Anticoagulants: Heparins and Direct Inhibitors

Heparins - Clot Busters Classic

• Mechanism: Potentiate Antithrombin III (ATIII).
• Unfractionated Heparin (UFH):
  • Via ATIII: inactivates Thrombin (IIa) & Factor Xa (1:1).
  • Monitor: aPTT (1.5-2.5x control).
  • Reversal: Protamine Sulfate.
  • ⚠️ Risk: Heparin-Induced Thrombocytopenia (HIT).
• Low Molecular Weight Heparins (LMWHs): (e.g., Enoxaparin, Dalteparin)
  • Via ATIII: Xa > IIa inhibition.
  • Monitor: Anti-Xa assay (usually not needed).
  • Reversal: Protamine Sulfate (partial).
  • ↓ HIT risk vs UFH.
• Fondaparinux:
  • Synthetic pentasaccharide; Via ATIII: selectively inhibits Factor Xa.
  • No routine monitoring; no reversal agent.
  • Minimal HIT risk.

⭐ UFH acts via antithrombin to inhibit factors IIa (thrombin) and Xa almost equally. LMWHs preferentially inhibit Factor Xa over IIa (approx. 2-4:1 ratio). Fondaparinux selectively inhibits only Factor Xa via antithrombin activation.

Direct Thrombin Inhibitors - Thrombin Tamers

Directly bind to and inhibit thrombin (Factor IIa), preventing fibrin formation. Independent of antithrombin; act on free & clot-bound thrombin.

• Parenteral DTIs:
  • Bivalirudin: Rapid onset, short half-life. Use: PCI. Monitor: ACT/aPTT.
  • Argatroban: Hepatic metabolism. Use: HIT (especially with renal impairment). Monitor: aPTT.
• Oral DTI:
  • Dabigatran Etexilate: Prodrug.
    • Uses: Stroke prevention in non-valvular AF, VTE treatment/prophylaxis.
    • Reversal: Idarucizumab.
    • AEs: Bleeding, dyspepsia.
    • 📌 "DABigatran directly ATTACKS thrombin."

⭐ Argatroban is preferred for HIT in patients with renal dysfunction as it undergoes hepatic metabolism, unlike lepirudin (renal excretion).

Direct Factor Xa Inhibitors - Xa Zappers

Oral anticoagulants directly inhibiting Factor Xa. Common suffix "-xaban".

• Agents: Rivaroxaban, Apixaban, Edoxaban, Betrixaban.
  • 📌 Mnemonic: "-xa-ban" indicates Xa inhibition.
• Mechanism: Directly bind to active site of Factor Xa (free & clot-bound).
  • Blocks conversion of prothrombin to thrombin.
• Key Features:
  • Oral, fixed dosing.
  • Predictable effect, no routine INR monitoring.
  • Rapid onset/offset.
• Uses:
  • DVT/PE treatment & prophylaxis.
  • Stroke prevention in non-valvular Atrial Fibrillation (NVAF).
• Side Effects: Bleeding (main concern).
• Antidote: Andexanet alfa (for rivaroxaban, apixaban).
• Caution: Renal impairment (dose adjustment/avoid).

⭐ Rivaroxaban (≥15 mg doses) should be taken with food for optimal absorption; apixaban can be taken with or without food.

Anticoagulant Crossroads - Clinical Choices & Cautions

• Choice by Condition:
  • Pregnancy: LMWH (safe). Warfarin (X: teratogenic), DOACs (avoid).
  • Renal Impairment (CrCl < 30 mL/min): UFH (esp. if CrCl < 15 mL/min), adjust LMWH/DOACs; Warfarin usable.
  • Heparin-Induced Thrombocytopenia (HIT): STOP heparins! Use Argatroban, Bivalirudin, Fondaparinux.
  • Mechanical Valves: Warfarin. NVAF: DOACs preferred.
• Reversal Agents:
  • Heparin/LMWH: Protamine Sulfate.
  • Warfarin: Vit K (slow), PCC/FFP (rapid).
  • Dabigatran (DTI): Idarucizumab.
  • Factor Xa Inhibitors: Andexanet Alfa; PCC.
• Monitoring:
  • UFH: aPTT. LMWH: Anti-Xa (select cases). Warfarin: PT/INR (target 2-3; 2.5-3.5 for high-risk valves). DOACs: Generally no routine monitoring.
• Warfarin Interactions: High risk. Affected by CYP2C9 inhibitors (e.g., Amiodarone ↑INR) & inducers (e.g., Rifampin ↓INR), antibiotics, diet (Vitamin K).

⭐ For patients on Warfarin requiring urgent surgery, reversal with Prothrombin Complex Concentrate (PCC) is faster than FFP and Vitamin K alone.

High‑Yield Points - ⚡ Biggest Takeaways

• UFH acts via Antithrombin III on Factors Xa & IIa; monitor aPTT. LMWH mainly targets Factor Xa via AT-III.
• Protamine sulfate reverses heparin. Heparin-Induced Thrombocytopenia (HIT) is a critical risk.
• Direct Thrombin Inhibitors (e.g., oral Dabigatran) directly block thrombin (IIa).
• Direct Factor Xa Inhibitors (e.g., oral Rivaroxaban, Apixaban) directly block Factor Xa.
• DOACs offer fixed dosing, predictable effects, and no routine monitoring.
• Antidotes: Idarucizumab for Dabigatran; Andexanet alfa for Xa inhibitors.