Spectral analysis of laser speckle contrast imaging and infrared thermography to assess skin microvascular reactive hyperemia.

BACKGROUND: Post-occlusive reactive hyperemia (PORH) test with signal spectral analysis coupled provides potential indicators for the assessment of microvascular functions. OBJECTIVE: The objective of this study is to investigate the variations of skin blood flow and temperature spectra in the PORH test. Furthermore, to quantify the oscillation amplitude response to occlusion within different frequency ranges. MATERIALS AND METHODS: Ten healthy volunteers participated in the PORH test and their hand skin temperature and blood flow images were captured by infrared thermography (IRT) and laser speckle contrast imaging (LSCI) system, respectively. Extracted signals from selected areas were then transformed into the time-frequency space by continuous wavelet transform for cross-correlation analysis and oscillation amplitude response comparisons. RESULTS: The LSCI and IRT signals extracted from fingertips showed stronger hyperemia response and larger oscillation amplitude compared with other areas, and their spectral cross-correlations decreased with frequency. According to statistical analysis, their oscillation amplitudes in the PORH stage were obviously larger than the baseline stage within endothelial, neurogenic, and myogenic frequency ranges (p < 0.05), and their quantitative indicators of oscillation amplitude response had high linear correlations within endothelial and neurogenic frequency ranges. CONCLUSION: Comparisons of IRT and LSCI techniques in recording the reaction to the PORH test were made in both temporal and spectral domains. The larger oscillation amplitudes suggested enhanced endothelial, neurogenic, and myogenic activities in the PORH test. We hope this study is also significant for investigations of response to the PORH test by other non-invasive techniques.

Heat transfer analysis of blood perfusion in diabetic rats using a genetic algorithm.

Diabetes is frequently associated with structural and functional impairment of the microcirculation. Blood perfusion is an important indicator of both physiological and pathological conditions of the microcirculation. Given that temperature is closely related to blood perfusion and is more easily measured, blood perfusion can be estimated from variations in skin temperature using an inverse method. The aim of this paper was to develop a thermal analysis method for estimation of blood perfusion and apply it in the assessment of skin blood perfusion in diabetic rats. First, diabetes was induced in the rat models of the experimental group. Skin temperature from the rats left hind paws was measured during a 10-min local heating period followed by a 15-min cooling period. A simple one-dimensional heat transfer model, including an arteriolar vessel node, was used to describe the skin heat transfer process. The blood perfusion of the arteriole was estimated by correlating the calculated skin temperature with known experimental temperatures using a genetic algorithm. The results indicated that the average blood perfusion in the control group was higher during local heating and decreased faster during the cooling period, showing dynamic responses to the thermal stimuli. In contrast, the blood perfusion of diabetic rats was reduced compared with that of the control rats during the heating phase and the rate of decrease in perfusion during the cooling stage was similarly reduced, implying a slower response to thermal stimulation in these rats. It is interesting to note that diabetic rats fed a normal diet showed a similar blood perfusion pattern to that in the control rats, implying that diet may be important in the treatment of diabetes-associated microvascular dysfunction.

Computational analysis of nitric oxide biotransport in a microvessel influenced by red blood cells.

It is pivotal that endothelium-dependent Nitric Oxide (NO) consumed by hemoglobin (Hb) inside red blood cells (RBCs) membrane, regulates the vascular tone. The whole processes of NO transport in vessel containing flowing RBCs is still not clear, such as NO production in endothelium, diffusion in plasma and consumption inside RBCs. In this work, the motion of RBCs in a microvessel is investigated by using immersed boundary lattice Boltzmann method (IB-LBM) first and the deformability of RBCs is expressed by using spring network model which is based on the minimum energy principle. Furthermore, the interaction between RBCs is considered. Based on the wall shear stress (WSS), NO production rate originated from endothelium was obtained by using a hyperbolic model. NO distribution inside the microvessel with multiple RBCs was computed by using immersed boundary finite difference method (IB-FDM). The result shows that a large (small) WSS exists at locations with a relatively wide(narrow) gap between the wall and cell. In terms of mass transfer, an increase of RBC membrane permeability leads to a decrease of NO concentration in the vessel and the surrounding endothelium significantly. In addition, with the increasing of hematocrit (Hct) value, NO concentration distribution in the whole vessel decreases both in the lumen and vascular wall. Finally, the thickness of RBCs-depleted layer gradually decreases with the weakened deformability of RBCs membrane, and the change degree of cell free layer (CFL) thickness decreases as the bending stiffness is relatively higher. Thus, when bending stiffness is higher, the NO concentration in vascular wall is reduced resulting from the thinner CFL.

Numerical modeling of fluid and oxygen exchanges through microcirculation for the assessment of microcirculation alterations caused by type 2 diabetes.

Type 2 diabetes mellitus (DM2) is frequently accompanied by microcirculation complications, including structural and functional alterations, which may have serious effects on substance exchanges between blood and interstitial tissue and the health of organs. In this paper, we aim to study the influence of microcirculation alterations in DM2 patients on fluid and oxygen exchanges through a model analysis. A fluid flow and oxygen transport model were developed by considering the interplay between blood in capillary network and interstitial tissue. The two regions were separately represented by 1D network model and 3D volume model, and the immersed boundary method (IBM) was adopted to solve fluid and mass transfer between these two regions. By using the model, the steady flow field and the distributions of oxygen in capillary network and surrounding tissue were firstly simulated. In the interstitial volume, fluid pressure and oxygen tension decreased with the increase of distance from the network; in the network, oxygen tension in blood plasma dropped from 100 mm Hg at the entrance to about 40 mm Hg at the exit. We further tested several structural and functional disorders related to diabetic pathological conditions. Simulated results show that the impaired connectivity of the network could result in poor robustness in maintaining blood flow and perfused surface; under high fluid permeability conditions of capillary walls, the pressure gradient was much larger around the capillary bed, and this alteration led to a saturation level of the interstitial pressure when lymphatic flow drainage can't work effectively; the variations in network connectivity and permeability of capillary wall also had unfavorable influence on oxygen distributions in interstitial tissue. In addition, when the oxygen releasing capacity of hemoglobin was confined by glycosylated hemoglobin (HbA1) in the case of diabetes, the plasma could not be complemented with adequate oxygen and thus the hypoxic tissue range will be extended. This study illustrates that when microcirculation disturbances, including the structure of capillary network, the wall osmosis property and the capacity of blood binding oxygen occur in DM2, some negative impacts are raised on microvascular hemodynamics and metabolism circumstance of interstitial tissue.