Orthostatic tolerance and autonomous nervous functions before and after 20-days bed rest.

To determine the critical factor that induces tachycardia during orthostasis after bed rest, we tested some autonomous nervous functions and the reaction to lower body negative pressure before and after 20 days horizontal bed rest. Subjects were 5 young healthy males and one female. Carotid vagal baroreflex sensitivity was evaluated by cardiac response to a gradual increase in carotid distending pressure of 150 mmHg within 10 s by neck suction after neck positive pressure. Combined effects of cardiopulmonary and aortic baroreflexes were assessed by the cardiovascular responses during Valsalva manoeuvre. Sympathetic reactivity was evaluated by a cold pressor test. The carotid vagal baroreflex tended to be diminished after bed rest in supine rest, but unchanged in seated position. Cardiac responses to Valsalva manoeuvre did not change after bed rest, although blood pressures decreased more during the manoeuvre after bed rest. Cold pressor response did not change after bed rest. Heart rate response during lower body negative pressure was increased after bed rest. In conclusion, combining our results from several autonomous nervous functions suggested that the exaggerated orthostatic tachycardia after bed rest could not be explained by the changes in sensitivity of carotid, aortic, and cardiopulmonary reflexes and sympathetic reactivity to cold stress.