The pneumothorax (also leads to lung collapse) is the result of air or gas accumulation in the pleural cavity, i.e. inside the pleura.
The pleura is the outer skin of the lungs and is formed from two layers (or leaves):
- External or parietal – in contact with the chest,
- Inside or visceral, in contact with lung tissue.
The trapped air originates:
- from outside the body,
- from the lungs themselves.
The pneumothorax can be:
- monolateral,
- bilateral (if both lungs are affected).
Between the visceral pleura and parietalis is the pleural cavity, which is a very small space containing a small amount of serous fluid (2-3 ml for each lung).
The pleura visceralis remains in constant contact with the parietal pleura, because there is a negative pressure (-3mmHg), also called Donders negative pressure, between them.
This negative pressure is important for correct breathing: if it does not exist, we would die after a few minutes due to asphyxia.
Why negative pressure plays a fundamental role: it holds the lung tissue and chest together, so the lungs automatically expand with chest movement.
Usually the air is located in the lungs:
- in the bronchi
- in the alveoli.
In patients with pneumothorax, there is no longer negative pressure between the pleural sheets:
- In the inhalation phase, no air enters the lungs.
- When exhaling, no air comes out.
The air is free in the pleural space, therefore:
- there is no Donders pressure,
- the lungs collapse.
The result is respiratory insufficiency of varying degrees, depending on the severity of the pneumothorax.
Contents
Development of pneumothorax (pathophysiology)
Breathing mechanics
The lungs have a natural tendency to collapse due to the presence of forces that tend to retract the tissues:
The fibers of lung tissue consist of:
- elastin (which can lengthen slightly),
- Collagen (quite tight).
In the resting position, the fibers experience only a slight pull, but during inspiration the tension increases.
For this reason:
- muscle contraction (work) is required when inhaling,
- it happens automatically when exhaling that the elastic fibers return to their resting position.
Surface tension is the force used to increase the surface area of a liquid upon contact with air.
There are attractive forces among the molecules of a liquid that are greater than the forces between liquid and gas (in this case, the air in the alveoli).
In order to increase the size of the alveoli during inspiration, the liquid surface must be increased upon contact with air, but this involves a great deal of force to overcome the surface tension of the liquid.
In the alveoli there are:
- lots of air,
- a thin film of liquid.
The attraction between the water molecules tends to cause the lungs to collapse.
However, the presence of surfactants (a liquid consisting of fats and proteins) in the alveoli serves to reduce surface tension.
The surfactant is located on the surface between air and water, where:
- the hydrophilic component directed towards water,
- the hydrophobic portion in contact with the air.
The elastic forces, acting in the opposite direction, cause negative pressure in the “virtual” pleural space relative to atmospheric pressure.
If there is an anatomical connection between the alveoli and the pleural cavity, the air from the alveoli enters the pleural space (where the pressure is lower).
The air entered causes an increase in pressure in the pleural cavity, and thus a decrease in the dimensions of the lung tissue.
The lungs:
- withdraws,
- Collapses.
If the amount of air entering the pleural space is significantly large, the increasing intrapleural pressure displaces:
- the mediastinum (which is the middle region between the two lungs) to the opposite side.
- the diaphragm downwards.
All this can lead to:
- a reduction in the overall “vital capacity” of the lungs,
- subsequently to a change in gas exchange.
If intrapleural pressure increases sharply, venous return flow to the heart (the amount of blood that returns to the heart) decreases with a decrease in cardiac output.
Classification of the pneumothorax
An open or external pneumothorax is present when there is an injury to the chest and there is a constant exchange of air between the lungs, pleural space and the external environment.
In case of a serious open wound, air enters the pleural cavity when the patient breathes:
- from the lungs,
- from the outside.
This phenomenon is called a blowing chest wound.
The result is a total pneumothorax, namely that the lungs:
- detaches from the pleura,
- collapses in itself and towards the middle of the chest.
In this case, therapy consists in occlusion of the injury and drainage of air from the pleural cavity.
A closed pneumothorax is when air enters the pleural cavity but there is no air or gas exchange with the outside.
It is a partial pneumothorax.
Tension pneumothorax
The valve or voltage pneumothorax is caused by the formation of a monodirectional valve that allows the flow of air only from the lungs to the pleural cavity inwards. When breathing, the amount of air that enters is greater than the one that comes out, because it accumulates in the pleural cavity and causes greater pressure than outside.
It is the most serious type and can lead to a complete collapse of the lungs.
This is a serious and fatal disease and concerns:
- Breathing: the collapsed lung shifts to the opposite side and compresses the healthy lung (right figure).
- This leads to severe respiratory insufficiency.
- Blood circulation: the two vessels that suffer most from the displacement of the mediastinum are:
- superior vena cava,
- Inferior vena cava.
Both end in the right atrium.
In the case of pneumothorax with cardiac displacement, the openings into which the veins enter remain fixed and thus a displacement and compression of the vena cava occurs.
Consequences are:
- a marked reduction in venous return flow to the right atrium.
- a reduction in blood flow to the arteries and veins of the lungs because the right heart pumps less blood.
The left heart receives less blood, thereby reducing cardiac output to the tissues of the body: “syndrome of reduced cardiac output“.
First, a high heart rate is triggered in the organism in order to supply sufficient blood to the rest of the body.
Then the heart rate drops until:
- to bradycardia,
- to death due to cardiac arrest.
These patients can die quickly due to acute respiratory failure.
Why is a higher pressure reached in the pleural cavity than in the air, which is contained in the respiratory tract and in the external environment?
A valve mechanism allows the air to enter the pleural cavity, but not to exit again. Therefore, air penetrates during inspiration and increases the pressure.
Development of tension pneumothorax
Many patients with spontaneous pneumothorax may develop tension pneumothorax due to cough.
Cough leads to a significant increase in pressure in the airways.
The result is a worsening of the pneumothorax, because even more air enters the pleural cavity until a tension pneumothorax occurs.
- Pneumothorax: drainage, surgery and healing process
- Spontaneous or trauma pneumothorax: symptoms and causes
- Pulmonary embolism: symptoms and causes
