Mechanism of action
Standard unfractionated heparin is a heterogeneous mixture of fractions with a molecular weight ranging from 5,000 to 30,000 Da (average weight around 15,000 Da). The drug is synthesized by mast cells in the body and stored in the liver, lungs, and intestinal mucosa (industrially, it is extracted from porcine intestinal mucosa or bovine lung).
The pharmacological effect is based on the activation of antithrombin III. Standard heparin molecules have a sufficient chain length (about 40 monosaccharide units) to simultaneously bind antithrombin III and thrombin. As a result, standard heparin preparations inhibit thrombin (factor IIa) and factor Xa equally (activity ratio 1:1). Due to its macromolecular structure and high hydrophilicity, heparin has a very small volume of distribution ($V_d \approx 3$–3.6 L), residing predominantly within the vascular bed.
Pharmacological effects
In addition to its anticoagulant effect, heparin exhibits pronounced pleiotropic (non-anticoagulant) effects:
- Antiatherosclerotic action: stimulates the release of lipoprotein lipase, which hydrolyzes triglycerides within VLDL, and inhibits the proliferation of vascular smooth muscle cells.
- Anti-inflammatory action: inhibits hyaluronidase, reducing vascular wall permeability.
- Fibrinolytic activity: stimulates the release of tissue plasminogen activator ($tPA$) from the endothelium.
- Effect on primary hemostasis: may reduce platelet aggregation, which, combined with fibrinolysis, increases the risk of bleeding.
Indications and dosage
The drug is used in conditions requiring a rapid reduction in blood coagulability, for the prevention and treatment of thromboses.
- Parenteral administration: I.v., i.m., and s.c. Administered intravenously at 5,000–10,000 IU every 4–6 hours.
- Topical application: Heparin ointment (1 g contains 100 IU). A 3–4 cm strip of ointment is applied to the skin in a thin layer 1–3 times a day.
Side effects and complications
Unfractionated heparin therapy is associated with several serious adverse reactions:
- Heparin-induced thrombocytopenia (HIT): Early benign (on days 2–4) and late immune (on days 5–12) thrombocytopenia are distinguished. The late form is dangerous due to the formation of the "heparin-platelet factor 4" complex and the synthesis of IgG class antibodies, leading to paradoxical thrombin generation, thrombosis (myocardial infarction, stroke, limb gangrene), and skin necrosis.
- Effect on bone tissue: Increases parathyroid hormone activity, stimulating bone resorption and creating a risk of osteoporosis with prolonged use.
- Electrolyte disturbances: Inhibits aldosterone synthesis in the adrenal glands, creating a risk of hyperkalemia.
- Other effects: Reversible alopecia, allergic reactions (skin rash, arthralgia), and elevated liver transaminase levels.
Administration features and antagonism
In the event of hemorrhagic complications or overdosage, the mechanism of chemical antagonism is used. The specific antidote for heparin is protamine sulfate, which chemically binds to the anionic centers of heparin, neutralizing its anticoagulant effect.
Important clinical limitation: in confirmed heparin-induced thrombocytopenia (HIT), the administration of low-molecular-weight heparins is strictly contraindicated due to the risk of cross-reactive immune responses.
Briefly: answers to topical questions
What laboratory parameters must be monitored to control unfractionated heparin therapy?
- aPTT (activated partial thromboplastin time) — mandatory repeated determinations to monitor therapy efficacy.
- Platelet count (platelet count) in blood — determination before starting therapy and on days 1, 3, 5, 7, 9, 11–14 of treatment.
- Additional tests — antithrombin level, thrombin time, and thrombin-heparin clotting time (indicated when aPTT changes do not correspond to the drug dose).
In which specific diseases is intravenous administration of heparin sodium indicated?
- Deep vein thrombosis and venous thromboembolic complications — intravenous infusion of unfractionated heparin is the method of choice in relevant clinical situations.
- Pulmonary embolism — intravenous infusion is indicated in DVT with high-risk PE of early mortality when reperfusion is indicated.
- Acute coronary syndrome (unstable angina, myocardial infarction) — these indications for heparin are stated in sources, but the specific intravenous route of administration is not detailed in the provided sources.
What are the absolute contraindications to the systemic administration of heparin sodium?
- Ongoing bleeding (haemorrhagia) — an absolute contraindication for the heparin group.
- Thrombocytopenia (thrombocytopenia) — blood platelet count below 25×10⁹/L.
- Heparin-induced thrombocytopenia (thrombocytopenia inducta a heparino) — history of HIT.
- Renal failure (insufficientia renalis) — severe degree.
HEPARin: GEnerates Paradoxical Risk (HIT and thrombosis during anticoagulant therapy).
Class and exam questions
What is the main cellular source of endogenous heparin?
Endogenous heparin is synthesized by mast cells and stored in their secretory granules (primarily in the liver, lungs, and intestinal mucosa).
What is the pathogenesis of late immune heparin-induced thrombocytopenia (HIT)?
It develops on days 5–12 of therapy and is associated with the release of platelet factor 4 (PF4), complex formation with heparin, synthesis of IgG class antibodies, and endothelial damage, leading to paradoxical thrombus formation.
Which drug is the specific chemical antidote for heparin sodium?
Protamine sulfate. It engages in a chemical antagonism reaction, binding to the anionic centers of heparin and completely neutralizing its anticoagulant effect.
What anti-Xa to anti-IIa activity ratio is characteristic of standard heparin?
Standard heparin inhibits thrombin (factor IIa) and factor Xa equally, meaning their activity ratio is 1:1.
What else to study on this topic
- Low-molecular-weight heparins (LMWHs): production via depolymerization, selectivity, and pharmacokinetic differences
- Heparinoids as a separate group (sulodexide, danaparoid sodium)
- Comparative analysis of the bioavailability of unfractionated heparin and LMWHs upon subcutaneous administration
- Laboratory monitoring criteria (aPTT) during standard heparin therapy
- Pleiotropic effects of heparin: impact on lipid metabolism and vascular wall proliferation
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