Tension pneumothorax can occur without any prior lung conditions or diseases. In most cases, a tension pneumothorax results from the rupture of blebs (air-filled sac on the lung). As a result, air escapes from the lung and enters the chest cavity causing the lung to collapse. It differs from spontaneous pneumothroax in that the collapse is much more severe and can cause many serious medical problems.
As the air escapes from the lung and enters the pleural space, increased pressure can cause a mediastinal shift, in which the heart, trachea, and great vessels are pushed towards the unaffected side of the chest.