A comparison of sympathetic vasomotor and cardiovascular responses to head-up tilt and to head-up suspension in humans.
Muscle sympathetic nerve activity (MSNA) from the tibial nerve, heart rate (HR), respiration and blood pressure (BP) changes were recorded in 7 healthy young adults to investigate the role of antigravity muscle activity on the reflex sympathetic vasomotor and circulatory responses during orthostatic stress. The conventional passive graded head-up tilt (HUT) with a foot-board and the graded head-up suspension (HUS) with a body harness that provide orthostasis without weight bearing by the legs were applied for 3 consecutive, sequential 5-min periods at the levels of 20, 40 and 60 degrees. The resting values and responses to the lower level (20 degrees) of orthostasis by both techniques were similar. However, the changes in MSNA and HR during the HUS at higher levels (40 and 60 degrees) were smaller with a slight increase in mean BP at 60 degrees than the HUT at the same levels and duration (p<0.05). Antigravity muscle activity during the HUT generates tension in the muscles and may activate muscle mechanoreceptors. Therefore, the reflex sympathetic vasomotor activity and circulatory responses to the HUT in humans are probably due to the additive effects of both the baroreflexes and muscle mechanoreflexes.
Shamsuzzaman
A S
AS
Department of Autonomic Neuroscience, Nagoya University, Japan.
Sugiyama
Y
Y
Mano
T
T
D000328:Adult; D017704:Baroreflex; D001794:Blood Pressure / Q000502:physiology*; D018475:Head-Down Tilt; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D008297:Male; D008465:Mechanoreceptors; D018482:Muscle, Skeletal / Q000098:blood supply / Q000294:innervation* / Q000502:physiology; D011187:Posture / Q000502:physiology*; D013564:Sympathetic Nervous System / Q000502:physiology*; D013979:Tibial Nerve / Q000502:physiology; D018667:Tilt-Table Test; D014666:Vasomotor System / Q000502:physiology; D016474:Weight-Bearing