Title : Hemodynamics in the Human Microcirculation
Abstract: Microcirculation is the largest part of the human cardiovascular system performing the primary function of this system which is chemical exchange. However, due to the small size of the microvessels and the difficulty of access, the hemodynamic status of the human microcirculation remained for a long time unknown. This situation changed with the advent of state of the art image acquisition and processing techniques which can be applied at appropriate “windows” to the microcirculation such as the conjunctival tissue of the eye. In this non-invasive and non-contact way, blood flow velocity was measured in the capillaries and post-capillary venules of physiological subjects and later, velocity pulsation was quantified in the pre-capillary arterioles using the Resistive Index (RI) and the Pulsatility Index (PI). Another important hemodynamic parameter implicating in many physiological and pathological phenomena is Wall Shear Stress (WSS) acting on the inner microvessel wall. WSS can be quantified in the human microcirculation using blood flow velocity measurements. After the successful “mapping” of the normal human conjunctival microvascular hemodynamics, there were a series of studies on systemic disorders and cardiology pathologies such as diabetes mellitus, hypertension, sickle cell disease, ischaemic stroke, cyanotic congenital heart disease and acute myocardial infarction that found statistically significant changes in conjunctival microcirculation. Significant changes were also found in normal pregnancy and in diabetic retinopathy patients after the administration of drugs. In addition, microvascular blood velocity was significantly correlated to albuminuria in sickle cell disease patients and to cardiovascular risk (Framingham risk score) showing good potential for prediction. There are also reports showing severe alterations of the microcirculation in patients with COVID-19. In conclusion, it seems there is a bright future of microvascular hemodynamics not only for basic research but also for clinical applications in diagnosis and prediction of various pathologies.