Antiplatelet management

Pathophysiology - The Clot Thickens

• Adhesion: Endothelial damage exposes von Willebrand Factor (vWF), which binds to platelet glycoprotein (Gp) Ib receptors.
• Activation: Platelets change shape and release pro-thrombotic factors:
  • Thromboxane A₂ ($TXA_2$) from arachidonic acid via COX-1.
  • Adenosine diphosphate (ADP) from dense granules.
• Aggregation: $TXA_2$ and ADP activate other platelets, leading to conformational change in GpIIb/IIIa receptors, which bind fibrinogen to cross-link platelets.

⭐ The GpIIb/IIIa receptor is the final common pathway for platelet aggregation, making it a potent target for antiplatelet therapy (e.g., abciximab, eptifibatide, tirofiban).

Antiplatelet Agents - The Platelet Punchers

• Aspirin (ASA): Irreversibly inhibits COX-1, preventing Thromboxane A₂ ($TXA_2$) synthesis. The effect lasts for the platelet's lifespan (7-10 days).

• P2Y₁₂ Receptor Blockers: Prevent ADP-mediated platelet activation and aggregation.

  • Irreversible: Clopidogrel, Prasugrel.
  • Reversible: Ticagrelor, Cangrelor.

  ⭐ Clopidogrel is a prodrug activated by hepatic CYP2C19. Genetic polymorphisms can cause variable efficacy and treatment failure.

• Glycoprotein (GP) IIb/IIIa Inhibitors: Directly block the final common pathway of aggregation.

  • Abciximab, Eptifibatide, Tirofiban.
  • Primarily used in high-risk patients during percutaneous coronary intervention (PCI).

• Phosphodiesterase (PDE) Inhibitors: Increase cAMP, which inhibits platelet function.

  • Dipyridamole, Cilostazol.
  • Cilostazol also acts as a direct arterial vasodilator, used for intermittent claudication.

Treatment Algorithm - DAPT Decision Dash

• Core Principle: Balance ischemic risk vs. bleeding risk.
• Default Duration (Post-ACS or Elective PCI): Aspirin + P2Y12 inhibitor for 6-12 months.

⭐ The DAPT Score predicts risk/benefit of continuing therapy. A score ≥2 favors extending DAPT, while a score <2 favors stopping after the initial period due to bleeding risk outweighing ischemic benefit.

Complications & Caveats - Bleeding & Beyond

• Bleeding: Most common complication. Risk ↑ with age, CKD, low body weight, and invasive procedures.
  • Sites: GI (most frequent), intracranial (most feared), access site.
  • Management: Varies by severity; may involve holding therapy, transfusion, or reversal agents (rarely).
• Other Adverse Effects:
  • Ticagrelor: Dyspnea (common, usually transient), ↑uric acid, ventricular pauses.
  • Clopidogrel/Prasugrel: ⚠️ Thrombotic Thrombocytopenic Purpura (TTP).

⭐ TTP is a rare but life-threatening complication associated with thienopyridines, especially clopidogrel, typically occurring within the first 2 weeks of therapy.

High-Yield Points - ⚡ Biggest Takeaways

• Dual antiplatelet therapy (DAPT) with Aspirin and a P2Y12 inhibitor is the cornerstone of ACS management.
• Administer Aspirin immediately; it irreversibly inhibits COX-1, blocking thromboxane A2 production.
• P2Y12 inhibitors like Clopidogrel, Prasugrel, and Ticagrelor prevent ADP-mediated platelet aggregation.
• Prasugrel is contraindicated in patients with a history of stroke or TIA.
• Ticagrelor is reversible and a common side effect is dyspnea.
• Continue DAPT for at least 12 months post-ACS, particularly after stenting.