Pneumothorax

Overview

Pneumothorax is the accumulation of air in the pleural cavity. It can occur on its own or as a result of stress or damage to the lung due to an underlying pathology. Air commonly enters the pleural space via a hole in the chest wall, or via a hole in the surface of the lung. Both will tend to collapse the affected lung. In a tension pneumothorax, a flap of pleura acts as a one way valve and allows air to accumulate in the pleural cavity under pressure. This is life-threatening.

Pneumothorax

A pneumothorax is the presence of air within the pleural cavity, which is otherwise filled with a small volume of fluid and under slightly negative pressure. The presence of air interferes with the usual cycle of pressure changes within the pleural cavity during breathing and hence impairs ventilation of the lungs. Clinically, a presentation of pneumothorax is divided into two types, depending on the history of the patient:

Mechanisms

The pleural cavity is sealed from the external environment and has a slightly lower pressure than barometric pressure (PB). This is due to the combined properties of the chest wall being inclined to expand (if possible) and the elastic recoil properties of the lung. These forces pull in opposite directions, creating a negative pressure within the pleural cavity. Inflation of the lungs depends upon the diaphragm being able to drive intrapleural pressure to more subatmospheric pressures. If the chest or the lung is breached, air can flow into the pleura, making this difficult to impossible to achieve. Such a breach may occur during trauma to the chest wall (e.g. stabbing) or to the visceral pleura surrounding the lung (e.g. the effect of an inwardly directed broken rib after a chest contusion). Many cases of primary pneumothorax are thought to involve the bursting of weak points on the visceral pleura. Such cases are often tall, lean men.

Figure 1: Mechanisms by which a pneumothorax commonly arises, using a simple model of a balloon in a sealed jar. A. In the normal thorax, the opposing forces of the chest wall and lung recoil generate a negative pressure in the thorax, holding the alveoli slightly open. The mechanism of breathing requires that the pleura remain isolated from the external environment so that more negative pressures can be generated to drive air flow. If the chest wall is pierced (B) or the surface of the lung is torn (C), a pneumothorax occurs and breathing becomes difficult.

Because the mediastinum separates the two pleural spaces surrounding the lungs, a pneumothorax may only affect the one side of the lungs. Breathing will be difficult and there will be discomfort, but it is not necessarily life-threatening.

Tension pneumothorax

A more worrying situation is a tension pneumothorax, where a flap of damaged pleura acts as a one way valve, allowing air into the pleura to build up pressure that exceeds atmospheric pressure. This will cause a large, pressurised void of air in the pleural space that pushes against the contralateral side of the thorax, restricting the ventilation of the other lung. If on the left side, such a disturbance will press against the veins returning blood to the heart, causing a reduction in cardiac output. This is life-threatening.

Figure 1: In a tension pneumothorax, a flap of tissue acts as a one way valve over a hole in the lung (as shown), or a hole in the chest wall. During inspiration, air is allowed into the pleural cavity, but at the end of inspiration the flap snaps shut and no air can leave. This cycle repeats itself with each breath, until the developed pressure collapses the affected lung. If left untreated, the pressure on one side of the thorax can push against the other, making breathing on that side difficult too, and possibly shifting the heart and collapsing the great vessels.

Treatment

Initial treatment involves inserting a chest drain and withdrawing the air within the pleura. The drain is connected to a one-way valve so that air may be expelled from the thorax on inspiration, but cannot return.