11. Heparin

Mechanism of action: potentiates the action of antithrombin and thereby inactivates thrombin and prevents the conversion of fibrinogen to fibrin.

  • Activity of unfractionated heparin may be monitored by aPTT.
  • Low molecular weight heparin directly targets factor Xa without modulating antithrombin. Thus, low molecular weight heparin does not affect the aPTT. When necessary, monitoring may be done by anti-factor Xa levels.

Risk Factors for Bleeding on Heparin

  • Surgery, trauma, or stroke within the previous 14 days.
  • History of peptic ulcer disease, GI bleeding or GU bleeding.
  • Platelets <150K.
  • Age >70 years.
  • Hepatic failure, uremia, bleeding diathesis, brain metastases (especially hypervascular tumors such as renal cell, melanoma, thyroid).
  • Concomitant anti-thrombotic therapy (thrombolytics, anti-platelet therapy).
  • Recent or anticipated central or arterial catheterization.

LWMH

  • LMWH eliminates the need to monitor PTT and adjust dosages. You may send selected patients home with enoxaparin and warfarin, rather than keeping them in-house on heparin for 5 days or until they are therapeutic on warfarin (see Hematology/Oncology: DVT). Moreover, LMWH is associated with a lower incidence of heparin-associated thrombocytopenia. 
  • LMWH is preferable to oral anticoagulation with warfarin in patients with malignancy-associated venous thromboembolism.
  • Consider a hematology consult before using LMWH in the following situations:
    • Weight >150 kg.
    • Creatinine >2, or creatinine clearance <30 ml/min.
    • Pregnancy.

Key Points

  • Unfractionated heparin is preferable when rapid reversal of anticoagulation might be necessary (e.g., possible surgery).
  • For heparin sliding scale, see Appendix C: Sliding Scales.

Hirsh J, Warkentin TE, Shaughnessy SG, et al.  Heparin and low-molecular-weight heparin: mechanisms of action, pharmacokinetics, dosing, monitoring, efficacy, and safety.  Chest 2001;119:64S-94S.

Lee et al.  Low-molecular-weight heparin versus coumadin for the prevention of recurrent venous thrombolembolism in patients with cancer.  N Engl J Med 2003 Jul 10;349(2): 146-53.