The microcirculatory system is situated at the end of the vascular network and consists of a myriad of micro-vessels measuring less than 20-microns in diameter including capillaries, arterioles, post-capillary venules, and other cellular constituents. The smallest capillaries measure approximately 6–8 µm1. Being at the terminal end of the cardiovascular system, the microvasculature plays a major role in the transfer of oxygen from red blood cells in the capillaries to the local parenchymal cells as well as the collection of cellular waste.
Furthermore, microcirculation is responsible for the regulation of the concentration of solutes, nutrients, and hormones in fluid transferring between intravascular and tissue cells. The microcirculatory movement of fluid and cells is an essential part of the cardiovascular system2.
Cardiovascular disease (CVD) is known as the top cause of death over the world, and an estimated 18 million people die from CVD every year. Microcirculatory dysfunction has been shown to appear during earlier stages of CVD, and therefore would prove to be a crucial factor to monitor in patients who are at risk. To clarify, microcirculatory health can serve as a method for medical practitioners to assess the progression of CVD in an individual and their risk of experiencing more severe cardiovascular dysfunction.
Studies have centered on imaging and evaluating three primary regions of microvasculature: conjunctiva vessels3, lingual vessels, and nailfold vessels. Several studies have reported that remodeling of the microvasculature is correlated with CVD and related health issues such as hypertension and diabetes. Multiple assessments of microcirculation in the 3 different regions have affirmed this correlation.
Relevant microcirculation parameters and characteristics
In one example of microcirculation study on the eye, a slit-lamp biomicroscopy and a specialized retinal function imager were used to study blood flow in the conjunctiva and the retina9. Quantitative parameters such as diameter (D), blood volume flow (Q), wall shear rate/stress (WSR/WSS), and flow rate (Va) would be evaluated using a non-invasive slit-lamp biomicroscope. These parameters can be quantitatively measured by analyzing a short recording of capillary vessels10.
Other assessments, particularly for microvasculature in the nailfold and tongue, have been devised to quantify microcirculation characteristics such as blood vessel density, capillary perfusion, and heterogeneity11.
By compiling these existing metrics for evaluating microcirculatory health, it is possible to quantitatively describe the cardiovascular health and the progression of CVD in an individual.