The role of capillaries in determining coronary blood flow reserve: Implications for stress-induced reversible perfusion defects

In summary, unlike tracers such as radiolabeled microspheres that are trapped in the microcirculation (and therefore have a 100% extraction fraction), the uptake of diffusible tracers is determined by the status of myocardial capillaries. When the coronary arterioles and venules are fully dilated, the ceiling of hyperemia CBF (or CBF reserve) is determined by the capillaries in the absence of coronary stenosis. When a coronary stenosis is present, its resistance is only partially responsible for the attenuation of the hyperemic response. During hyperemia, the capillaries derecruit distal to a stenosis in order to maintain a constant hydrostatic pressure, which is the main reason for the reduced CBF reserve seen in the presence of coronary stenosis. This derecruitment results in a decrease in capillary surface area, which causes reduced extraction of the isotope and the consequent perfusion defect. The degree of capillary derecruitment distal to the stenosis during hyperemia is proportional to stenosis severity. It is for this reason that the presence, extent, and magnitude of reversible perfusion defects have such strong prognostic value.