Tension pneumothorax occurs when there is a flap-like pleural defect creating a valve effect, allowing air to enter the pleural space with inspiration which cannot escape with expiration. The cause is usually traumatic, but may be spontaneous. The side of the tension pneumothorax is usually lucent, with flattening of the hemidiaphragm. There is shift of the mediastinum and heart to the contralateral side, which can eventually lead to impaired venous return to the heart. Tension pneumothorax is a life-threatening emergency which requires immediate chest tube placement.