Spaceflight alters autonomic regulation of arterial pressure in humans.

Spaceflight is associated with decreased orthostatic tolerance after landing. Short-duration spaceflight (4-5 days) impairs one neural mechanism: the carotid baroreceptor-cardiac reflex. To understand the effects of longer-duration spaceflight on baroreflex function, we measured R-R interval power spectra, antecubital vein plasma catecholamine levels, carotid baroreceptor-cardiac reflex responses, responses to Valsalva maneuvers, and orthostatic tolerance in 16 astronauts before and after shuttle missions lasting 8-14 days. We found the following changes between preflight and landing day: 1) orthostatic tolerance decreased; 2) R-R interval spectral power in the 0.05 to 0.15-Hz band increased; 3) plasma norepinephrine and epinephrine levels increased; 4) the slope, range, and operational point of the carotid baroreceptor cardiac reflex response decreased; and 5) blood pressure and heart rate responses to Valsalva maneuvers were altered. Autonomic changes persisted for several days after landing. These results provide further evidence of functionally relevant reductions in parasympathetic and increases in sympathetic influences on arterial pressure control after spaceflight.