Assessment Of Macro- And Microvascular Function And Reactivity

The arterial system consists of large conduit vessels that branch continuously into smaller and smaller vessels ending in arterioles and capillaries. The role of the larger (macro) arteries is to distribute flow to various organs and to provide compliance to minimize the load on the heart and to maximize pressure during diastole. The role of the smaller (micro) vessels is to control regional blood flow by varying their resistance in response to local demand. When all the cardiovascular sensors, control mechanisms, and actuators are working properly, the input impedance of the vascular system remains matched to the output impedance of the heart to maximize efficiency and energy transfer both at rest and during exercise or stress. When disease or aging alters part of the system, overall function is compromised and other parts adapt to maintain basic function but at reduced efficiency and reserve. Thus, when evaluating macrovascular versus microvascular function, reserve, and reactivity to isolate the primary culprit, one must recognize that the entire arterial system may be altered. There are a number of methods and techniques available for assessing vascular function and reactivity, the most accepted being flow-mediated dilation. In this method, microvessels in a limb are stimulated via ischemia during inflation of a blood pressure cuff and assessed by measuring flow or velocity during reactive hyperemia after cuff deflation. Macrovessels are stimulated during the reactive hyperemia and assessed by measuring the change in diameter induced by the increased flow. Other methodologies evaluate different aspects of macrovascular and microvascular function and reactivity, and macrovascular may not be separated from microvascular function unambiguously. Thus, a multifaceted approach may be necessary for a comprehensive assessment of peripheral vascular function.