Capillary plugging by granulocytes and the no-reflow phenomenon in the microcirculation

Granulocytes are large, stiff viscoelastic cells that adhere naturally to the vascular endothelium. On their passage through the capillary network they have to be deformed, and recent evidence indicates that they may impose a significant hemodynamic resistance. The entry time of granulocytes into capillaries is about three orders of magnitude longer than that for red cells. Inside the capillary the granulocytes move with a lower velocity than red cells. Under conditions when the capillary perfusion pressure is reduced and/or elevated levels of inflammatory products are present that increase the adhesion stress to the endothelium, granulocytes may become stuck in the capillary. In such a situation, the granulocytes form a large contact area with the capillary endothelium, they obstruct the lumen, and they may initiate tissue injury. After the restoration of the perfusion pressure the granulocytes may not be removed from the capillary owing to the adhesion to the endothelium. Capillary plugging by granulocytes appears to be the mechanism responsible for the no-reflow phenomenon, and together with oxygen free radical formation and lysosomal enzyme activity may constitute the origin for ischemic injury as well as other microvascular occlusive diseases.