Is a water soluble heterogenous mucopolysaccharide organic acid with molecular weight 3,000-30,000 daltons. It is endogenously present in mast cells in the liver and lung. Commercial preparations are from bovine and porcine sources.

Heparin acts by reversibly binding to antithrombin III (ATIII) accelerating the ability of ATIII to neutralize thrombin (IIa) and Xa. The heparin/ATIII complex also inactivates IXa, XIa, XIIa and plasmin but to lesser degrees.

Heparin is administered intravenously or subcutaneously. The duration of action depends on temperature, dose, liver and renal function. The average T1/2 is 1.2-2 h and after 4-6 h there is no therapeutic effect. Monitoring of effect occurs with PTT, ACT and heparin levels.

Platelet factor 4 (PF4) easily neutralizes the effects of heparin by binding in a 1:1 complex which is inactive and cleared. Blood products do not reverse heparin.

Common side effects include hemorrhage, allergic reactions, thrombocytope-nia, altered protein binding, hypotension, and a decrease in ATIII levels.

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