Effect of prolonged head-down bed rest on complex cardiovascular dynamics

We postulated that a change in complex dynamics of the cardiovascular syste m could be involved in the orthostatic intolerance observed after simulated weightlessness. Supine recordings of 1024 consecutive pulse intervals and systolic blood pressures were obtained on 7 subjects adapted to a 42 day he ad-down bed rest (day 22 and 42) but also before and 6 days after head-down bed rest (-6 degrees). Coarse graining spectral analysis was used to extra ct the non-harmonic (fractal) component from each time series, The power sp ectral densities of this fractal component are inversely proportional to th eir frequency (l/f(beta)). We fitted an inverse power law estimate to the f ractal component to determine the spectral exponent beta. The complex dynam ics of blood pressure and heart rate variability were also analyzed by corr elation dimension and non-linear prediction. Bed rest induced orthostatic i ntolerance in 4 subjects. There was a significant increase in the spectral exponent beta of RR-interval variability during and after head-down bed res t (before: 1.039+/-0.090; during: 1.552+/-0.080 and 1.547+/-0.100; after: 1 .428+/-0.040). Analysis of the blood pressure dynamics indicated lower corr elation dimensions during head-down bed rest and higher coefficients of pre dictability after head-down bed rest. Complexity alterations of RR-interval and blood pressure variability were not linked with one another during hea d-down bed rest. These alterations seemed to be correlated with the orthost atic intolerance observed after bed rest. These results suggest a change of the integration level of cardiovascular autonomic regulation. (C) 2001 Pub lished by Elsevier Science B.V.