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OVERVIEW OF THROMBOSIS

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GENERAL OVERVIEW

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Thrombosis, the obstruction of blood flow due to the formation of clot, may result in tissue anoxia and damage, and it is a major cause of morbidity and mortality in a wide range of arterial and venous diseases and patient populations. In 2009 in the United States, an estimated 785,000 people had a new coronary thrombotic event, and about 470,000 had a recurrent ischemic episode. Each year, approximately 795,000 people have a new or recurrent stroke. It is estimated that 300,000–600,000 people each year have a pulmonary embolism or deep venous thrombotic event. In the nondiseased state, physiologic hemostasis reflects a delicate interplay between factors that promote and inhibit blood clotting, favoring the former. This response is crucial as it prevents uncontrolled hemorrhage and exsanguination following injury. In specific settings, the same processes that regulate normal hemostasis can cause pathologic thrombosis, leading to arterial or venous occlusion. Importantly, many commonly used therapeutic interventions may also alter the thrombotic–hemostatic balance adversely.

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Hemostasis and thrombosis primarily involve the interplay among three factors: the vessel wall, coagulation proteins, and platelets. Many prevalent acute vascular diseases are due to thrombus formation within a vessel, including myocardial infarction, thrombotic cerebrovascular events, and venous thrombosis. Although the end result is vessel occlusion and tissue ischemia, the pathophysiologic processes governing these pathologies have similarities as well as distinct differences. While many of the pathways regulating thrombus formation are similar to those that regulate hemostasis, the processes triggering thrombosis and, often, perpetuating the thrombus may be distinct and can vary in different clinical and genetic settings. In venous thrombosis, primary hypercoagulable states reflecting defects in the proteins governing coagulation and/or fibrinolysis or secondary hypercoagulable states involving abnormalities of blood vessels and blood flow or stasis lead to thrombosis. By contrast, arterial thrombosis is highly dependent on the state of the vessel wall, the platelet, and factors related to blood flow.

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ARTERIAL THROMBOSIS

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OVERVIEW OF ARTERIAL THROMBOSIS

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In arterial thrombosis, the platelets and abnormalities of the vessel wall typically play a key role in vessel occlusion. Arterial thrombus forms via a series of sequential steps in which platelets adhere to the vessel wall, additional platelets are recruited, and thrombin is activated (Fig. 22-1). The regulation of platelet adhesion, activation, aggregation, and recruitment will be described in detail below. In addition, while the primary function of platelets is regulation of hemostasis, our understanding of their role in other processes, such as immunity, wound healing, and inflammation, continues to grow.

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FIGURE 22-1

Platelet activation and thrombosis. Platelets circulate in an inactive form in the vasculature. Damage to the endothelium and/or external stimuli activates platelets that adhere to the exposed subendothelial von Willebrand factor and collagen. This adhesion leads to activation of the platelet, shape change, and the synthesis and release of thromboxane (TxA2), serotonin ...

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