Final answer:
The immediate management of an open pneumothorax involves sealing the chest wound with an airtight material to restore negative pressure in the chest cavity, essential for lung expansion.
Step-by-step explanation:
The most important, immediate step in the management of an open pneumothorax is to seal the chest wound with an airtight material to prevent further air from entering the chest cavity and to reestablish the negative pressure necessary for lung expansion. In an open pneumothorax, air enters the pleural space and disrupts the negative pressure that normally keeps the lungs inflated; hence, the priority is to stop the air inflow. After sealing the wound, medical personnel should continue monitoring the patient for signs of respiratory distress and prepare for further treatment, which may include chest tube insertion to fully re-expand the lung and evacuate any collected air.