Obstructive shock results from a physical obstruction to blood flow, impairing cardiac output. Among the causes, tension pneumothorax is the most common because it rapidly increases intrathoracic pressure, compressing the vena cava and reducing venous return to the heart. This leads to decreased preload and subsequent shock. Immediate recognition and needle decompression or chest tube insertion are critical surgical interventions to relieve pressure and restore circulation. This urgency and frequency make tension pneumothorax the leading cause of obstructive shock in emergency settings.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.