A pneumothorax is a collection of gas or air within the pleural cavity (Fig. 3.42). When air enters the pleural cavity the tissue elasticity of the parenchyma causes the lung to collapse within the chest, impairing the lung function. Occasionally, the gas within the pleural cavity may accumulate to such an extent that the mediastinum is “pushed” to the opposite side, compromising the other lung. This is termed a tension pneumothorax and requires urgent treatment.