Pleural fluid

Definition of pleural fluid

It defines itself pleural fluid the fluid interposed between the two serous sheets that make up the pleura, that double layer of connective tissue having the function of supporting and covering the lungs. An adequate amount of pleural fluid is essential to facilitate breathing: acting as a lubricant, this liquid ensures the sliding of the two serous sheets.

The pleural fluid should amount to no more than 10-20 ml: maintaining an amount equal to that reported prevents the collapse of the lung. This negligible quantity of pleural fluid is continuously filtered and reabsorbed between the vascular and extravasal compartments: if the flow direction is oriented towards the outside of the capillaries, therefore towards the pleural fluid, we speak of filtration, while when the flow is directed from the pleural space to the capillaries we speak of resorption.
Some pathologies can favor the accumulation of fluid in the pleural cavity: in such situations, the analysis of the pleural fluid is essential to identify the triggering cause. The chemical-physical, microbiological and morphological examination of the pleural fluid is very useful for tracing a definitive diagnosis, excluding or confirming the clinical suspicion formulated through the pre-tests.

Formation and reabsorption

The production of pleural fluid, like that of all fluids interposed between a vascular and an extravascular side, is heavily conditioned by Starling's law. This law describes the role of hydrostatic pressure and oncotic pressure in the movement of fluid (pleural fluid) across capillary membranes.

1. The hydrostatic pressure favors the filtration, therefore the escape of the liquid from the capillaries towards the pleural cavity; this pressure depends on the acceleration of gravity on the blood imposed by the heart and the vascular patency, so the higher the arterial pressure and the higher the hydrostatic pressure, and vice versa. As shown in the figure, the hydrostatic pressure prevails at the level of the arterial extremity of the capillaries.
2. The colloidosmotic pressure (or simply oncotic) of the plasma proteins draws the liquid towards the inside of the capillaries, thus favoring the reabsorption of the pleural fluid. As the protein concentration of the blood increases, the oncotic pressure increases and the extent of reabsorption; vice versa, in a blood poor in proteins the oncotic pressure is low and the reabsorption less → greater quantities of fluid accumulate in the pleural cavity, as happens in the presence of severe liver diseases with reduced synthesis of plasma proteins in the liver.
   It is important to underline that the oncotic pressure of the plasma proteins is always higher than that exerted by the proteins of the pleural fluid, present in much lower concentrations. As shown in the figure, the oncotic pressure prevails at the level of the venous end of the capillaries.

Under physiological conditions, the entity of the two processes (hydrostatic and oncotic) is balanced → there is NO variation of the pleural fluid

The pulmonary circulation that irrigates the visceral pleura has an oncotic pressure identical to that of the general circulation, but in its capillaries the hydrostatic pressure is significantly lower, estimated around 20 cm H2O less.

• In the visceral pleura, the pleural fluid tends to be drawn from the pleural cavity towards the capillaries: for this reason, the forces of recall of the fluid towards the intravascular compartment prevail.

The delicate interweaving between the reabsorption and filtration forces, combined with the permeability of the capillary wall, the total surface of the two pleural membranes and the filtration coefficient, guarantees the balance between production and reabsorption of the liquids contained in the pleural cavity.

The breaking of the balance of these forces can send all the mechanisms of regulation and control into a tailspin. An increase in hydrostatic pressure, associated with a decrease in oncotic pressure and pressure inside the pleural space, can also favor serious diseases, such as pleural effusion.

Starling's law

Starling's law Q = K [(Pi cap - Pi pl) - σ (π cap-π pl)]

Q → liquid flow [ml / min]
K → filtration constant (proportionality constant) [ml / min mmHg]
Pi → hydrostatic pressure [mmHg]
π (pi) → oncotic pressure [mmHg]
σ (sigma) → reflection coefficient (useful for evaluating the ability of the capillary wall to oppose the flow of proteins with respect to water)

