A 58-year-old woman presents to the emergency department with acute onset dyspnea and chest pain. She underwent total knee replacement surgery 5 days ago. On examination, her heart rate is 118 bpm, respiratory rate 28/min, blood pressure 92/58 mmHg, and oxygen saturation 88% on room air. Unilateral leg swelling is noted on the operated side. ECG shows sinus tachycardia and T-wave inversions in leads V1–V4. Chest X-ray is normal. D-dimer is markedly elevated at 2.8 μg/mL. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Risk Factors **Key Point:** This patient presents with the classic triad of acute PE: dyspnea, chest pain, and hemodynamic instability (hypotension, tachycardia, hypoxemia). Risk factors present: - Recent major orthopedic surgery (knee replacement) — **highest risk for VTE** - Immobilization post-operatively - Unilateral leg swelling (DVT sign) ## Diagnostic Findings | Finding | Interpretation | |---------|----------------| | **Hypoxemia (88%)** | Right-to-left shunting or V/Q mismatch from PE | | **Hemodynamic instability** | Indicates massive PE with RV strain | | **ECG: T-wave inversion V1–V4** | RV strain pattern; classic for acute PE | | **Normal CXR** | Typical in PE; rules out pneumonia/pneumothorax | | **Elevated D-dimer** | Highly sensitive but nonspecific; confirms thromboembolism | | **Unilateral leg swelling** | Source of thrombus (DVT → PE) | ## Why This Is Massive PE **High-Yield:** Hemodynamic instability (SBP <90 mmHg) defines **massive PE** and carries mortality >30% without treatment. 1. Acute RV afterload increase from pulmonary vascular obstruction 2. RV dilatation → septal shift → LV compression → reduced cardiac output 3. Hypotension + hypoxemia = shock state ## Pathophysiology of PE-Induced Shock ```mermaid flowchart TD A[Thrombus in pulmonary artery]:::outcome --> B[Acute increase in RV afterload]:::action B --> C[RV dilatation & dysfunction]:::action C --> D{Severity?}:::decision D -->|Massive| E[Hemodynamic collapse]:::urgent D -->|Submassive| F[RV strain without shock]:::outcome E --> G[Hypotension, shock, RV strain ECG]:::outcome F --> H[Tachycardia, hypoxemia, normal BP]:::outcome ``` **Clinical Pearl:** Post-orthopedic surgery patients have the **highest PE incidence** (up to 40–60% if prophylaxis not given). This is a board-favorite scenario. ## Next Steps 1. **Immediate:** Anticoagulation (unfractionated heparin bolus) — do NOT delay for imaging 2. **Imaging:** CT pulmonary angiography (CTPA) or bedside echocardiography to confirm RV dilatation 3. **Consider:** Thrombolysis or embolectomy given hemodynamic instability **Mnemonic — PE Risk Factors (THROMBOSED):** **T**ravel/immobilization, **H**ypercoagulable states, **R**ecent surgery, **O**besity, **M**alignancy, **B**irthcontrol/HRT, **O**lder age, **S**tasis (AF, heart failure), **E**ndothelial injury, **D**iabetes/DVT history. [cite:Harrison 21e Ch 297]