Unfractionated heparin is the anticoagulant you can shut off in an hour — and the one whose worst complication makes patients clot instead of bleed.
Unfractionated heparin (UFH) is an indirect anticoagulant: it binds antithrombin III (AT-III) and accelerates AT-III's inactivation of thrombin (factor IIa) and factor Xa roughly 1,000-fold. Without adequate AT-III, heparin cannot work. UFH prevents a clot from propagating — it does NOT dissolve an existing clot (that is thrombolysis, e.g. alteplase). Therapeutic UFH is given as a weight-based continuous IV infusion on a programmable pump (a high-alert medication), titrated to an aPTT of 1.5–2.5 times the control value, commonly about 46–70 seconds. The aPTT is drawn 6 hours after initiation and 6 hours after any rate change, because that is when steady state is reached. Low-dose subcutaneous UFH (5,000 units every 8–12 hours) is used for prophylaxis, and dilute heparin flushes maintain line patency. UFH's short half-life (~60–90 minutes IV) and complete reversibility with protamine sulfate (1 mg per 100 units of heparin) make it the choice whenever rapid on-off control is needed: cardiac bypass, an unstable patient who may go to surgery, or significant renal impairment, where LMWH accumulates. The most dangerous adverse effect is heparin-induced thrombocytopenia (HIT), a type II immune reaction in which antibodies against the heparin–platelet factor 4 complex activate platelets. It appears 5–10 days into exposure, drops the platelet count by 50% or more from baseline, and causes paradoxical thrombosis rather than bleeding. The nursing response is to stop ALL heparin — infusion, flushes, and heparin-coated lines — notify the provider, and anticipate a non-heparin anticoagulant such as argatroban or bivalirudin. Never transfuse platelets in HIT; they fuel the thrombosis. Baseline and serial platelet counts every 2–3 days are standard during heparin therapy.
Key Distinctions
Match the drug to its lab: aPTT monitors UFH, PT/INR monitors warfarin, and anti-Xa is the level used for LMWH — swapping these is a classic exam trap. Match the drug to its antidote: protamine sulfate reverses heparin, vitamin K reverses warfarin. Do not confuse anticoagulation with thrombolysis: heparin prevents clot extension, alteplase breaks clot down. And do not read HIT as a bleeding problem — the platelet count falls, but the patient clots, so the danger is a new DVT, PE, or limb ischemia, not hemorrhage. UFH differs from LMWH in every practical way: IV titratable versus fixed-dose subcutaneous, aPTT-monitored versus not routinely monitored, fully reversible versus partially reversible.
Clinical Pearl
Heparin is the bodyguard, not the assassin — it stops the clot from recruiting reinforcements while the body dissolves it. Monitor with aPTT (1.5–2.5× control), reverse with protamine, and remember the cruel twist of HIT: the platelets fall but the patient CLOTS.