Tension Pneumothorax

This illustration shows a tension pneumothorax, which develops when a lung injury (shown in the illustration as a small dark area on the left lung) or chest wall injury acts as a one-way valve mechanism that allows air into the pleural space but not out of it. (Small black arrows indicate air leaking out of the lung; large black arrows indicate pressure from air in the pleural space.) As a result, air accumulates and intrathoracic pressure increases, compressing the affected lung. Eventually, all the midline thoracic structures shift, including the mediastinum; the contralateral lung is compressed; and venous return to the heart decreases, resulting in shock. These complications can develop rapidly, particularly in patients receiving positive pressure ventilation.