The availability of handheld, noninvasive sublingual video-microscopes allows for visualization of the microcirculation in critically ill patients. Recent studies demonstrate that reduced numbers of blood-perfused microvessels and increased penetration of erythrocytes into the endothelial glycocalyx are essential components of microvascular dysfunction. The aim of this study was to identify novel microvascular variables to determine the level of microvascular dysfunction in sepsis and its relationship with clinical variables.
This observational, prospective, cross-sectional study included 51 participants, of which 34 critically ill sepsis patients were recruited from intensive care units of a university hospital. Seventeen healthy volunteers served as controls. All participants underwent sublingual videomicroscopy by sidestream darkfield imaging. A new developed version of the Glycocheck™ software was used to quantify vascular density, perfused boundary region (PBR-an inverse variable of endothelial glycocalyx dimensions), red blood cell (RBC) velocity, RBC content, and blood flow in sublingual microvessels with diameters between 4 and 25 µm.
A detailed analysis of adjacent diameter classes (1 µm each) of vessels between 4 and 25 µm revealed a severe reduction of vascular density in very small capillaries (5–7 µm), which correlated with markers of sepsis severity. Analysis of RBC velocity (VRBC) revealed a strong dependency between capillary and feed vessel VRBC in sepsis patients (R2 = 0.63, p < 0.0001) but not in healthy controls (R2 = 0.04, p = 0.43), indicating impaired capillary (de-)recruitment in sepsis. This finding enabled the calculation of capillary recruitment and dynamic capillary blood volume (CBVdynamic). Moreover, adjustment of PBR to feed vessel VRBC further improved discrimination between sepsis patients and controls by about 50%. By combining these dynamic microvascular and glycocalyx variables, we developed the microvascular health score (MVHSdynamic™), which decreased from 7.4 [4.6–8.7] in controls to 1.8 [1.4–2.7] in sepsis patients (p < 0.0001) and correlated with sepsis severity.
We introduce new important diameter-specific quantification and differentiated analysis of RBC kinetics, a key to understand microvascular dysfunction in sepsis. MVHSdynamic, which has a broad bandwidth to detect microvascular (dys-) function, might serve as a valuable tool to detect microvascular impairment in critically ill patients.
Alterations of microvascular perfusion caused by endothelial cell dysfunction, glycocalyx degradation, increased leukocyte adhesion, microthrombus formation, and regional redistribution of blood flow contribute to organ failure in critically ill patients [1,2,3]. The use of handheld, noninvasive sublingual video microscopes, such as incident dark field (IDF) or sidestream dark field (SDF) microscopy, allows researchers to visualize and analyze microvascular perfusion alterations in vivo [4, 5]. Over the past decade, several measures of microvascular perfusion variables have been proposed to improve risk stratification, prognostication and eventually to individualize the therapy, especially in critically ill patients with sepsis and septic shock [6,7,8,9,10,11,12,13]. Despite tremendous efforts to improve video quality and standardize reporting, the quantitative analysis of functional measures remains challenging and is often performed manually . Several visual scoring systems and specialized (semi-)automated analysis tools have been developed to shorten time-to-result and improve accuracy [14,15,16,17]. Common to all these approaches is the (automated) selection and pooled analysis of microvessels with a diameter < 20 (rarely < 10) µm. An improved spatiotemporal resolution within this < 20 µm range is desirable to further dissect and understand functional microvascular alterations in sepsis and critical illness.
We have previously used the Glycocheck™ System to analyze the perfused boundary region (PBR), an inverse variable of endothelial glycocalyx (eGC) dimensions, in sublingual microvessels from sepsis patients [18,19,20]. The software automatically detects, records, and analyzes the PBR according to diameter classes (each 1 µm) in microvessels with diameters between 4 and 25 μm. The methodology showed very good inter- and intra-observer reproducibility under real-life conditions  and excellent accuracy compared to other in vitro and ex vivo estimations of eGC thickness [18, 20, 21]. The aim of this study was to identify novel dynamic microvascular variables generated by a new diameter-class-wise approach to determine the level of microvascular dysfunction in sepsis and its relationship with established clinical variables and the number of dysfunctional organs.