The wavelet transform-based correlation analysis has been used to study skin temperature fluctuations caused by periodic changes in blood flow resulting from oscillations in vasomotor smooth muscle tone. We considered two cases, one in which temperature measurements and blood flow recordings by laser Doppler flowmetry are made simultaneously and another in which two temperature signals are measured concurrently. Twelve healthy subjects participated in our study. The gapped wavelet technique was used to suppress artifacts caused by boundary effects. Simultaneous recordings of skin temperature fluctuations and the signal of the laser Doppler flowmeter provided correlation coefficients essentially exceeding the values obtained for noise signals within three spectral ranges corresponding to myogenic (0.05-0.14 Hz), neurogenic (0.02-0.05 Hz), and endothelial (0.0095-0.02 Hz) regulation mechanisms. Within the frequency range from 0.14 to 2 Hz the values of the correlation function are compatible with the values of noise correlations. The same results were obtained for two concurrently measured temperature signals. Reduction in the amplitude of temperature fluctuations and in the level of correlations with the frequency arises because the skin has the properties of a low-frequency filter. As temperature fluctuations propagate their amplitude decays as an exponential function of frequency. Hence small oscillations in the spectral range reflecting the influence of heartbeat and respiration cannot be distinguished from external thermal noise.

Photoplethysmography (PPG) and laser Doppler flowmetry (LDF) are two recognized optical techniques that can track low-frequency perfusion changes in microcirculation. The aim of this study was to determine, in healthy subjects, the correlation between the techniques for specific low-frequency bands previously defined for microcirculation. Twelve healthy male subjects (age range 18 to 50 years) were studied, with PPG and LDF signals recorded for 20 min from their right and left index (PPG) and middle (LDF) fingers. Wavelet analysis comprised dividing the low-frequency integral wavelet spectrum (IWS) into five established physiological bands relating to cardiac, respiratory, myogenic, neurogenic, and endothelial activities. The correlation between PPG and LDF was quantified using wavelet correlation analysis and Spearman correlation analysis of the median IWS amplitude. The median wavelet correlation between signals (right-left side average) was 0.45 (cardiac), 0.49 (respiratory), 0.86 (myogenic), 0.91 (neurogenic), and 0.91 (endothelial). The correlation of IWS amplitude values (right-left side average) was statistically significant for the cardiac (ρ = 0.64, p < 0.05) and endothelial (ρ = 0.62, p < 0.05) bands. This pilot study has shown good correlation between PPG and LDF for specific physiological frequency bands. In particular, the results suggest that PPG has the potential to be a low-cost replacement for LDF for endothelial activity assessments.

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The objective of this study is to explore changes in microvascular tone during a contralateral cold pressor test and to compare the results obtained in healthy subjects and in patients with impaired glucose tolerance (IGT) and type 2 diabetes. Low-amplitude fluctuations of skin temperature in the appropriate frequency ranges were used as a characteristic for the mechanism for vascular tone regulation. In total, 13 adults with type 2 diabetes aged 40-67 years and 18 adults with IGT aged 31-60 years participated in this pilot study. The control group included 12 healthy men and women aged 39-60 years. The response to the cold pressor test in patients with type 2 diabetes and with IGT differs essentially from that of healthy subjects in the endothelial frequency range. Endothelial dysfunction occurs in the preclinical stage of diabetes and manifests, in particular, as a disturbance of the endothelial part of vascular tone regulation.

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