Loss of negative pleural pressure allows the lung to collapse and the chest wall to spring out
Expansion
Normally the lung’s inward recoil and the chest wall’s outward recoil oppose one another, generating a negative pleural pressure. Introducing air abolishes it.
- Transpulmonary pressure falls to zero, so the lung collapses inward
- The chest wall springs outward, so the hemithorax may look larger
- The collapsed lung is still perfused, creating a shunt and hypoxaemia
- Ventilation-perfusion matching partly improves with time as hypoxic pulmonary vasoconstriction reduces flow to the collapsed lung
Tension pneumothorax adds a one-way valve, so pleural pressure becomes progressively positive, shifting the mediastinum, kinking the great veins and obstructing venous return. Death results from circulatory failure rather than hypoxia alone.
Two management points follow from the physiology. Positive pressure ventilation can convert a simple pneumothorax into a tension pneumothorax, so a drain is required first. And breathing high flow oxygen speeds resorption of a small pneumothorax, by washing nitrogen out of the pleural space and steepening the gradient for absorption.