A comparison of autonomic responses in humans induced by two simulation models of weightlessness: lower body positive pressure and 6° head-down tilt

Abstract

Six-degree head-down tilt (HDT) is well accepted as an effective weightlessness model in humans. However, some researchers utilized lower body positive pressure (LBPP) to simulate the cardiovascular and renal effects of a decreased gravitational stress. In order to determine whether LBPP was a suitable model for simulated weightlessness, we compared the differences between these two methods. Ten healthy males, aged 21–41 years, were subjected to graded LBPP at 10, 20 and 30 mm Hg, as well as 6° HDT. Muscle sympathetic nerve activity (MSNA) was microneurographically recorded from the tibial nerve along with cardiovascular variables. We found that MSNA decreased by 27% to a similar extent both at low levels of LBPP (10 and 20 mm Hg) and HDT. However, at a high level of LBPP (30 mm Hg), MSNA tended to increase. Mean arterial pressure was elevated significantly by 11% (10 mm Hg) at 30 mm Hg LBPP, but remained unchanged at low levels of LBPP and HDT. Heart rate did not change during the entire LBPP and HDT procedures. Total peripheral resistance markedly increased by 36% at 30 mm Hg LBPP, but decreased by 9% at HDT. Both stroke volume and cardiac output tended to decrease at 30 mm Hg LBPP, but increased at HDT. These results suggest that although both LBPP and HDT induce fluid shifts from the lower body toward the thoracic compartment, autonomic responses are different, especially at LBPP greater than 20 mm Hg. We note that high levels of LBPP (>20 mm Hg) activate not only cardiopulmonary and arterial baroreflexes, but also intramuscular mechanoreflexes, while 6° HDT only activates cardiopulmonary baroreflexes. We conclude that LBPP is not a suitable model for simulated weightlessness in humans.