Nonlinear Analysis of Blood Cell Flux Fluctuations in the Rat Brain Cortex during Stepwise Hypotension Challenge

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Abstract
Blood flow through a region of interest in the brain cortex (cerebral blood flow (CBF)) as measured by laser-Doppler flowmetry (LDF) shows a complex temporal pattern, which can be either merely random or a manifestation of segmental chaotic dynamics of vasomotion-induced flowmotion in the arterial tree, a deterministic phenomenon; or that of a fractal, self-similar correlation order that emerges from the set of segmental perfusion events on statistical ground. Fractal content (F%) was determined by coarse-graining spectral analysis and their self-similar exponent, H, estimated by bridge-detrended Scaled Windowed Variance (bdSWV) method and a variant of the power Spectral Density method (lowPSDw,e). Chaotic dynamics were assessed by computing the correlation dimension (Dcorr) and the largest Lyapunov exponent (Λmax) on unfiltered raw and surrogate datasets. In 10 Sprague–Dawley rats anesthetized by halothane, CBF was measured through the thinned calvarium by the LDF method. Blood pressure (BP) was reduced from 100 to 40 mm Hg in steps of 20 mm Hg maintained for 2 mins by the lower body negative pressure method. Fractal and chaotic patterns coexisted in tissue perfusion. CBF did not show autoregulation. At every BP step, F% remained high (72% to 88%) and was independent of BP. Laser-Doppler flowmetry signals proved to be nonstationary fractional Brownian motions. Their Hs by bdSWV and lowPSDw,e (0.29 ± 0.006 and 0.25 ± 0.012, respectively) were independent of BP. Neither Dcorr nor Λmax varied with hypotension. Their values were characteristic of a chaotic system, but surrogate data analysis rendered some of them inconclusive. Hence, CBF fluctuations can be regarded as a robust phenomenon that is not abolished even by sustained hypotension at 40 mm Hg.