Pneumothorax II: Pathophysiology

During a pneumothorax, air enters the pleural space through a rupture in the lung or chest wall injury, eliminating the normal negative intrapleural pressure. This loss of negative pressure allows the lung to recoil inward and collapse. The affected lung separates from the chest wall, disrupting the delicate balance that normally keeps lungs expanded during breathing.

When the lung collapses, the alveoli collapse as well, significantly reducing ventilation and oxygen exchange. This decreased gas exchange leads to low oxygen levels in the blood, causing shortness of breath and hypoxemia. The severity depends on how much of the lung is affected and whether air continues accumulating in the pleural space.

Tension pneumothorax occurs when air continuously enters the pleural space without escape, causing pressure to rise progressively. This increased intrathoracic pressure compresses the heart and great veins, shifting the mediastinum to the opposite side. The compression reduces venous return and cardiac output, leading to hypotension and obstructive shock, which requires immediate medical intervention.

Pneumothorax typically presents with reduced chest movement on the affected side, decreased or absent breath sounds, and a hollow sound on percussion. Severe cases, especially tension pneumothorax, may show tracheal deviation, swollen neck veins, and low blood pressure. Patients often report sudden sharp chest pain and shortness of breath as initial symptoms.

In tension pneumothorax, rising intrathoracic pressure shifts the mediastinum and compresses the great veins returning blood to the heart. This compression reduces venous return, decreasing cardiac output and causing hypotension. The resulting circulatory failure can rapidly progress to shock if not treated immediately, making tension pneumothorax a medical emergency.

Normal negative intrapleural pressure creates a delicate balance between the chest wall's outward recoil and the lungs' inward elastic recoil. This pressure gradient keeps the lungs expanded against the chest wall, enabling smooth breathing and proper oxygen and carbon dioxide exchange. When this negative pressure is lost, the lung collapses and ventilation fails.

Pneumothorax presents with sudden onset sharp chest pain and shortness of breath, accompanied by reduced breath sounds and hollow percussion sounds. Unlike conditions such as chronic obstructive pulmonary disease clinical manifestations, pneumothorax develops acutely from air entering the pleural space. Tension pneumothorax additionally causes tracheal deviation, neck vein distension, and shock symptoms requiring emergency treatment.