Pulmonary embolism (PE) occurs when a thrombus (most commonly from the deep veins of the legs - DVT) dislodges and travels to the pulmonary arterial system, causing partial or complete obstruction. Virchow triad describes the three factors predisposing to thrombosis: (1) Venous stasis (immobility, prolonged bed rest, long flights), (2) Endothelial injury (surgery, trauma, central line placement), and (3) Hypercoagulability (malignancy, pregnancy, oral contraceptives, Factor V Leiden, antiphospholipid syndrome). The hemodynamic impact depends on the degree of obstruction: a high-risk PE (>50% of pulmonary vasculature) causes acute right ventricular pressure overload, leading to RV dilation, interventricular septum bowing into the LV, decreased LV filling, and ultimately cardiogenic shock. V/Q mismatch occurs as blood flow is redirected away from the obstructed segment while alveolar ventilation is preserved (dead space ventilation), causing hypoxemia and increased A-a gradient. Classification: intermediate-risk PE involves RV dysfunction without systemic hypotension (BP >90 systolic), while high-risk PE causes hemodynamic instability (systolic BP <90 for 15+ minutes).