, November 1, 2004; 97(5): 1660 - 1666. [Abstract] [Full Text] [PDF]