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

We examined whether the altered orthostatic tolerance following 14 days of head-down tilt bed rest (HDBR) was related to inadequate sympathetic outflo w 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 sympa thetic nerve activity (MSNA, microneurography), and stroke volume blood vel ocity (SW, Doppler ultrasound) were assessed during supine 30 degrees (5 mi n) and 60 degrees (5-10 min) HUT positions in 15 individuals who successful ly 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 SW durin g supine and HUT postures were not altered in the OT group. Hypotension dur ing 60 degrees HUT in the post-bed rest test for the OI group (P < 0.05) wa s associated with a blunted increase in MSNA (P < 0.05). SW was reduced fol lowing HDBR in the OI group (main effect of HDBR, P < 0.02). The data suppo rt the hypothesis that bed rest-induced orthostatic intolerance is related to an inadequate increase in sympathetic discharge that cannot compensate f or a greater postural reduction in stroke volume.