Slow head-up tilt causes lower activation of muscle sympathetic nerve activity: loading speed dependence of orthostatic sympathetic activation in humans

Many earlier human studies have reported that increasing the tilt angle of head-up tilt (HUT) results in greater muscle sympathetic nerve activity (MSNA) response, indicating the amplitude dependence of sympathetic activation in response to orthostatic stress. However, little is known about whether and how the inclining speed of HUT influences the MSNA response to HUT, independent of the magnitude of HUT. Twelve healthy subjects participated in passive 30 degrees HUT tests at inclining speeds of 1 degrees (control), 0.1 degrees (slow), and 0.0167 degrees (very slow) per second. We recorded MSNA (tibial nerve) by microneurography and assessed nonstationary time-dependent changes of R-R interval variability using a complex demodulation technique. MSNA averaged over every 10 degrees tilt angle increased during inclination from 0 degrees to 30 degrees , with smaller increases in the slow and very slow tests than in the control test. Although a 3-min MSNA overshoot after reaching 30 degrees HUT was observed in the control test, no overshoot was detected in the slow and very slow tests. In contrast with MSNA, increases in heart rate during the inclination and after reaching 30 degrees were similar in these tests, probably because when compared with the control test, greater increases in plasma epinephrine counteracted smaller autonomic responses in the very slow test. These results indicate that slower HUT results in lower activation of MSNA, suggesting that HUT-induced sympathetic activation depends partially on the speed of inclination during HUT in humans.