In pneumothorax, air entering the pleural space shifts the intrapleural pressure from negative to positive and decreases the suctioning or pulling effects of the pleural cavity on the elastic lung tissue. In tension pneumothorax, air leaks into the pleural space and the volume of air continues to grow. As the tension pneumothorax builds, it begins to put pressure on the mediastinum and the trachea shifts.
Pressure builds on the heart and great vessels, diminishing venous return and cardiac output.
Eventually, pressure may compress the opposite lung, further compromising ventilation.