Noninvasive assessment of human microvascular function in health and disease using incident dark-field microscopy.

Reduced peripheral microvascular reactivity is associated with an increased risk for major adverse cardiac events (MACEs). Tools for noninvasive assessment of peripheral microvascular function are limited, and existing technology is poorly validated in both healthy populations and patients with cardiovascular disease (CVD). Here, we used a handheld incident dark-field imaging tool (CytoCam) to test the hypothesis that, compared with healthy individuals (no risk factors for CVD), subjects formally diagnosed with coronary artery disease (CAD) or those with ≥2 risk factors for CAD (at risk) would exhibit impaired peripheral microvascular reactivity. A total of 17 participants (11 healthy, 6 at risk) were included in this pilot study. CytoCam was used to measure sublingual microvascular total vessel density (TVD), perfused vessel density (PVD), and microvascular flow index (MFI) in response to the topical application of acetylcholine (ACh) and sublingual administration of nitroglycerin (NTG). Baseline MFI and PVD were significantly reduced in the at-risk cohort compared with healthy individuals. Surprisingly, following the application of acetylcholine and nitroglycerin, both groups showed a significant improvement in all three microvascular perfusion parameters. These results suggest that, despite baseline reductions in both microvascular density and perfusion, human in vivo peripheral microvascular reactivity to both endothelial-dependent and -independent vasoactive agents remains intact in individuals with CAD or multiple risk factors for disease.NEW & NOTEWORTHY To our knowledge, this is the first study to comprehensively characterize in vivo sublingual microvascular structure and function (endothelium-dependent and -independent) in healthy patients and those with CVD. Importantly, we used an easy-to-use handheld device that can be easily translated to clinical settings. Our results indicate that baseline microvascular impairments in structure and function can be detected using the CytoCam technology, although reactivity to acetylcholine may be maintained even during disease in the peripheral microcirculation.

Blood flow velocity comparison in the eye capillaries and postcapillary venules between normal pregnant and non-pregnant women.

BACKGROUND: There is no consensus on how much and at what diameters the blood flow velocity changes in the female microcirculation during normal pregnancy. METHODS: A non-contact, digital slit-lamp biomicroscopy system was used to measure axial blood velocity (Vax) and diameter (D) in the conjunctival microcirculation of 28 normal non-pregnant women (Control Group), 17 women in the first semester of their normal pregnancy (Group 1) and 16 women in the third trimester of their normal pregnancy (Group 2). Blood volume flow (Q) was estimated from Vax and D. Microvessels were classified as "capillaries" (CAP) with D < 9 µm, "postcapillary venules of size 1" (PC1) with 9 ≤ D < 14 µm and "postcapillary venules of size 2" (PC2) with 14 ≤ D ≤ 24 µm. RESULTS: The women groups did not differ significantly in age, diastolic and systolic pressure and diameter of each size. Taking as baseline the capillary Vax of 0.51 mm/s of the Control Group, there was a statistically significant (p < 0.001) increase to 0.74 mm/s (45%) in Group 1 and to 0.95 mm/s (86%) in Group 2. This significant Vax increase in capillaries (CAP) was a consistent finding irrespective of the exact vessel size cut-off value for discriminating CAP from PC1. There was no statistical difference in Vax among groups at postcapillary venules of size 2 (PC2). Statistical conclusions for blood volume flows were similar to velocities. CONCLUSIONS: Normal pregnancy increases significantly axial blood velocity (Vax) in capillaries (CAP) with diameter <9 µm.

Identification and quantification of human microcirculatory leukocytes using handheld video microscopes at the bedside.

Leukocyte recruitment and adhesion to the endothelium are hallmarks of systemic inflammation that manifest in a wide range of diseases. At present, no method is available to directly measure leukocyte kinetics at the bedside. In this study, we validate a new method to identify and quantify microcirculatory leukocytes observed by handheld vital microscopy (HVM) using space-time diagram (STD) analysis. Video clips ( n = 59) containing one capillary-postcapillary venule unit where leukocytes could be observed emanating from a capillary into a venule in cardiac surgery patients ( n = 20) were included. STD analysis and manual counting were used to quantify the number of leukocytes (total, rolling, and nonrolling). Pearson's correlation and Bland-Altman analysis were used to determine agreement between the STDs and manual counting. For reproducibility, intra- and interobserver coefficients of variation (CVs) were assessed. Leukocyte (rolling and nonrolling) and red blood cell velocities were assessed. The STDs and manual counting procedures for the quantification of rolling leukocytes showed good agreement ( r = 0.8197, P < 0.0001), with a Bland-Altman analysis mean difference of -0.0 (-6.56; 6.56). The overall intraobserver CV for the STD method was 1.5%. The overall interobserver CVs for the STD and the manual method were 5.6% and 9.4%, respectively. The nonrolling velocity was significantly higher than the rolling velocity (812 ± 519 µm/s vs. 201 ± 149 µm/s, P = 0.001). STD results agreed with the manual counting procedure results, had a better reproducibility, and could assess the leukocyte velocity. STD analysis using bedside HVM imaging presented a new methodology for quantifying leukocyte kinetics and functions in the microcirculation. NEW & NOTEWORTHY In this study, we introduce space-time diagram analysis of sublingual microcirculation imaging using handheld vital microscopy to identify and quantify the presence and kinetics of human microcirculatory leukocytes. We validated the methodology by choosing anatomical units consisting of a capillary connected to a venule, which allowed precise identification of leukocytes.

Wall shear stress in the human eye microcirculation in vivo, segmental heterogeneity and performance of in vitro cerebrovascular models.

Wall shear stress (WSS) is a very important hemodynamic parameter implicated in many physiological and pathological phenomena. In order to study these phenomena, it is more convenient to use in vitro models before testing on animals and humans. Dynamic in vitro cerebrovascular models are considered capable of simulating the in vivo hemodynamic conditions, but only few of them seem to meet the criteria for this task. It is now clear that in the human eye microcirculation a significant pulsation exists at the pre-capillary arterioles with average WSS values more than twice those in the venular side, for the same diameters. WSS heterogeneity is in support of segmental heterogeneity i.e. the endothelial phenotypic and functional difference among arterioles, capillaries and venules. In this review paper, the importance of WSS is described in detail and two more microvascular segments are proposed: a pre-capillary arteriolar and a post-capillary venular segment. The accurate hemodynamic simulation in all microvascular segments seems to be a prerequisite step in the development of dynamic in vitro blood brain barrier (BBB) models and microfluidic platforms on chip. Endothelial cells in the cardiovascular system seem to have sophisticated role acting like cardiovascular processing sensors (CPSs).