Heparins: The Fast-Acting Blood Thinners
When a clot lodges in the lungs or a leg vein and every hour counts, doctors reach for a blood thinner that works in minutes, not days: heparin. It comes in two flavours — an old, powerful, IV drip that needs constant monitoring, and a modern injection that's so predictable you can send patients home with it. And it hides a strange, dangerous paradox: a clot-preventing drug that can, rarely, cause catastrophic clotting.
A pregnant woman develops a painful, swollen calf — a deep vein thrombosis, a clot that could break off and travel to her lungs. She needs a blood thinner immediately, but the usual oral one (warfarin) can harm her baby. So she's started on heparin, which works within minutes and, crucially, does NOT cross the placenta — safe for the fetus. This one case captures why heparin exists: it's fast, it's the anticoagulant of choice in pregnancy, and it comes in forms tailored to the situation.
How heparin works
Heparin supercharges the body's own clotting brake. The body has a natural clotting inhibitor called antithrombin. Heparin binds to it and dramatically boosts its activity, so it shuts down key clotting factors — especially thrombin (factor IIa) and factor Xa — stopping the clotting cascade in its tracks. Because it acts on factors already circulating in the blood, its effect is immediate, unlike warfarin (next article), which takes days. Heparin is used to treat and prevent venous clots (DVT and pulmonary embolism), in acute coronary syndromes, and to 'bridge' patients until slower oral anticoagulants take effect.
Two kinds: unfractionated vs low-molecular-weight
Unfractionated heparin (UFH) is the original — given as a continuous intravenous drip, it acts instantly and wears off fast, which is ideal when you may need to switch it off quickly (before surgery, or if bleeding). Its downside is that its effect is unpredictable, so it must be monitored with a blood test (the aPTT) and the dose constantly adjusted. Low-molecular-weight heparin (LMWH) — enoxaparin, dalteparin — is a refined version that mainly inhibits factor Xa. It's injected under the skin once or twice a day, has a predictable effect that needs no routine monitoring, and lasts longer — so it's preferred for most patients and can be used at home. A pure factor Xa inhibitor, fondaparinux, is a related option. UFH is cleared quickly and reversible; LMWH is cleared by the kidney (caution in kidney failure).
The dangerous paradox is heparin-induced thrombocytopenia (HIT). In some patients, the immune system reacts to heparin, forming antibodies that both destroy platelets (dropping the platelet count) AND activate the remaining ones — so, bizarrely, a drug given to PREVENT clotting triggers dangerous new clots. If the platelet count falls a few days into heparin, stop ALL heparin immediately and switch to a non-heparin anticoagulant. HIT is the classic trap: falling platelets on heparin means clotting risk, not just bleeding risk.
- Heparin boosts antithrombin → inhibits thrombin (IIa) and factor Xa; acts immediately.
- UFH: IV, monitored by aPTT, short-acting, reversible by protamine.
- LMWH (enoxaparin): SC, predictable, no routine monitoring, renally cleared; preferred for most VTE.
- Heparin is safe in pregnancy (doesn't cross the placenta), unlike warfarin.
- Watch for HIT — falling platelets on heparin means clotting risk; stop all heparin.
- Continuing heparin when platelets fall (possible HIT). Stop it and use a non-heparin agent.
- Monitoring LMWH with an aPTT. It has a predictable effect and isn't routinely monitored that way.
- Using warfarin instead of heparin in pregnancy. Warfarin is teratogenic; heparin is safe.
- Full-dose LMWH in severe kidney failure. It accumulates — UFH may be safer.
A patient's platelet count drops several days into heparin therapy. What should you do?
- Heparin boosts antithrombin to inhibit thrombin & factor Xa — immediate anticoagulation.
- UFH: IV, aPTT-monitored, reversible by protamine; LMWH: SC, predictable, renally cleared.
- Heparin is the anticoagulant of choice in pregnancy (doesn't cross the placenta).
- HIT: antibodies drop platelets AND cause clots — stop all heparin immediately.
- Katzung BG. Basic & Clinical Pharmacology — Drugs Used in Disorders of Coagulation (heparins).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Anticoagulant drugs: heparin.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Anticoagulants.
- ACCP / clinical guidelines — Heparin, LMWH & heparin-induced thrombocytopenia.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Anticoagulants (heparin).
