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Am J Physiol Regul Integr Comp Physiol 281: R778-R785, 2001;
0363-6119/01 $5.00
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Vol. 281, Issue 3, R778-R785, September 2001

Effects of lower body positive pressure on muscle sympathetic nerve activity response to head-up tilt

Qi Fu, Satoshi Iwase, Yuki Niimi, Atsunori Kamiya, Jun Kawanokuchi, Jian Cui, Tadaaki Mano, and Akio Suzumura

Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan

The present study was performed to test the hypothesis that application of lower body positive pressure (LBPP) during orthostasis would reduce the baroreflex-mediated enhancement in sympathetic activity in humans. Eight healthy young men were exposed to a 70° head-up tilt (HUT) on application of 30 mmHg LBPP. Muscle sympathetic nerve activity (MSNA) was microneurographically recorded from the tibial nerve, along with hemodynamic variables. We found that in the supine position with LBPP, MSNA remained unchanged (13.4 ± 3.3 vs. 11.8 ± 2.3 bursts/min, without vs. with LBPP; P > 0.05), mean arterial pressure was elevated, but arterial pulse pressure and heart rate did not alter. At 70° HUT with LBPP, the enhanced MSNA response was reduced (33.8 ± 5.0 vs. 22.5 ± 2.2 bursts/min, without vs. with LBPP; P < 0.05), mean arterial pressure was higher, the decreased pulse pressure was restored, and the increased heart rate was attenuated. We conclude that the baroreflex-mediated enhancement in sympathetic activity during HUT was reduced by LBPP. Application of LBPP in HUT induced an obvious cephalad fluid shift as well as a restoration of arterial pulse pressure, which reduced the inhibition of the baroreceptors. However, the activation of the intramuscular mechanoreflexes produced by 30 mmHg LBPP might counteract the effects of baroreflexes.

cardiopulmonary baroreceptors; arterial baroreceptors; intramuscular pressure-sensitive receptors; fluid shift


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