PATHOPHYSIOLOGY OF SIRS
A mild systemic inflammatory response to an
injury, infection, or other bodily insult may nor-mally have salutatory
effects. However, a marked or prolonged response, such as that associated with
severe infections, is often deleterious and can result in widespread organ
dysfunction. Although gram-negative organisms account for most cases of
infection-related SIRS, many other infectious agents are capable of inducing
the same syndrome. These organisms either elaborate toxins or stimu-late
release of substances that trigger this response. The most commonly recognized
initiators are the lipopolysaccharides, which are released by gram-negative
bacteria. Lipopolysaccharide is composed of an O polysaccharide, a core, and
lipid A. The O polysaccharide distinguishes between different types of
gram-negative bacteria, whereas lipid A, an endo-toxin, is responsible for the
compound’s toxicity. The resulting response to endotoxin involves a complex
interaction between macrophages/monocytes, neu-trophils, lymphocytes,
platelets, and endothelial cells that can affect nearly every organ.
The central mechanism in initiating SIRS appears to be the abnormal secretion of cyto-kines. These low-molecular-weight peptides and glycoproteins function as intercellular mediators regulating such biological processes as local and systemic immune responses, inflammation, wound healing, and hematopoiesis. The most important cytokines released during SIRS are interleukin-6, adrenomedullin, soluble CD 14, the adhesion molecule sELAM-1, macrophage inflammatory protein-1α, extracellular phospholipase A 2, and C-reactive pro-tein. The resulting inflammatory response includes release of potentially harmful phospholipids, attrac-tion of neutrophils, and activation of the comple-ment, kinin, and coagulation cascades.
Increased phospholipase A2 levels release ara-chidonic acid from cell
membrane phospholip-ids. Cyclooxygenase converts arachidonic acid to
thromboxane and prostaglandins, whereas lipoxy-genase converts arachidonic acid
into leukotrienes (slow-reacting substances of anaphylaxis). Increased
phospholipase A 2 and acetyltransferase activities result in the formation of another
potent proinflam-matory compound, platelet-activating factor. Attrac-tion and
activation of neutrophils releases a variety of proteases and free radical
compounds that dam-age vascular endothelium. Activation of monocytes causes
them to express increased amounts of tissue factor, which in turn can activate
both the intrinsic and extrinsic coagulation cascades.
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