Sample entropy of laser Doppler flowmetry signals increases in patients with systemic sclerosis.

Associated to reactivity tests, laser Doppler flowmetry (LDF) emphasizes abnormal skin microvascular function in diseases affecting digits, such as Raynaud's phenomenon (RP) and systemic sclerosis (SSc). However, baseline perfusion value does not discriminate between disease states. We study if LDF sample entropy (SampEn) allows distinguishing healthy subjects, RP and SSc patients. LDF measurements were performed on finger pad and forearm of 108 subjects (27 controls, 28 RP patients, 53 SSc patients), before and after local thermal hyperemia. We also assessed the reproducibility of SampEn [expressed as within-subject coefficients of variation (CV) and intra-class correlation coefficients (ICC)]. Baseline SampEn is significantly increased in patients with SSc compared to RP and controls on finger pad [0.49 (0.19), 0.38 (0.14) and 0.36 (0.15), respectively; P<0.002], but not on forearm. However, local thermal hyperemia increased SampEn at all sites and for all groups. Finally, reproducibility of SampEn computed on two baseline segments was acceptable (CV=26%, ICC=0.63). SampEn of skin blood flow at rest is increased on finger pad of patients with SSc but not on forearm. This is consistent with the pathophysiology of the disease, which predominantly affects digital microcirculation in most patients. SampEn of LDF signal could be a reproducible tool to predict digital microvascular impairment.