Mechanism of Action

Heparin is an indirect anticoagulant: it binds antithrombin III (AT-III) and accelerates AT-III's inactivation of thrombin (factor IIa) and factor Xa ~1,000-fold. Without adequate AT-III, heparin cannot work. Unfractionated heparin (UFH) inhibits BOTH thrombin and factor Xa; LMWHs (enoxaparin) preferentially inhibit factor Xa. Heparin prevents clot propagation — it does NOT dissolve existing clots (that is thrombolysis, e.g. alteplase). Therapeutic IV UFH is titrated to an aPTT of 1.5–2.5× the control value (commonly ≈46–70 sec).

Sub-therapeutic

Therapeutic

Supratherapeutic (bleeding risk)

1.5

2.5

3.5

× control aPTT

Common Medications

heparin (unfractionated)Prototype

IV or subcutaneous; short half-life (~60–90 min IV); aPTT-monitored

enoxaparin

LMWH; fixed-dose subcutaneous; no routine lab monitoring

dalteparin

LMWH

Indications

deep vein thrombosis treatment and prevention

pulmonary embolism

UFH preferred when rapid on/off control is needed

acute coronary syndrome

atrial fibrillation bridging to warfarin

cardiac bypass anticoagulation

UFH — rapid, reversible

IV line patency

low-dose heparin flushes

Side Effects

injection-site bruising

minor bleeding

gums, easy bruising

injection-site hematoma

worsened by aspirating or massaging the SC site

Contraindications & Interactions

Contraindications

active major bleeding

history of heparin-induced thrombocytopenia

severe thrombocytopenia

intramuscular injection

causes severe hematoma — heparin is never given IM

antithrombin III deficiency

heparin cannot work without AT-III

Interactions

antiplatelet agents

aspirin, clopidogrel — additive bleeding

NSAIDs

additive bleeding risk

thrombolytics

additive bleeding risk

Administration & Monitoring

give IV UFH on a programmable infusion pump

high-alert medication; prevents accidental bolus

verify concentration and rate against weight-based protocol

required safety check before initiating

obtain baseline aPTT

monitor aPTT every 6 hours during initiation1.5–2.5× control

until therapeutic, then per protocol

monitor PT/INR for warfarin bridging

PT/INR tracks warfarin — NOT heparin

monitor platelet countwatch for ≥50% drop

screen for HIT around day 5–10

keep protamine sulfate available

reverses UFH completely; only partially reverses LMWH

do not aspirate or massage SC injection sites

leave the enoxaparin air bubble in; inject ≥2 inches from umbilicus

Patient Teaching

report bleeding gums or nosebleeds

report black or tarry stools

sign of GI bleeding

report pink or red urine

report new leg swelling or pain

could signal HIT-related clotting

use a soft toothbrush and electric razor

inject enoxaparin into the abdomen

≥2 inches from umbilicus; keep the air bubble; do not rub the site

avoid aspirin and NSAIDs unless prescribed

Report Nowescalate immediately

heparin-induced thrombocytopenia Hallmarkplatelets < 100,000/mm³ or ≥50% drop

HIT, ~day 5–10; paradoxical CLOTTING (new DVT/limb swelling), not bleeding — stop heparin

major hemorrhage

GI bleed, intracranial bleed, hematuria, falling H&H; antidote = protamine sulfate

supratherapeutic aPTT> 2.5× control

high bleeding risk — hold infusion and notify provider

Clinical Pearl

Heparin is the bodyguard, not the assassin — it stops the clot from recruiting reinforcements while the body dissolves it. Monitor UFH with aPTT (1.5–2.5× control), reverse with protamine, and remember the cruel twist of HIT: the platelets fall but the patient CLOTS.

Unfractionated Heparin