A large number of vascular abnormalities have been described in patients with septic shock. Many vascular beds are dilated, but some are constricted, resulting in the maldistribution of blood flow. The aggregation of neutrophils and platelets may reduce blood flow; neutrophil margination occurs along the vascular endothelium, resulting in the release of many mediators and the migration of neutrophils into tissues23. Neutrophils can release active oxygen species, such as superoxide radicals, that can directly damage cells. Many of these neutrophil and endothelial-cell interactions are mediated by selectins and their receptors on endothelial surfaces24. Components of the complement system such as C5a are activated25; they can attract neutrophils and result in the release of locally active agents such as leukotrienes.
Inflammatory mediators derived from arachidonic acid -- prostaglandins and leukotrienes -- are released from multiple types of cells and can cause local vasoconstriction or vasodilatation and the accumulation of inflammatory cells. Another important local mechanism of vascular control is endothelium-derived relaxing factor, or nitric oxide, a potent endogenous vasodilator in the microcirculation26,27. Cytokines also have effects on the microvasculature. Tumor necrosis factor dilates vascular smooth muscle, an action that is not affected by the inhibition of cyclooxygenase but is reduced by the removal of endothelial cells,28 suggesting that tumor necrosis factor-induced vasodilatation is partially dependent on the formation of nitric oxide by the endothelium. Thus, a large number of mediators and inflammatory cells interact with endothelial and vascular smooth-muscle cells to interfere with blood flow and ultimately lead to microvascular failure. A central question in the pathogenesis of sepsis is whether decreased perfusion due to microvascular dysregulation is a primary cause or only an associated event in sepsis-induced organ failure.