Generic Name

Heparin

Mechanism

• Augments antithrombin III (AT‑III) activity: Heparin binds AT‑III, accelerating its inhibition of thrombin (factor IIa) and factor Xa.
• Inhibits clotting cascade: By blocking factor Xa and IIa, it prevents the conversion of fibrinogen to fibrin and inhibits platelet‑mediated thrombus formation.
• Reversible: Antithrombin‑heparin complex has a short half‑life; anticoagulation can be promptly reversed with protamine sulfate.

Pharmacokinetics

• Administration: IV or subcutaneous (SC).
• Absorption: SC bioavailability ~70 %; IV 100 %.
• Distribution: Bound ~30 % to plasma proteins; extracellular fluid volume usage.
• Metabolism: Penetrates into the extravascular space; cleared primarily by the liver and kidneys.
• Half‑life: 1.5–2 h (IV), 3–5 h (SC); depends on dose and route.
• Elimination: Renal excretion of heparin‑AT‑III complexes; hepato‑renal system handles non‑excreted fractions.

Indications

• Acute coronary syndromes (e.g., unstable angina, STEMI) – IV infusion.
• Peri‑operative anticoagulation (cardiac, vascular, orthopedic surgery).
• Deep vein thrombosis (DVT)/ pulmonary embolism (PE) – IV or SC.
• Continuous renal replacement therapy (CRRT) – anticoagulate circuit.
• Transit of blood products and septic shock frequently require prophylactic IV heparin.

Contraindications

• Absolute: known hypersensitivity to heparin or AT‑III; active bleeding or high bleeding risk.
• Relative: thrombocytopenia, severe renal or hepatic dysfunction, pregnancy (except for specific indications).
• Warning: Heparin‑induced thrombocytopenia (HIT) – monitor platelet counts; treat with alternate anticoagulants once HIT suspected.
• Drug interactions: Coumadin, vitamin K antagonists, DOACs, antiplatelet agents → increased bleeding risk.

Dosing

• Transient tachycardia or skin irritation may occur; adjust dose accordingly.
• Use *ultra‑filtration* preparations when potassium monitoring, as potassium@heparin can be falsely low.

Adverse Effects

• Common
• Bleeding (bruising, epistaxis, GI bleed).
• Thrombocytopenia (HIT).
• Skin irritation at SC site.
• Hyperkalemia (especially with SC products).
• Serious
• Severe or life‑threatening hemorrhage.
• HIT with microvascular thrombosis or organ infarction.
• Bruising in patients on anticoagulants.
• Hypersensitivity reactions (rash, anaphylaxis).

Monitoring

• Activated partial thromboplastin time (aPTT) – 1.5–2.5× baseline for IV dosing.
• Platelet count – every 4‑5 days during prolonged therapy; weekly if risk for HIT.
• Anti‑Xa activity – in patients with renal failure or obesity where aPTT may be unreliable.
• Coagulation profile (PT/INR) – if combined with vitamin K antagonists.
• Signs of bleeding – physical exam, hemoglobin/hematocrit levels.

Clinical Pearls

• Prophylaxis vs. Treatment: For surgical prophylaxis, SC dosing of 5 000 U bid is adequate; for treatment of thrombosis, weight‑based IV infusion is needed.
• Protamine Sulfate: 1 mg protamine reverses 1 000 U heparin; dose ceiling 5 mg for <2‑hour infusions, 10 mg for ≥2‑hour infusions.
• Renal Impairment: SC formulations are preferred; monitor anti‑Xa to avoid bleeding.
• HIT Work‑up: Use 4T score, serologic testing, and if positive, switch to argatroban or bivalirudin.
• Pregnancy: Heparin does not cross the placenta; safe when benefits outweigh risks.
• Record all doses meticulously; heparin's narrow therapeutic window and biotransformation variability necessitate strict documentation.

*This drug card offers a precise yet comprehensive snapshot of heparin for medical students and clinicians—ready for quick reference or deeper study.*