Vital signs reflect circulatory function and hence hemodynamics on a macroscopic scale and are often unreliable or late indicators of hemodynamic instability. Previous studies support that alterations in the microcirculation may provide early indicators of deterioration and impending shock. Microcirculation is also restored late in the recovery process. Hence, monitoring microcirculation is important since treatments based on normalizing classical vital signs will not always restore microvascular hemodynamics and the microcirculation may remain in shock although e.g., blood pressure seems normal.

The aim of this study was to investigate alterations in skin microcirculation dynamics during lower body negative pressure as a model of shock and central hypovolemia. By using spatial frequency domain imaging (SFDI) and polarized reflectance imaging, we investigated the association between micro- and macrovascular function during these conditions. Furthermore, we evaluated microvascular reactivity using the capillary refill test.

We present in vivo optical properties of skin tissue assessed using spatially resolved diffuse reflectance spectroscopy. Data was collected in a sub-study of the Swedish CArdioPulmonary bioImage Study (SCAPIS). Measurements were performed on 3,809 subjects, 50-65 years, on the volar side of the forearm with the PeriFlux 6000 EPOS system.

The analysis consisted of an inverse Monte Carlo method where modeled spectra were non-linearly fitted to measured diffuse reflectance spectra at 0.4 and 1.2-mm source-detector fiber separations, respectively, between 475 and 850 nm. The model consisted of one epidermis layer with adaptable thickness and absorption caused by melanin, and two dermis layers. The upper dermis had a fixed thickness of 0.2 mm and the lower an infinite thickness. The absorption in the dermis layers depended on varying amounts of hemoglobin and its oxygen saturation. The reduced scattering coefficient, with three adaptable parameters, was equal in all layers in the model.

The PeriFlux 6000 EPOS system combines diffuse reflectance spectroscopy (DRS) and laser Doppler flowmetry (LDF) for the assessment of oxygen saturation (expressed in percentage), red blood cell (RBC) tissue fraction (expressed as volume fraction, %RBC), and perfusion (%RBC  ×  mm  /  s) in the microcirculation. It also allows the possibility of separating the perfusion into three speed regions (0 to 1, 1 to 10, and >10  mm  /  s). We evaluate the speed-resolved perfusion components, i.e., the relative amount of perfusion within each speed region, using a blood-flow phantom. Human blood was pumped through microtubes with an inner diameter of 0.15 mm. Measured DRS and LDF spectra were compared to Monte Carlo-simulated spectra in an optimization routine, giving the best-fit parameters describing the measured spectra. The root-mean-square error for each of the three speed components (0 to 1, 1 to 10, and >10  mm  /  s, respectively) when describing the blood-flow speed in the microtubes was 2.9%, 8.1%, and 7.7%. The presented results show that the system can accurately discriminate blood perfusion originating from different blood-flow speeds, which may enable improved measurement of healthy and dysfunctional microcirculatory flow.