Variability and cardiovascular homeostasis.

The inability to maintain the upright posture due to a failure in the arterial blood pressure regulatory mechanisms on return from space travel or after a period of head down tilt bed rest (HDBR) is the ultimate sign of cardiovascular deconditioning. Yet, the final response of syncope is potentially heralded by a series of precursor events that can be quantified and analyzed in new, more insightful ways to attempt to understand the integrative nature of autonomic control of the cardiovascular system.