Contributions of MSNA and stroke volume to orthostatic intolerance following bed rest

J. K. Shoemaker, C. S. Hogeman, L. I. Sinoway

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We examined whether the altered orthostatic tolerance following 14 days of head-down tilt bed rest (HDBR) was related to inadequate sympathetic outflow or to excessive reductions in cardiac output during a 10- to 15-min head-up tilt (HUT) test. Heart rate, blood pressure (BP, Finapres), muscle sympathetic nerve activity (MSNA, microneurography), and stroke volume blood velocity (SVV, Doppler ultrasound) were assessed during supine 30°(5 min) and 60°(5-10 min) HUT positions in 15 individuals who successfully completed the pre-HDBR test without evidence of orthostatic intolerance. Subjects were classified as being orthostatically tolerant (OT, n = 9) or intolerant (OI, n = 6) following the post-HDBR test. MSNA, BP, and SVV during supine and HUT postures were not altered in the OT group. Hypotension during 60° HUT in the post-bed rest test for the OI group (P < 0.05) was associated with a blunted increase in MSNA (P < 0.05). SVV was reduced following HDBR in the OI group (main effect of HDBR, P < 0.02). The data support the hypothesis that bed rest-induced orthostatic intolerance is related to an inadequate increase in sympathetic discharge that cannot compensate for a greater postural reduction in stroke volume.

Original languageEnglish (US)
Pages (from-to)R1084-R1090
JournalAmerican Journal of Physiology - Regulatory Integrative and Comparative Physiology
Issue number4 46-4
StatePublished - Oct 1999

All Science Journal Classification (ASJC) codes

  • Physiology
  • Physiology (medical)


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