Answer:USMLE step 2 Mcq 66:Heparin is a commonly used anti-coagulant agent


Correct Answer: B
Heparin is a commonly used anti-coagulant agent and is specifically indicated in acute pulmonary emboli. Heparin acts indirectly at multiple sites in both the intrinsic and extrinsic blood clotting systems to potentiate the inhibitory action of antithrombin III.
Patients with acute pulmonary emboli require larger doses during the first few days due to increased hepatic clearance. Pulmonary emboli cause the obstruction of intravascular blood flow due to which the blood flow gets slowed and the blood spends a longer time in the hepatic circulation. This causes increased breakdown of heparin thus decreasing its half life. Due to this larger doses of heparin are required when the therapy with heparin is started.'A' is incorrect and pulmonary emboli are not more resistant to thrombolysis.Larger doses are not required to prevent recurrence. Maintenance doses of 10-15 units/kg/hr are adequate to prevent the recurrence.Pulmonary emboli maybe multiple in number but this does not require a larger dose.


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