Clinical meaning
Obstructive shock results from a mechanical barrier that prevents adequate cardiac filling or ejection, causing acute reduction in cardiac output despite adequate intravascular volume and myocardial function. The three primary causes are: (1) Cardiac tamponade: accumulation of fluid, blood, or air in the pericardial space compresses the heart, preventing diastolic filling. As little as 100-200 mL of rapid fluid accumulation can cause hemodynamic collapse (chronic effusions can tolerate 1-2 L due to gradual pericardial stretching). Equalization of diastolic pressures (RA = RV = PA diastolic = PAWP) is the hemodynamic hallmark. (2) Tension pneumothorax: air enters the pleural space through a one-way valve mechanism and cannot escape, progressively compressing the ipsilateral lung and shifting mediastinal structures (heart, great vessels, trachea) to the contralateral side. This kinks the great veins, preventing venous return and dramatically reducing preload. (3) Massive pulmonary embolism: a large clot (or saddle embolus) obstructs the pulmonary vasculature, preventing right ventricular outflow to the pulmonary circulation. Acute right ventricular pressure overload causes RV dilation and failure, interventricular septum bowing into the left ventricle, and reduced left ventricular filling and output. All three causes share the common mechanism of impaired cardiac filling or ejection, elevated central venous pressure (JVD), and rapid cardiovascular collapse. The definitive treatment is ALWAYS removing the obstruction (pericardiocentesis, needle decompression, thrombolysis/embolectomy); fluids and vasopressors are temporizing measures only.