Pulmonary embolism causes acute right ventricular pressure overload from mechanical vascular obstruction and vasoactive mediator release. When > 30-50% of the pulmonary vascular bed is occluded, the thin-walled RV fails to generate adequate systolic pressures against the increased afterload. RV dilation causes interventricular septal shift (D-sign), impairs LV filling, and reduces cardiac output. V/Q mismatch from perfused but non-ventilated and ventilated but non-perfused lung units causes hypoxemia. Dead space ventilation increases (high V/Q regions), producing tachypnea. The Wells criteria pre-test probability score guides diagnostic testing: low probability (< 2 points) + negative age-adjusted D-dimer safely excludes PE. CTPA is the definitive imaging test. Risk stratification (PESI/sPESI) combined with RV function assessment and biomarkers determines management intensity from outpatient anticoagulation to systemic thrombolysis.