Underlying lung disease e.g. Marfan’s syndrome, Ehlers-Danlos syndrome, obstructive lung disease etc.
Traumatic
Iatrogenic e.g. insertion of a central line or positive pressure ventilation
Trauma e.g. a penetrating trauma or blunt trauma with rib fracture
Pathophysiology
Intrapleural pressure is normally negative (less than atmospheric pressure) because of inward lung and outward chest wall recoil
The intra-alveolar pressure is equal to the atmospheric pressure, hence the collapsing force of the lungs is equal and opposite to the expanding force of the chest wall
This creates a negative intrapleural pressure which is responsible for opposing the lungs elastic recoil force and preventing collapse of the alveoli
In pneumothorax, air enters the pleural space from outside the chest or from the lung itself via mediastinal tissue planes or direct pleural perforation
Intrapleural pressure increases, and lung volume decreases → the intrapleural pressure is equal to the atmospheric pressure
The opposing forces are lost, hence the lungs collapse and the chest wall springs out
Pneumothorax abolishes the transmural pressure gradient
Eventually, the intrapleural pressure becomes greater than the atmospheric pressure
This means that the ipsilateral lung collapses and is unable to expand due to the pressure
Mediastinal shift and compression of the superior vena cava can also occur, further compromising the cardiopulmonary function, leading to an obstructive shock