lung, but it also compresses the contralateral lung ( Fig 115.2 ) These patients may present in severe respiratory distress with decreased breath sounds on the side of the pneumothorax There is also a shift of the mediastinal structures to the contralateral side Two-thirds of the blood supply to the body is returned to the heart via the inferior vena cava Because the inferior vena cava is relatively fixed in place as it passes through the diaphragm and cannot shift as much as the superior vena cava, venous return to the heart is reduced FIGURE 115.2 Tension pneumothorax with a mediastinal shift Some patients with a pneumothorax or hemothorax demonstrate tension physiology: tachycardia and peripheral vasoconstriction If left untreated, this will progress to shock Initial treatment for tension physiology consists of needle decompression An immediate release of air should be noted with tension pneumothorax and the patient’s hemodynamic status should improve The needle decompression is only a temporizing measure and must be followed by tube thoracostomy Open Pneumothorax An open pneumothorax is the result of penetrating trauma There is a direct connection between the pleural space and the atmosphere, impeding ventilation As in a bronchial tear or lung parenchymal injury, air may enter but not leave the pleural space, creating a ball-valve effect Initial treatment includes placement of