Pneumothorax means presence of air between the layers of pleura. Tension pneumothorax develops when a lung or chest wall injury is such that it allows air into the pleural space but not out of it. As a result, air accumulates and compresses the lung, eventually shifting the mediastinum compressing the contralateral lung and increasing intra thoracic pressure enough to decrease venous return to the heart, causing shock. In most cases, a pneumothorax is caused by a traumatic injury, such as a rib fracture or penetrating injury (stab or gunshot wound).