Heparin (unfractionated)
Heparin is an unfractionated injectable anticoagulant used in veterinary medicine to prevent and treat thromboembolic disease and to maintain the patency of intravenous catheters. It relies on the presence of endogenous antithrombin III (AT III) to exert its effects. > **Clinical Warning:** Heparin carries a significant risk of hemorrhage. It does not lyse existing clots but prevents their extension. **Clinical Pearl:** Because heparin requires AT III to function, it may be ineffective in patients with severe protein-losing nephropathy or enteropathy where AT III is depleted.
Mechanism: Heparin exerts its effects primarily by enhancing the binding of **antithrombin III (AT III)** to **factors IIa, IXa, Xa, XIa and XlIa**. * **AT III/clotting factor complex** formation → subsequent removal by the liver. * Inactivates **thrombin** → blocks the conversion of **fibrinogen** to **fibrin**. * Inhibition of **Factor XII** activation → prevents the formation of stable fibrin clots. > **Note:** Heparin is only effective if adequate AT III is present. It does not significantly change the concentrations of clotting factors, nor does it lyse pre-existing clots.
Routes of administration
Contraindications
- Active bleeding or severe bleeding disorders
- Severe thrombocytopenia
- Recent neurosurgery or ocular surgery
- Known hypersensitivity to heparin
Adverse effects
- Hemorrhage (most common)
- Heparin-induced thrombocytopenia (HIT)
- Injection site reactions (hematoma, pain)
- Hypersensitivity reactions
Drug interactions
- Aspirin · Increased risk of bleeding due to platelet inhibition · major
- NSAIDs · Increased risk of gastrointestinal ulceration and bleeding · major
- Warfarin · Synergistic anticoagulant effect; increased bleeding risk · major
- Clopidogrel · Increased risk of bleeding · major
Monitoring
- aPTT (Activated Partial Thromboplastin Time) - typically target 1.5 to 2.5 times baseline
- Platelet count
- Packed Cell Volume (PCV) and Total Protein (TP)
- Clinical signs of bleeding (mucous membranes, feces, urine)
Overdose
The primary sign of overdosage is **hemorrhage** (e.g., epistaxis, hematuria, melena, petechiae). * **Mild to moderate overdose:** Discontinue heparin therapy; due to its short half-life, coagulation parameters often normalize quickly. * **Severe, life-threatening hemorrhage:** Administer **protamine sulfate** as a specific reversal agent (antidote). Administer slowly IV to avoid hypotension and anaphylactoid reactions.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.