A 58-year-old man with acute onset dyspnea and pleuritic chest pain is diagnosed with acute pulmonary embolism on CT pulmonary angiography. He has no contraindications to anticoagulation. What is the drug of choice for initial anticoagulation in this patient?

Explanation

## Initial Anticoagulation in Acute Pulmonary Embolism **Key Point:** Unfractionated heparin (UFH) is the first-line anticoagulant for acute PE because it has the shortest half-life, is reversible with protamine, and allows rapid dose adjustment—critical in hemodynamically unstable patients who may require thrombolysis or embolectomy. ### Mechanism of UFH in PE Management UFH works by: 1. Potentiating antithrombin III 2. Inhibiting factors IIa and Xa 3. Preventing thrombus propagation and new clot formation 4. Allowing time for endogenous fibrinolysis ### Why UFH is Preferred Over Alternatives | Feature | UFH | LMWH | Warfarin | Aspirin | |---------|-----|------|----------|----------| | **Onset** | Immediate (IV) | 3–4 hrs (SC) | 48–72 hrs | No anticoagulant effect | | **Reversibility** | Yes (protamine) | Partial | No (takes days) | No | | **Hemodynamic instability** | Ideal | Relative CI | Contraindicated | Inadequate | | **Thrombolysis-ready** | Yes | Requires washout | No | No | | **Monitoring** | aPTT-guided | Not needed | INR-guided | Not applicable | **High-Yield:** In hemodynamically unstable PE (shock, RV dysfunction), UFH is mandatory because: - It can be rapidly reversed if bleeding occurs during thrombolysis - It allows immediate escalation to systemic thrombolysis or embolectomy - LMWH cannot be rapidly reversed and is contraindicated in shock **Clinical Pearl:** The target aPTT for PE is 1.5–2.5 × control (or 60–100 seconds), achieved with a bolus of 80 units/kg IV followed by 18 units/kg/hr infusion. ### Transition to Warfarin Warfarin is started concurrently (day 1) but takes 48–72 hours to achieve therapeutic INR (2–3). UFH is continued until INR is therapeutic for ≥24 hours (overlap period typically 5–7 days). **Mnemonic: HEPARIN FIRST** — **H**emodynamic instability, **E**arly reversibility, **P**erfect for thrombolysis, **A**djustable dosing, **R**apid onset, **I**ntermediate duration, **N**eeds monitoring. [cite:Harrison 21e Ch 297]