الفهرس 
PHYSIIOLOGY OF PLATELETSOnly 14 pages are availabe for public view
 |  | from | 101 |  |  |
| | | from | 101 | | |
Abstract
Platelet are cell fragments that function in the clotting system. Platelets half-life is from 7 to 10 days. Platelet count is normally 140,000 to 440,000/L. Platelets arise from hematopoietic cells which differentiate into megakaryocytes through exposure to thrombopoietin (Tpo). Megakaryocyte maturation involves a process of endomitosis, nuclear duplication without cell division. The fragmentation of megakaryocyte cytoplasm into individual platelets results from shear force of circulating blood.
The platelet’s major function is to seal openings in the vascular tree. The initiation signal for platelet deposition and activation is exposure of underlying protions of the blood vessel wall that normally are concealed from circulating platelets by an intact endothelial lining.
A whole-blood platelet count is the first test that should be performed in any patient suspected of having a platelet disorder. For more than a century, physicians have screened for defects in primary hemostasis using the bleeding time test. Although physiologically relevant, the bleeding time is nonspecific. Other tests to evaluate platelet functions are PFA-100, Platelet Aggregometry, Platelet Secretion Assays and Platelet Flow Cytometry.
Critically ill patients often present with thrombocytopenia. The incidence of thrombocytopenia (platelet count <150 x 109/L) in critically ill medical patients is 35 to 44%. Causes of thrombocytopenia in ICU patients include disseminated intravascular coagulation, heparin-induced thrombocytopenia, hemophagocytic syndrome, sepsis, thrombotic thrombocytopenia purpura, liver disease, drug-induced thrombocytopenia, HELLP syndrome, catastrophic antiphospholipid antibody syndrome, post-transfusion purpura and pseudo-thrombocytopenia.
Drugs and many systemic diseases can lead to acquired platelet function defects. Causes of platelet dysfunction in ICU patients include cardiac bypass, liver disease, drug-induced platelet dysfunction and uremia.
Management of platelet disorders in ICU patients in addition to the specific management of underlying disease may include platelet transfusion.
Platelet products are either prepared from whole blood donations (platelet concentrates), or are collected by aphaeresis. One platelet concentrate should increase the 1-hour post-transfusion platelet count by 7000 to 10,000 platelets/L in a 75-kg man.
Platelet counts of at least 50,000/L are generally considered safe for general surgery. However, lower counts are accepted by some. Platelet counts of70,000 to 100,000/L are considered appropriate for neurosurgery.