The pleura is the outer membrane of the lung and consists of two layers (or leaves):

  1. External or parietal – in contact with the rib cage,
  2. Internal or visceral – in contact with lung tissue.

The trapped air may come:

  • From outside the body,
  • From the lung itself.

The pneumothorax may be:

  • Unilateral,
  • Bilateral (if affecting both lungs).

Between the visceral and parietal pleuraes there is the pleural cavity, that is, a very small space where a small amount of serum fluid is contained (2-3 ml for each lung).

The visceral pleura is always in contact with the parietal pleura because there is a negative pressure (-3 mmHg) between them , called the Donders negative pressure. 
This negative pressure is essential for proper breathing: if it did not exist, we would die from suffocation after a few minutes. 
This is because negative pressure plays a key role: it keeps the lung tissue and the rib cage together, so that the expansion of the chest also expands the lungs automatically.

Usually air is contained within the lungs:

  • In the bronchi,
  • In the alveoli.

In the patient with pneumothorax, there is no more negative pressure between the pleural leaflets:

  • In inspiration, the air does not enter the lungs,
  • During exhalation, the air does not go out.

The air is free in the pleural cavity, then:

  • There is no more internal negative pressure,
  • The lung is collaborative.

The consequence is variable respiratory failure depending on the severity of the pneumothorax.

 

Development of pneumothorax (pathophysiology)

Respiratory mechanics
The lung has a natural tendency to collapse because there are forces that tend to retract the tissue:

1. Pulmonary tissue fibers are composed of:

  • Elastin (which can stretch easily),
  • Collagen (fairly rigid).

At rest, the fibers undergo only a slight elongation, but during inspiration the tension increases. 
For this reason:

  • For inspiration, muscle contraction (work) is necessary,
  • The expiration is automatic because the elastic fibers return to the state of rest.

2. Surface tension is the force required to raise the surface of a liquid in contact with air. 
There are forces of attraction between the molecules of a liquid that are larger than the forces between liquid and gas (in this case, the air in the alveoli).

Superficial tension

To increase the size of the wells during inspiration, the liquid surface in contact with air needs to be increased, but important force is needed to overcome the surface tension of the liquid. 
In the alveoli there are:

  • Lots of air,
  • A thin film of liquid.

The attraction between water molecules tends to make the lungs collapse. 
However, the presence of surfactant (a liquid composed of fats and proteins) in the alveoli reduces surface tension. 
The surfactant is located on the surface between air and water, with:

  • The hydrophilic head facing the water,
  • The hydrophobic tail is in contact with the air.

The elastic forces acting in the opposite direction cause the formation of a “virtual” pleural space to have a negative pressure in relation to the atmospheric pressure. 
When anatomical communication occurs between the pulmonary alveoli and the pleural cavity, air passes from the alveolus to the pleural space (where the pressure is less). 
The penetrated air causes an increase in pressure in the pleural cavity and, therefore, a reduction in the size of the lung tissue.

The lung

  • Withdraw,
  • It collapses.

If the amount of air flowing into the pleural cavity is noticeable, the increase in intrapleural pressure tends to press:

  • The mediastinum (the central region, between the two lungs) towards the opposite hemithorax,
  • The diaphragm down.

All this can lead to:

  • Initially, a reduction in the overall “vital capacity” of the lungs,
  • Subsequently, a change of the gas exchanges.

If intrapleural pressure increases greatly, the venous return to the heart (the amount of blood returning to the heart ) is reduced, with a reduction in cardiac output.

 

Classification of pneumothorax

The  open pneumothorax occurs in case of injury of the thoracic cage and remains a continuous exchange of air between the lung, pleural space and the outside.

In the case of a severe open wound, when the patient breathes, air enters the pleural cavity:

  • Through the lungs,
  • Outside hair.

This phenomenon is called the aspiration chest wound. 
The consequence is the total pneumothorax, ie the lung:

  • Separate the pleura,
  • It collapses on itself toward the center of the thorax.

In this case, the treatment provides for the closure of the lesion and drainage of air into the pleural cavity.

The closed pneumothorax occurs when air enters the pleural cavity, but does not occur an exchange of air or gas to the outside. 
It is a partial pneumothorax.

 

Hypertensive pneumothorax

The tension pneumothorax  is caused by the formation of a one-way valve allowing air passage only of the lung to the pleural cavity. 
When the person breathes, the amount of air entering is greater than that which exits because it accumulates in the pleural cavity and causes a greater pressure than the external one. 
It is the most serious type and can cause total collapse of the lung.

It is a serious and fatal disease because it affects:

  1. Breathing : The collapsed lung moves to the opposite side and compresses the healthy lung (right image). 
    This causes severe respiratory failure.
  2. Circulation : the two vessels most affected by the displacement of the mediastinum are:
    • A veia superior cava,
    • A veia cava inferior.

Both end in the right atrium.

In the case of pneumothorax with displacement of the heart, the orifices in which the veins arrive remain fixed, then there is displacement and compression of the cavas veins . 
The consequences are:

  • An important reduction of venous return in the right atrium.
  • A decrease in blood flow to the pulmonary arteries and veins because the right heart has less blood to pump.

The left heart receives less blood, therefore reduces the cardiac output to the tissues of the body: “low cardiac output syndrome”. 
Initially, the body causes a high heart rate to bring more blood to the rest of the body. 
Then your heart rate falls until you reach:

These patients can die quickly due to acute respiratory failure.

Why is there a greater pressure in the pleural cavity than the air contained in the respiratory tract and in the external environment ? 
A valve mechanism allows air to enter but does not leave the pleural cavity. Thus, during inspiration, the air enters and increases the pressure.

 

Evolution for hypertensive pneumothorax

Many patients who present with spontaneous pneumothorax may develop a hypertensive pneumothorax due to cough . 
Coughing causes a significant increase in pressure in the respiratory tract. 
The consequence is the worsening of pneumothorax because no more air enters the pleural cord until it causes a hypertensive pneumothorax.

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