Heart rate and blood pressure response to short-term head-down bed rest: Anonlinear approach

Although it is well-known that prolonged exposure to microgravity environme nt such as in space travel results in derangements of orthostasis, recent e vidence suggests that even short-term exposure may have similar effects and parallels such common examples as prolonged bed rest. Whereas spectral ana lysis of heart rate and systolic blood pressure have been unable to detect changes, we hypothesized that nonlinear indexes may be better able to uncov er such perturbations. Eighteen healthy subjects were exposed to 4-hour hea d-down ti It, a nd of these, 4 exhibited fainting. Two nonlinear indexes, m utual information and recurrence quantification were used to analyze the da ta. Only recurrence quantification was able to detect a "decoupling" of hea rt rate and systolic blood pressure at rest using discriminant analysis (p < 0.05). These results suggest that orthostatic intolerance may be due to a decoupling of heart rate from systolic blood pressure reflexive activity o ccurring at rest.