Long COVID Mechanisms, Microvascular Effects, and Evaluation Based on Incidence.

Since the initial reports of Long COVID symptoms, numerous pathophysiological mechanisms have been proposed to explain them; nevertheless, no consensus has been reached. Some of these mechanisms are directly linked to microcirculation, while others are related indirectly. Those with a direct connection involve the respiratory system (such as pulmonary embolism), the cardiovascular system (including cardiac arrest, heart failure, myocardial inflammation, stroke, endothelial dysfunction, and microangiopathy), hematological conditions (like coagulopathy, deep vein thrombosis, microclots, and endothelial irregularities), and brain function. However, few of these mechanisms are grounded in quantitative data and fundamental physiological principles. Furthermore, diagnostic and therapeutic methods remain inadequate. This report provides a brief overview of these processes, focusing primarily on quantitative data, recently proposed mechanisms, and advances in microcirculation, with a special emphasis on the tissue blood supply reduction (TBSR or SR in short) mechanism. Then, the SR pathophysiological mechanism is assessed based on the total incidence rate of the Long COVID symptoms that can be directly attributed to this mechanism. The proposed SR mechanism can account for seven principal Long COVID symptoms with a total normalized incidence of 76%.