Relationship between nonlinear properties of sacral skin blood flow oscillations and vasodilatory function in people at risk for pressure ulcers

The purposes of this study were to quantify the nonlinear properties of sacral skin blood flow oscillations (BFO) and to explore their relationships with impaired vasodilatory function in people at risk for pressure ulcers. A total of 25 people with various levels of vasodilatory functions were studied, 10 people with normal vasodilatory function (Biphasic thermal index, BTI (5.5, 4.5, 10.1)), 10 people with slight impaired vasodilatory function (BTI (3.7, 3.2, 6.7)), and 5 people with severe impaired vasodilation (BTI (2.4, 1.7, 4.5)). A non-painful fast heating protocol was applied to the sacral region to induce biphasic vasodilation, axon reflex mediated and nitric oxide mediated. Biphasic thermal index is defined as ratios of first peak, nadir, and second peak to baseline blood flow. Laser Doppler flowmetry was used to record the BFO signals. Nonlinear properties of BFO were quantified based on self-similarity using Hurst exponent (HE) and detrended fluctuation analysis (DFA), regularity using sample entropy (SampEn), complexity using correlation dimension (CD), and chaotic behavior using largest Lyapunov exponent (LLE). The Wilcoxon signed rank tests were used to examine the differences between groups. Our results showed that local heating reduces the self-similarity and increases complexity of skin blood flow oscillations. Vasodilatory function has an inverse relationship with nonlinear properties in sacral skin baseline BFO. Nonlinear indexes, including HE, DFA, CD, and LLE, are appropriate tools to quantify nonlinear properties of BFO to study the microvascular dysfunction (p<0.05), and that SampEn may not be appropriate for this purpose (p>0.05). Our study supports the use of nonlinear indexes to predict the vasodilatory function, which can complement current analysis of blood flow control mechanisms using spectral (wavelet) analysis.