An open pneumothorax occurs when:

An open pneumothorax occurs when air enters the pleural space from outside the body. This typically happens when there is a penetrating injury to the chest, such as a stab wound or a gunshot wound, creating a direct pathway for air to flow into the thoracic cavity. This condition is also known as a "sucking chest wound," as the open wound allows air to be sucked in during inhalation, which can compromise lung function and cause respiratory distress.

In this scenario, the presence of an open wound means that air can bypass the usual barrier that the thoracic cavity provides, leading to an imbalance in the pressure that can affect breathing. Immediate treatment often involves sealing the wound to prevent further air entry and to stabilize the patient’s condition.