Anticoagulants & Antiplatelets
On board day, you will be asked to separate venous clots from arterial clots and match each to the right drug. Anticoagulants — warfarin, heparin, the DOACs (apixaban, rivaroxaban, dabigatran, edoxaban) — interrupt the coagulation cascade and prevent venous thromboembolism, atrial fibrillation stroke, and mechanical valve thrombosis. Antiplatelets — aspirin, clopidogrel, ticagrelor — block platelet aggregation in arterial disease (post-MI, post-stent, ischemic stroke). The exam tests CHA2DS2-VASc thresholds for AFib anticoagulation, INR goals for warfarin, the lack of routine monitoring for DOACs, and the reversal agents (vitamin K, idarucizumab, andexanet alfa) by name.
📖 Overview
Anticoagulants prevent fibrin formation (used in a-fib, VTE, mechanical valves). Warfarin requires INR monitoring; DOACs (apixaban, rivaroxaban) don't. Antiplatelets (aspirin, clopidogrel) prevent platelet aggregation — used for stroke/MI primary and secondary prevention and after stent placement. Bleeding is the unified risk; HAS-BLED quantifies it in a-fib.
⚙️ Mechanism of Action
Warfarin: inhibits vitamin K epoxide reductase → ↓ factors II, VII, IX, X + proteins C/S. Apixaban/rivaroxaban: direct factor Xa inhibitors. Aspirin: irreversible COX-1 inhibition → ↓ thromboxane A2. Clopidogrel: irreversible P2Y12 inhibition → ↓ platelet activation.
💎 Board Pearls
- 📊 Warfarin INR goals: 2–3 for most (a-fib, VTE); 2.5–3.5 for mechanical mitral valve.
- 🥬 Warfarin + vitamin K (leafy greens): don't avoid — keep INTAKE CONSISTENT.
- 💊 Apixaban = cleanest DOAC (least renal clearance, twice daily).
- ⚠️ DOACs: renal dose adjustments; apixaban preferred in CKD (eGFR 15–29).
- 🫀 Stroke prevention in a-fib: CHA₂DS₂-VASc ≥2 (men) or ≥3 (women) → anticoagulate.
- 🩸 Bleeding risk: HAS-BLED ≥3 = high risk, not a reason to avoid — a prompt for reversible risks.
- 🆘 Warfarin reversal: vitamin K (mild), 4-factor PCC (emergent). DOAC reversal: andexanet (Xa), idarucizumab (dabigatran).
- 👶 Aspirin <16 yrs → Reye syndrome (avoid in viral illness).
- 🧬 Clopidogrel = prodrug activated by CYP2C19 → poor metabolizers (30% Asian pop) have ↓ effect.
💊 Drugs in This Class
- Warfarin — Coumadin, JantovenA-fib, mechanical heart valves (only approved option), VTE treatment/prevention, antiphospholipid syndrome.
- Apixaban — EliquisA-fib stroke prevention, VTE treatment + prevention, post-surgical VTE prophylaxis.
- Rivaroxaban — XareltoA-fib stroke prevention, VTE treatment + prevention, post-surgical VTE prophylaxis.
- Aspirin — Bayer, EcotrinSecondary ASCVD prevention, post-MI, post-stroke (non-cardioembolic), post-stent (DAPT), Kawasaki disease, (limited primary prevention).
- Clopidogrel — PlavixPost-stent DAPT (+ aspirin), post-ACS, secondary stroke prevention, PAD.
Practice Questions
You are reviewing the chart of a 72-year-old man newly diagnosed with nonvalvular atrial fibrillation. He has hypertension, type 2 diabetes, and prior transient ischemic attack, giving him a CHA₂DS₂-VASc score of 4. His HAS-BLED score is 2, largely from age and hypertension. Which is the most appropriate plan?
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