Acute effects of simulated microgravity on heart rate variability

Microgravity is known to stress the heart and blood vessels and to perturb the normal neural regulation of the cardiovascular system. In an effort to gain greater insight into the adjustments of neural control of the cardiovascular system in space, we have used a model of simulated microgravity, the -6 degrees head-down tilt body position (HDT). As a common method for the description of the functional state of the cardiovascular system the power spectral analysis of the heart rate can be used. The following working hypotheses were postulated: 1. HDT causes an acute activation of the parasympathetic nerve traffic to the heart, which is detectable by a reduction in heart rate and by the spectral power distribution of the heart rate variability. 2. HDT induces changes in the autonomic nervous response to upright posture which can be detected after a 45-minute period of head-down tilt.