Autonomic tone is impaired by prolonged bedrest and sympathetic tone before bedrest predicts tolerance to tilt after bedrest

Microgravity-induced orthostatic intolerance continues to be a primary concern of the human space program. Reduced autonomic tone has been shown to contribute to this phenomenon. However, the techniques that have been utilized for quantitating autonomic tone are limited. In this paper, we present an improved, noninvasive method for quantitating distinctly parasympathetic and β-sympathetic tone and demonstrate its validity with respect to postural changes. We then apply the method to data collected from 20 human subjects before and after exposure to simulated microgravity (16-day head down bedrest). Statistical analyses show that parasympathetic and β-sympathetic tone are both impaired by bedrest and reduced 0-sympathetic tone before bedrest can predispose subjects to presyncope following bedrest.