Heparin and protamine

Anticoagulation during bypass is achieved with heparin which is reversed at the end by protamine. The anticoagulant effect is monitored using the activated clotting time, which is a modification of the whole blood clotting time, and protamine is titrated to return it to the pre-bypass level. The activated clotting time is also affected by low levels of coagulation factors and platelets and by hypothermia, so that protamine reversal of heparin is inexact. Heparin levels can also be used to monitor reversal. A prolonged activated clotting time soon after bypass should be treated with a further dose of protamine.

Uncommonly, heparin rebound can occur following initial correction with protamine; the activated clotting time becomes prolonged some 4 to 6 h later. This is thought to be caused by release of heparin sequestered in the tissues into the circulation and is more likely after large doses of heparin.

Protamine may have an anticoagulant effect in large doses (three to four times those needed to reverse the effects of heparin) and thus may be a rare cause of postoperative bleeding.

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