Pneumothorax - Mechanism

Mechanism

The thoracic cavity is the space inside the chest that contains the lungs, heart and a number of major blood vessels. On each side of the cavity, a pleural membrane covers the surface of lung (visceral pleura) and also lines the inside of the chest wall (parietal pleura). Normally, the two layers are separated only by a small amount of lubricating serous fluid. The lungs are fully inflated within the cavity because the pressure inside the airways is higher than the pressure inside the pleural space. Despite the low pressure in the pleural space, air does not enter it because there are no natural connections to an air-containing passage, and the pressure of gases in the bloodstream is too low for them to be forced into the pleural space. Therefore, a pneumothorax can only develop if air is allowed to enter, through damage to the chest wall or damage to the lung itself, or occasionally because microorganisms in the pleural space produce gas.

Chest wall defects are usually evident in cases of injury to the chest wall, such as stab or bullet wounds ("open pneumothorax"). In secondary spontaneous pneumothoraces, vulnerabilities in the lung tissue are caused by a variety of disease processes, particularly by rupturing of bullae (large air-containing lesions) in cases of severe emphysema. Areas of necrosis (tissue death) may precipitate episodes of pneumothorax, although the exact mechanism is unclear. Primary spontaneous pneumothorax has for many years been thought to be caused by "blebs" (small air-filled lesions just under the pleural surface), which were presumed to be more common in those classically at risk of pneumothorax (tall males) due to mechanical factors. Various lines of evidence suggest that this hypothesis may not be sufficient to explain all episodes of primary spontaneous pneumothorax, such as the fact that pneumothorax may recur even after surgical treatment of blebs, and that blebs occur in 15% of healthy people. It has therefore been suggested that PSP may also be caused by areas of disruption (porosity) in the pleural layer, which are prone to rupture. Smoking may additionally lead to inflammation and obstruction of small airways, accounting for the markedly increased risk of PSP in smokers. Once air has stopped entering the pleural cavity, it is gradually resorbed spontaneously.

Tension pneumothorax occurs when the opening that allows air to enter the pleural space functions as a one way valve, allowing more air to enter with every breath and but not to escape. The body compensates by increasing the respiratory rate and tidal volume (size of each breath), worsening the problem. Unless corrected, hypoxia (decreased oxygen levels) and respiratory arrest eventually follow.