Pleural fluid

Definition of pleural fluid

Pleural fluid is defined as 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 promote breathing: acting as a lubricant, this liquid guarantees the flow of the two serous sheets.

The pleural fluid should amount to no more than 10-20 ml: maintaining a quantity equal to that reported in fact prevents the collapse of the lung. This paltry quantity of pleural fluid is continuously filtered and reabsorbed between the vascular compartment and the extravasal compartment: if the flow direction is oriented towards the outside of the capillaries, then towards the pleural fluid, we speak of filtration, whereas when the flow is direct from the pleural space to the capillaries we speak of resorption.

Some pathologies can favor the accumulation of fluid in the pleural cavity: in similar 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 to trace a definitive diagnosis, excluding or confirming the clinical suspicion formulated by the pre-tests.

Formation and reabsorption

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

The hydrostatic pressure favors filtration, therefore the leakage 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 by the vascular patency, so that the greater the arterial pressure and the greater 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.
The colloidosmotic (or simply oncotic) pressure of the plasma proteins draws the liquid towards the inside of the capillaries, therefore it favors the reabsorption of the pleural fluid. As the blood protein concentration increases, the oncotic pressure and the amount of resorption increase; vice versa, in a protein-poor blood the oncotic pressure is low and the reabsorption lower → greater quantities of liquid accumulate in the pleural cavity, as happens in the presence of serious liver diseases with reduced synthesis of plasma proteins in the liver.
It is important to emphasize that the oncotic pressure of plasma proteins is always higher than that exerted by pleural fluid proteins, present in clearly lower concentrations. As shown in the figure, the oncotic pressure prevails at the level of the venous end of the capillaries.

In 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 at around 20 cm H ₂ O 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 enclosed in the pleural cavity.

The breaking of the balance of these forces can cause all regulation and control mechanisms to go haywire. An increase in hydrostatic pressure, associated with the decrease in oncotic pressure and the pressure inside the pleural space, can 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 capacity 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, using a special needle inserted directly into the thoracic cavity (thoracentesis).

In terms of electrolytes, the composition of pleural fluid is very similar to that of plasma, but - unlike the latter - it contains a lower concentration of proteins (<1.5 g / dl).

In physiological conditions, a sub-atmospheric pressure is established in the pleural cavity, therefore negative (corresponding to -5cm H ₂ O). This pressure difference is indispensable to favor the adhesion between the two serous membranes of the pleura: in this way, the collapse of the lung is avoided.

Normally, the glucose content in the pleural fluid is similar to that of blood. The glucose concentration may decrease in the presence of rheumatoid arthritis, SLE (systemic lupus erythematosus), empyema, neoplasms and tuberculous pleuritis.

Also the pH values of the pleural fluid are very similar to those of the blood (pH ≈ 7). If this value is significantly reduced, the diagnosis of tuberculosis, hemothorax, rheumatoid arthritis, neoplasms, empyema or esophageal rupture is very likely. Otherwise, the pleural fluid assumes the characteristics of a transudate.

Pleural fluid amylase is elevated in case of neoplastic spread, esophageal rupture and pleural effusion associated with pancreatitis.

The pleural fluid appears, in 70% of the cases, with a citrine yellow color. A chromatic variation can be synonymous with pathology in place:

The presence of blood in the pleural fluid (reddish shades in the sample of fluid taken) can be a symptom of pulmonary infarction, tuberculosis and pulmonary embolism. This clinical condition is known as hemothorax.
A milky pleural fluid refers instead to the presence of kilo in the pleural cavity (chylothorax). A similar condition can originate from neoplasms, trauma, surgery or from any rupture of the thoracic duct. Pseudochylothorax (rich in lecithins-globulins) appears to be more often associated with tuberculosis 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, pleural fluid is rich in neutrophilic granulocytes.
When the pleural fluid becomes greenish or orange-colored, the presence of a high amount of cholesterol is very likely.

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

ESSUDATES can be an expression of both phlogistic processes of pleura and neoplasms. A pleural exudate has a high content in protein terms (> 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 typical effusions of bacterial infections, species supported by Staphylococcus aureus, Klebsiella and other gram negative bacteria (typical of the empyema). Detection of an 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 cancer.

Exudative pleural fluid

Protein pleural fluid / plasma protein ratio> 0.5

Proteins LP> 3g / dl

LDH in pleural fluid / LDH plasma> 0.6

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

pH 7.3-7.45

Transudative pleural fluid

A TRANSLATIVE-type pleural fluid is the result of the increase in hydrostatic pressure in the capillaries, associated with the reduction of the oncotic one. In similar situations, the pleurae are healthy. The detection of a transudative pleural fluid is often the expression of cirrhosis, congestive heart failure, nephrotic syndrome and pulmonary embolism, conditions linked by the reduction of plasma proteins (↓ oncotic pressure) and / or increase in arterial pressure (↑ hydrostatic pressure). The pH of the transudative pleural fluid is generally between 7.4 and 7.55.

The differential diagnosis between exudate and transudate is obtainable by measuring proteins and LDH in pleural fluid and serum.