Chapter 3: Antiplatelet Therapy

General Considerations

Atherosclerosis is a leading cause of cardiovascular disease. Atherosclerotic plaques can acutely rupture, exposing a necrotic core that sets off the coagulation cascade, ultimately culminating in vascular occlusion. Activation of platelets is the initial event in this cascade and, depending on the vascular bed involved, can cause acute coronary syndromes, ischemic stroke, mesenteric ischemia, or acute limb ischemia. Antiplatelet therapy forms the core of treatment for both acute and chronic atherosclerotic disease. In this chapter, we will discuss antiplatelet therapy for treatment of cardiovascular disease.

Role of Platelets in Thrombosis

After vascular injury, platelets bind to exposed collagen and von Willebrand factor (vWF) and are activated. Activated platelets then secrete thromboxane A₂ (TXA₂) and adenosine diphosphate (ADP), which leads to platelet aggregation and recruitment of more platelets. The final common pathway of platelet aggregation is mediated by glycoprotein (GP) II_bIII_a receptors that bind to fibrinogen and vWF, leading to platelet plug and clot formation. Antiplatelet agents target different pathways in this cascade (Figure 3–1).

Classification of Antiplatelet Drugs

A. Cyclooxygenase Inhibitors: Aspirin

1. Mechanism of action

Acetylsalicylic acid (ASA, aspirin) in low doses irreversibly inhibits cyclooxygenase-1 (COX-1), which is required for synthesis of TXA₂, a vasoconstrictor required for platelet aggregation (Figure 3–2). At higher doses, ASA also inhibits COX-2, which is required for prostacyclin production; prostacyclins are inhibitors of platelet aggregation and vasodilators. Thus, for optimal antiplatelet effect, an ASA dose between 75 and 325 mg is recommended. For rapid onset of action, in ASA-naïve patients, an initial dose of at least 162 mg should be used.

2. Contraindications

ASA is contraindicated in patients with a ...