Heparin

Dose requirement is variable to produce an activated partial thromboplastin time of 1.5 to 3 times control. This usually requires 500 to 2000 IU/h intravenously with an initial loading dose of 3000 to 5000 IU. In treatment of deep vein thrombosis, intermittent subcutaneous injections may be used (250 IU/kg every 12 h).

Heparin may be monitored by global tests or heparin assays, depending on the dosage and type given. Global tests for heparin include the whole-blood clotting time, activated coagulation time, and activated partial thromboplastin time. In patients not receiving heparin, a typical activated coagulation time might be 100 to 140 s; during cardiopulmonary bypass the safe range of heparin anticoagulation, as monitored by the activated coagulation time, is 400 to 500 s. In hemodialysis or hemofiltration an activated coagulation time of 180 to 200 s is usually acceptable.

However, other drugs or coexisting coagulation defects may interfere with coagulation monitoring. For instance, aprotinin inhibits contact activation and therefore can prolong the whole-blood clotting time, activated coagulation time, and activated partial thromboplastin time, thus giving an overestimation of the heparin level. In these cases heparin assays are required, the most specific of which are the anti-Xa assays.

The thrombin clotting time may be used for heparin control, and heparin levels may be titrated using various doses of protamine sulfate to find the dose that neutralizes heparin and normalizes the thrombin time. Typically, a thrombin clotting time of 1.5 times to twice control is acceptable.

For prevention of thromboembolism heparin is usually used subcutaneously in a dose of 5000 IU every 12 h. Control by monitoring coagulation tests is not necessary in these cases.

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