The anatomy and physiology of the microcirculation in human skin are complex. Normal cutaneous microcirculation is organized in two parallel plexuses with capillary loops extending perpendicularly from the superficial plexus. The physiological regulation of cutaneous microcirculation includes specific sympathetic activation, which causes vasoconstriction through the release of norepinephrine, neuropeptide Y, and ATP. A sympathetic cholinergic system is mainly involved in vasodilation through the co-transmission of acetylcholine, vasoactive intestinal peptide, and pituitary adenylate cyclase-activating peptide. Sensory nerves play a major role through the release of calcitonin gene-related peptide and substance P. Endothelium-dependent vasomotion implicates nitric oxide, prostacyclin, endothelium-dependent hyperpolarizing factors, and endothelin. Myogenic response also plays a role and explains why autoregulation is weak but exists in glabrous human skin. Variations in skin blood flow result from highly complex interactions between these mechanisms. In this article, we will detail the anatomy, physiology, and current methods of exploring the human microcirculation. We will further discuss the part played by cutaneous microvascular impairment in the pathophysiology of cardiovascular and metabolic diseases, or diseases more specifically affecting the skin. © 2020 American Physiological Society. Compr Physiol 10:1105-1154, 2020.

Human Skin Microcirculation.

Low-frequency component of photoplethysmogram reflects the autonomic control of blood pressure.

Abstract Dynamic light scattering (DLS) is a well known experimental approach uniquely suited for the characterization of small particles undergoing Brownian motion in randomly inhomogeneous turbid scattering medium, including water suspension, polymers in solutions, cells cultures, and so on. DLS is based on the illuminating of turbid medium with a coherent laser light and further analyzes the intensity fluctuations caused by the motion of the scattering particles. The DLS‐based spin‐off derivative techniques, such laser Doppler flowmetry (LDF), diffusing wave spectroscopy (DWS), laser speckle contrast imaging (LSCI), and Doppler optical coherence tomography (DOCT), are exploited widely for non‐invasive imaging of blood flow in brain, skin, muscles, and other biological tissues. The recent advancements in the DLS‐based imaging technologies in frame of their application for brain blood flow monitoring, skin perfusion measurements, and non‐invasive blood micro‐circulation characterization are overviewed. The fundamentals, breakthrough potential, and practical findings revealed by DLS‐based blood flow imaging studies, including the limitations and challenges of the approach such as movement artifacts, non‐ergodicity, and overcoming high scattering properties of studied medium, are also discussed. It is concluded that continued research and further technological advancements in DLS‐based imaging will pave the way for new exciting developments and insights into blood flow diagnostic imaging. 

Advances in Dynamic Light Scattering Imaging of Blood Flow

OBJECTIVE This study reveals the changes in brain activation due to different game interaction states based on functional near-infrared spectroscopy signals and discusses their significance for stroke rehabilitation. APPROACH The oxygenated hemoglobin concentration (Delta [HbO2]) signals and the deoxygenated hemoglobin (Delta [HbR]) signals were recorded from the prefrontal cortex (PFC), the motor cortex (MC), the occipital lobe (OL) and the temporal lobe of 21 subjects (mean age: 24.6 ± 1.9 years old) in three game interaction states: physical, motion-sensing, and button-push training. The subjects were also asked to complete user-satisfaction survey scales after the experiment. MAIN RESULTS Compared with the button-training state, several channels in the PFC and MC region of the physical-training state were significantly altered as were several channels in the RMC region of the motion-sensing training state (P < 0.05 after adjustment). The motion-sensing state of the PFC had a significant correlation with that of the MC and the OL. The subjective scale results show that the acceptability of the physical and motion-sensing states was greater than the acceptability of the button-push training state. SIGNIFICANCE The results show that the brain regions responded more strongly when activated by the physical and motion-sensing states compared with the button-push training state, and the physical and motion-sensing states are more conducive to the rehabilitation of the nervous system. The design of rehabilitation products for stroke patients is discussed and valuable insights are offered to support the selection of better interactive training methods.

Effects of three different rehabilitation games' interaction on brain activation using functional near-infrared spectroscopy.

Difficulty balance in the game level design is particularly important for user experience (UX). This study uses cognitive control and flow theory as a starting point, and based on the functional near-infrared spectroscopy technology, this research obtained oxygenated hemoglobin concentration (ΔHbO2) signals from three brain areas in the prefrontal cortex (PFC), namely the dorsolateral prefrontal cortex (DLPFC), the ventral prefrontal cortex (VLPFC), and the frontal area (FPA). Combined with the subjective flow state scale to explore the reaction difference between the video game players (VGPs) and the non-video game players (NVGPs) during the entire period of playing video games and the moment of game difficulty changes. The results show that VGPs and NVGPs have significant differences between flow state and HbO2 signals in different brain regions of the PFC. The players’ flow state is affected by the game difficulty. The HbO2 signals in the players’ DLPFC and FPA have a linear dependence relation with the flow state. This research proves that VGPs and NVGPs have different UX with game difficulty changes from the cognitive perspective. The results provide a reference for the future game level design about game difficulty balance to achieve a better UX in the game. 

