1

Describe the anatomy of the pleural space (Figure 1)

The pleural space is a potential area between the parietal and visceral pleural layers. In healthy individuals, a very small volume of pleural fluid is present in this space as a thin film.

The parietal pleura lines the inside of the chest wall, thoracic surface of the diaphragm and the lateral boundary of the mediastinum.

The visceral pleura covers the surface of the lung.

The two pleurae are in continuity by forming a reflective cuff around the hilum.

2

Describe the surface anatomy of the pleura (Figure 2)

Superiorly, the pleural margin starts 2cm above the medial third of the clavicle.

Anteriorly, it meets the contralateral pleura beneath the sternum until the 4th rib.

The lower limits of the pleura are:

a)

parasternally – 6th costal cartilage;

b)

mid-clavicular line – 8th rib;

c)

mid-axillary line – 10th rib;

d)

posteriorly – it travels along the 12th rib.

3

Describe the blood supply and lymphatic drainage of the pleura

The pleura has a dual blood supply from systemic and pulmonary arteries.

Parietal pleura:

a)

arterial supply – somatic arteries, such as intercostal, internal thoracic and musculophrenic arteries;

b)

venous drainage – intercostal, internal thoracic and musculophrenic veins;

c)

lymphatic drainage – via multiple microscopic stomata into the pleural space and the lymphatic circulation of the chest wall, draining into the intercostal, parasternal, posterior mediastinal and diaphragmatic lymph nodes.

The visceral pleura (via the lung parenchyma):

a)

arterial supply – branches of the bronchial and pulmonary arteries;

b)

venous drainage – tributaries of the bronchial and pulmonary veins;

c)

lymphatic drainage – pulmonary lymphatic vessels.

4

What is the composition of pleural fluid?

Pleural fluid is a clear ultrafiltrate of plasma and is composed of:

a)

cellular elements, which include a small number of lymphocytes, macrophages and mesothelial cells:

i)

no red blood cells;

ii)

white blood cell count <1000/mm³;

b)

non-cellular elements, which are similar in composition to interstitial fluid, containing proteins, glucose, ions (such as H⁺, Na⁺ and HCO₃^–) and enzymes (such as lactate dehydrogenase [LDH]):

i)

pH = 7.60-7.64;

ii)

protein content = 10-20g/L;

iii)

pleural fluid glucose level similar to plasma glucose level;

iv)

LDH <50% serum level.

5

What is the mechanism of pleural fluid turnover?

Pleural fluid is continuously produced and absorbed by the pleural surface. This amount ranges between 250-1000mL every day. In healthy individuals, 0.1-0.2mL/kg of fluid is present in the pleural space at any given time.

The turnover of the pleural fluid occurs at the level of the parietal pleural and is governed by the differences in hydrostatic and oncotic pressures in the capillaries and pleural space (Starling’s forces) (Figure 3).

Intrapleural pressures remain subatmospheric throughout the respiratory cycle and become more negative during the inspiratory phase. Within the pleural space, the apex is more negative than the base. The fluid filters from the capillaries of the parietal pleura and is drained by the lymphatic network that connects directly to the pleural cavity through the lymphatic stomata. Therefore, filtration mostly occurs at the apex and drainage towards the absorption sites at the bottom of the cavity and adjacent to the mediastinum (Figure 4).

These parameters change when disease processes affecting the adjacent lung or other tissues activate an immune response. An inflammatory increase in proteins and other large molecules will increase the oncotic pressure within the pleural space and cause fluid accumulation.

Pleural effusion can therefore develop as a result of:

a)

increased pulmonary capillary pressure;

b)

increased capillary membrane permeability;

c)

decreased intrapleural pressure;

d)

decreased plasma oncotic pressure;

e)

obstructed lymphatic drainage.

6

What is a pleural effusion?

A pleural effusion represents the presence of excessive fluid or fluid with an abnormal composition in the pleural space.

7

What are the types of fluid that may accumulate within the pleural space?

Serous fluid.

Blood.

Chyle.

Pus.

Contents of the digestive system (ruptured oesophagus).

Ascitic fluid (through the diaphragm).

Iatrogenic compounds (such as total parenteral nutrition).

8

What are the presenting symptoms of a patient with a pleural effusion?

Patients with small pleural effusions can be asymptomatic.

Otherwise they may present with dyspnoea, chest pain, cough or constitutional symptoms (such as malaise, fever and weight loss).

9

What are the signs of a pleural effusion?

Tracheal deviation to the contralateral side (with a very large pleural effusion).

Reduced ipsilateral chest movement.

Decreased tactile vocal fremitus.

Stony dull percussion note.

Reduced ipsilateral air entry.

Bronchial breathing over the lung compressed by the effusion.

As the presence of a pleural effusion can be a manifestation of other systemic diseases, symptoms and signs relevant to the affected system may be present, such as hepatomegaly, ascites, cardiomegaly, murmur of mitral regurgitation or raised jugular venous pulse.

10

How are pleural effusions classified?

Pleural effusions are generally differentiated on the basis of their protein content, provided the total serum protein is within normal limits:

a)

transudate – with a lower protein content (<30g/L), as capillary permeability is normal;

b)

exudate – with a higher protein content (>30g/L), due to increased capillary permeability.

11

What are Light’s criteria?

Light described three criteria that define an exudate. These are used especially when a patient has hypoproteinaemia, although they can also be applied to patients with normal serum protein levels.

An exudate should meet at least one of the following criteria:

a)

pleural/serum protein ratio >0.5;

b)

pleural/serum LDH ratio >0.6;

c)

LDH level more than two-thirds over the upper normal serum level.

12

What are the causes of a transudate?

Left ventricular failure.

Cirrhosis.

Hypoalbuminaemia.

Atelectasis.

Renal failure.

Peritoneal dialysis.

Pulmonary embolus (10-20%).

Malignancy (5%).

Mitral valve disease.

Constrictive pericarditis.

Ovarian hyperstimulation.

Meigs’ syndrome.

13

What are the causes of an exudate?

Malignancy (95%), such as mesothelioma (Figure 5) and lung carcinoma.