Abstract
The tissue fraction of red blood cells (RBCs) and their oxygenation and speed-resolved perfusion are estimated in absolute units by combining diffuse reflectance spectroscopy (DRS) and laser Doppler flowmetry (LDF). The DRS spectra (450 to 850 nm) are assessed at two source-detector separations (0.4 and 1.2 mm), allowing for a relative calibration routine, whereas LDF spectra are assessed at 1.2 mm in the same fiber-optic probe. Data are analyzed using nonlinear optimization in an inverse Monte Carlo technique by applying an adaptive multilayered tissue model based on geometrical, scattering, and absorbing properties, as well as RBC flow-speed information. Simulations of 250 tissue-like models including up to 2000 individual blood vessels were used to evaluate the method. The absolute root mean square (RMS) deviation between estimated and true oxygenation was 4.1 percentage units, whereas the relative RMS deviations for the RBC tissue fraction and perfusion were 19% and 23%, respectively. Examples of in vivo measurements on forearm and foot during common provocations are presented. The method offers several advantages such as simultaneous quantification of RBC tissue fraction and oxygenation and perfusion from the same, predictable, sampling volume. The perfusion estimate is speed resolved, absolute (% RBC×mm/s), and more accurate due to the combination with DRS.
Highlights
The microcirculation involves the smallest vessels in the tissue: the arterioles, the capillaries, and the venules
This article describes a method for measuring blood flow and oxygenation parameters in the microcirculation by integrating diffuse reflectance spectroscopy (DRS) and laser Doppler flowmetry (LDF)
The root mean square (RMS) deviation between estimated and true red blood cells (RBCs) tissue fraction 1⁄2hðRBCest − RBCtrueÞ2i1∕2 in the actual sampling volume in the evaluation simulations was 0.099 percentage units, which should be compared with an average RBC tissue (a)
Summary
The microcirculation involves the smallest vessels in the tissue: the arterioles, the capillaries, and the venules. These vessels are arranged into a refined microvascular network with a primary function of delivering oxygen to the surrounding tissue. This function depends strongly on the oxygen gradients in the microcirculation, a property that is governed by the balance between blood flow and tissue oxygen demand.[1] In tissues such as the brain, with a high-metabolic oxygen demand that is balanced by a high-blood flow, the oxygen transport to tissue mainly occurs in the capillaries. This article describes a method for measuring blood flow and oxygenation parameters in the microcirculation by integrating diffuse reflectance spectroscopy (DRS) and laser Doppler flowmetry (LDF)
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