[(Pi cap - Pi pl) - σ (π cap - π pl) → net filtration pressure

Generalities and types

A sample of pleural fluid is collected by aspiration, through a special needle inserted directly into the thoracic cavity (thoracentesis).
In terms of electrolytes, the composition of the pleural fluid is very similar to that of plasma, but - unlike the latter - it contains a lower protein concentration (<1.5 g / dl).
Under physiological conditions, a subatmospheric pressure is established in the pleural cavity, therefore negative (corresponding to -5cm H2O). This pressure difference is essential to favor the adhesion between the two serous membranes of the pleura: by doing so, the collapse of the pleura is avoided. lung.
Normally, the glucose content in the pleural fluid is similar to that of the blood. The glucose concentration may decrease in the presence of rheumatoid arthritis, SLE (systemic lupus erythematosus), empyema, neoplasms and tuberculous pleurisy.
The pH values ​​of the pleural fluid are also very similar to those of the blood (pH ≈ 7). If this value undergoes a significant reduction, the diagnosis of tuberculosis, hemothorax, rheumatoid arthritis, neoplasms, empyema or esophageal rupture is very likely. Otherwise, the pleural fluid takes on the characteristics of a transudate.
Pleural fluid amylase is elevated in cases of neoplastic spread, esophageal rupture and pleural effusion associated with pancreatitis.

The pleural fluid shows a citrine yellow color in 70% of cases. A chromatic variation can be synonymous with an ongoing pathology:

• The presence of blood in the pleural fluid (reddish tinges in the fluid sample) can be a symptom of pulmonary infarction, tuberculosis and pulmonary embolism. This clinical condition is known as hemothorax.
• A milky pleural fluid, on the other hand, refers to the presence of kilo in the pleural cavity (chylothorax). A similar condition can arise from cancer, trauma, surgery, or any rupture of the thoracic duct. Pseudochylothorax (rich in lecithin-globulins) seems to result more often from tuberculous diseases and rheumatoid arthritis.
• The purulent aspect of the pleural fluid takes on a further pathological significance: we speak of pulmonary empyema, expression of tuberculosis, subphrenic abscesses or bacterial infections in general. In this case, the pleural fluid is rich in neutrophilic granulocytes.
• When the pleural fluid takes on a greenish or orange color, it is very likely the presence of a high amount of cholesterol.

The analysis of the pleural fluid gives an idea of ​​the possible pathology that afflicts the patient: in this regard, a distinction is made between exudative and transudative pleural fluid.

Exudative pleural fluid

Definitions:

• The exudate is a liquid of variable consistency that forms during acute inflammatory processes of various kinds, accumulating in the tissue interstices or in the serous cavities (pleura, peritoneum, pericardium).
• the transudate is not formed as a result of inflammatory processes and as such is devoid of proteins and cells; instead it derives from the increase in venous pressure (therefore capillary), in the absence of increased vascular permeability.

EXUDATES can be the expression of both inflammatory processes of the pleura and neoplasms. A pleural exudate has a high protein content (> 3g / dl) and a density generally higher than 1.016-1.018.

An exudative pleural fluid is rich in lymphocytes, monocytes, neutrophils and granulocytes; these inflammatory cells are the expression of effusions typical of bacterial infections, species sustained by Staphylococcus aureus, Klebsiella and other gram negative bacteria (typical of empyema). Detection of exudative pleural fluid requires differential diagnosis. The most frequent causes of exudative pleural effusion are rheumatoid arthritis, cancer, pulmonary embolism, lupus erythematosus, pneumonia, trauma and tumor.

Exudative pleural fluid

Pleural fluid / plasma protein ratio> 0.5

LP proteins> 3g / dl

LDH in pleural fluid / LDH plasma> 0.6

Pleural fluid LDH> 200 IU (or in any case greater than 2/3 of the upper limit of the reference range for LDH in serum)

pH 7.3-7.45

Transudative pleural fluid

A transudative pleural fluid is the result of the increase in hydrostatic pressure in the capillaries, associated with the reduction of the oncotic pressure. In similar situations, the pleurae are healthy. The detection of a transudative pleural fluid is often an expression of cirrhosis, congestive heart failure, nephrotic syndrome and pulmonary embolism, conditions associated with a reduction in plasma proteins (↓ oncotic pressure) and / or an increase in blood pressure (↑ hydrostatic pressure). The pH of transudative pleural fluid is generally between 7.4 and 7.55.
The differential diagnosis between exudate and transudate can be obtained by measuring the proteins and LDH in the pleural fluid and in the serum.