| Abstract: | Wehypothesized that muscle sympathetic nerve activity (MSNA) andcardiovascular responses to the conventional head-up tilt (HUT) aredifferent from those to head-up suspension (HUS) because of antigravitymuscle activity. The MSNA from the tibial nerve, heart rate, bloodpressure, stroke volume, cardiac output, and calf blood flow weremeasured in 13 healthy young subjects. Left atrial diameter wasmeasured by two-dimensional echocardiography in another nine subjects.The resting MSNA and cardiovascular responses at a low level (20°)of orthostasis were similar during both modes. At higher levels (40 and60°), the responses of MSNA, heart rate, stroke volume, and cardiacoutput were significantly stronger and there was a smaller reduction incalf blood flow during HUT than during HUS(P < 0.05). Left atrial diameter was decreased significantly from the resting values during HUT and HUSwithout any significant difference between the modes of orthostasis. The results provide evidence that the engagement of antigravity musclesduring HUT may have additive effects on sympathetic vasoconstrictor andcardiovascular responses to orthostatic stress. |