Research on user experience of the video game difficulty based on flow theory and fNIRS

Race-specific differences in the level of glycated hemoglobin are well known. However, these differences were detected by invasive measurement of mean oxygenation, and their understanding remains far from complete. Given that oxygen is delivered to the cells by hemoglobin through the cardiovascular system, a possible approach is to investigate the phase coherence between blood flow and oxygen transportation. Here we introduce a noninvasive optical method based on simultaneous recordings using NIRS, white light spectroscopy and LDF, combined with wavelet-based phase coherence analysis. Signals were recorded simultaneously for individuals in two groups of healthy subjects, 16 from Sub-Saharan Africa (BA group) and 16 Europeans (CA group). It was found that the power of myogenic oscillations in oxygenated and de-oxygenated hemoglobin is higher in the BA group, but that the phase coherence between blood flow and oxygen saturation, or blood flow and hemoglobin concentrations is higher in the CA group.

Race-specific differences in the phase coherence between blood flow and oxygenation: A simultaneous NIRS, white light spectroscopy and LDF study.

OBJECTIVE To assess the performance of laser Doppler flowmetry (LDF) in measuring blood perfusion from darkly-pigmented skin, i.e. skin with high melanin concentration. LDF provides for the noninvasive monitoring of microvascular blood flow dynamics. It has been used extensively on light-skinned subjects, i.e. on skin with low melanin concentration, in both the healthy and pathological states. Because the optical properties of human skin might affect the reliability of optically-based diagnostic equipment, the effectiveness of LDF needs to be checked and evaluated on dark skin, too, if this method is to be useful in global healthcare. APPROACH Thirteen dark-skinned subjects and ten light-skinned subjects were included in the study. Microvascular blood flow dynamics was measured on both the right and left ankles using LDF with a laser diode of wavelength 780 nm. The characteristics of time-varying blood flow oscillations were investigated by wavelet analysis, nonlinear mode decomposition and wavelet phase coherence. An electrocardiogram (ECG), skin temperature, and respiratory effort were measured simultaneously with the LDF for each subject. MAIN RESULTS No significant differences were observed between the groups in the mean blood perfusion (p   >  0.1), or wavelet power (p   >  0.6). The instantaneous heart rate (IHR), extracted from the LDF at each of the recording sites, and from the ECG, did not differ significantly between the groups (p   >  0.8). Nor did the wavelet power of the IHR differ (p   >  0.8) between the groups. The only significant difference found between the groups lay in left/right ankle blood flow coherence near the cardiac frequency, attributable to known ethnic physiological differences. SIGNIFICANCE These results indicate that high melanin concentrations in skin exert no significant influence on the ability of LDF to monitor microvascular blood flow dynamics when using a laser diode of wavelength 780 nm. Hence LDF can help in the diagnosis and exploration of the pathogenesis of diseases such as diabetes, hypertension, or malaria in darkly pigmented patients across sub-Saharan Africa.

https://iopscience.iop.org/article/10.1088/1361-6579/ab2651/pdf

On the suitability of laser-Doppler flowmetry for capturing microvascular blood flow dynamics from darkly pigmented skin.

We show how a non-autonomous dynamics approach using time-resolved analyses of power spectra and phase coherence can help in the noninvasive diagnosis of malaria. The work is based on studying oscillations in blood flow and the variability of the heart and respiratory frequencies. The model used assumes that the heart and respiration are two oscillatory pumps with variable frequencies and that the vascular resistance also changes in an oscillatory manner. Red blood cells circulating through the system deliver oxygen to each cell. Malaria changes the red blood cells so that this delivery is compromised. The oscillatory properties of both pumps are also affected. We quantify the latter and compare three groups of subjects: febrile malaria patients (37); non-febrile malaria patients (10); and healthy controls (51). For each subject, time series of skin blood flow, respiratory effort, cardiac activity (ECG) and skin temperature were recorded simultaneously over an interval of 30 minutes. The oscillatory components within the range 0.005–2 Hz were analysed and their degree of coordination throughout the cardiovascular system was assessed by wavelet phase coherence analysis. It is shown that malaria, either febrile or non-febrile, substantially reduces the coordination.

Phase Coherence Between Cardiovascular Oscillations in Malaria: The Basis for a Possible Diagnostic Test

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Race-specific differences in the phase coherence between blood flow and oxygenation: A simultaneous NIRS, white light spectroscopy and LDF study.

On the suitability of laser-Doppler flowmetry for capturing microvascular blood flow dynamics from darkly pigmented skin.

Phase Coherence Between Cardiovascular Oscillations in Malaria: The Basis for a Possible Diagnostic Test