Disorders of the pleural space Flashcards
What is the pleural space
The pleural space is a potentiial space surrounding the lungs, containing a minimum of fluid (probably < 1.5 ml in the cat) for lubrication, allowing smooth movement of the lung lobes during respiration
- it is formed by the visceral and parietal pleura, which are serous membranes that line the outer surface of the lungs and inner surface of the thoracic cavity, respectively
The parietal pleura receives blood supply from the systemic circulation
The visceral pleural receives blood from the lower pressure pulmonary circulation
There is normally a flow of pleural fluid therefore, from the parietal pleura, into the pleural space from where it is rapidly absorbed by the visceral pleura
What are the pressures governing the flow of fluid in the pleural space
Pressures governing the flow of fluid are:
- hydrostatic (generated by the heart during systole) and
- oncotic (primarily generated by serum albuminn)
What are the mechanisms conducting to fluid accumulation in the pleural space
Alterations to the balance of pressures (hydrostatic and oncotic) will cause an accumulation of fluid in the pleural space
Fluid may accumulate as a result of:
- raised capillary hydrostatic pressure (e.g., congestive heart failure)
- lowered capillary oncotic pressure (e.g., hypoalbuminemia)
- increased capillary permeability (e.g., inflammation)
- obstruction to lymphatic drainage (e.g., neoplasia)
Because the mediastinum is fenestrated, fluid usually accumulates bilaterally unless it becomes loculated due to inflammation
What are the causes of pyothorax
Parapneumonic spread of infection following colonisation of lung tissue by oropharyngeal flora (i.e., obligate and facultative anaerobes) is suspected to be the most common source of infection, but pyothorax can occur from a number of sources:
- from a penetrating wound
- via hematogenous spread
- from a migrating foreign body
- from local extension of infection (e.g., esophageal rupture)
What is the cytological appearance of pyothorax
Cytological examination of the fluid reveals large numbers of neutrophils, including many degenerate neutrophils, and both intra- and extra-cellular bacteria are often visible
What is the difference between chylous effusions and pseudochylous effusions
Chylothorax is caused by leakage of chyle (rich in chilomicrons) from the thoracic duct into the pleural space
Chylous effusions have a milky-white appearncae
Psudochylous effusions are similar in appearnce to chyle but are reported to be due to the presence of cholesterol or lecithin in the fluid (probably derived from degenerated mesothelial cells)
- pseudochylous effusions are exceptionally rare in cats
How can you make a definitive daignosis between chylous effusion and pseudochylous effusion
Definitive diagnosis of chylothorax (and differentiation from pseudochyle) is achieved by demonstrating chylomicrons on agar lipoprotein electrophoresis
A more practical test is to measure serum and fluid triglyceride and cholesterol levels
- in chylothorax, because of the chyle present, the triglyceride levels in the fluid are typically higher than that of serum, and cholesterol levels are equivalent or lower
What are the four most common causes of chylothorax in cats
The four most common causes in cats are:
- anterior mediastinal neoplasia (e.g., thymic lymphoma)
- cardiomyopathy
- trauma
- idiopathic
Experience suggests that cardiomyopathy (usually hypertrophic or restrictive forms) is the single most common cause of chylothorax and unless another cause is obvious, echocardiography is always recommended for full cardiac assessment
What is the typical characterisation of breathing in a cat with pleural disease
Cats with significant pleural space disease adopt a sternal position with abducted elbows
A restrictive (rapid, shallow) respiratory pattern with increased inspiratory effort is typical
What are the main causes for a restrictive respiratory pattern
Pulmonary parenchymal pathology (e.g., pulmonary edema, pneumonia)
Disorders of the chest wall, diaphragm, peritoneal cavity or peripheral nerves
What should be your main differentials with unilateral effusion on a DV X-ray
Unilateral effusion should raise the index of suspicion for:
- pyothorax
- chylothorax
What are the radiographic signs of pleural effusion
Interlobar fissure lines
Rounding of the lung margins at the costophrenic angles
Retraction of the lobar borders from the thoracic wall
Widening of the mediastinum
Scalloping of the lung margins at the sternal border
Effacing of cardiac silhouette
Dorsal displacement of the trachea
What is the typical classification of pleural fluid based on total protein concentration and total nucleated cell count
Transudate
- < 25 g/L
- < 1000 TNCC/µL
- low numbers of mononuclear cells +/- nondegenerate neutrophils
- uncommon in cats. Rule out:
- CHF
- Hypoalbuminemia (albumine needs to be <15 g/L)
- Fluid overload
Modified transudate
- 25-35 g/L
- < 5000 TNCC/µL
- majority of mononuclear cells (macrophages, lymphocytes) +/- nondegenerate neutrophils
- differentials ranked according to clinical criteria and gross characteristics of fluid
Exudate
- > 30 g/L
- > 5000 TNCC/µL
- neutrophil is the predominant cell population
- may be subclassified as septic, non-septic, chylous, neoplastic
- rule out: FIP, infection, neoplasia
What are the specificity of chylous effusions
Chylous effusions may have TP/TNCC of a modified transudate or an exudate
They are defined as effusions with a triglyceride concentration of > 100 mg/dL (>1.12 mmol/L)
How can you confirm hemothorax
Hemothorax is variably defined as fluid with a hematocrit of at least 25% or 50% of the peripheral blood hematocrit
Where hemorrhage has been present for more than 1 h no platelets will be seen on pleural effusion smears and the blood will be defibrinated so it will not clot in a plain tube
What is the main differential for translucent, yellow effusions with a protein content > 30 g/L
FIP is a major differential
The effusion associated with FIP is typically viscous, froths on agitation due to its high protein content, and clots on standing
What are the characteristics of a pleural effusion due to FIP? How can you confirm the diagnosis
While the protein content is consistent with an exudate, the TNCC, comprising neutrophils and macrophages, is low, consistent with a modified (<5000 cells/µL) or pure (<1000/µL) transudate
The single most useful test for ruling in FIP as a cause of pleural effusion is the use of immunofluorescence to detect feline coronavirus antigen in macrophages
How would you explain a false negative result for FIP with a immunofluorescence test on an effusion sample
A negative result may occur in a cat with FIP if there are low numbers of macrophages or if epitope masking by patient antibody occurs
Rivalta’s test can be used where immunofluorescence is not available, or to support a negative finding on immunofluorescence
- this test identifies exudates based on their ability to retain their shape in a dilute acetic acid solution
What parameters can help diffferentiate cardiogenic, neoplastic and atypical infectious effusions
LDH
- is a marker of pleural inflammation
- LDH < 226 IU/L = cardiogenic effusion
- LDH > 226 IU/L = neoplastic effusion
Infectious effusions
- LDH typically > 200 IU/L
- pH is =<6.9
- glucose is usually < 0.5 g/L
- > 85% neutrophils
Neoplastic effusions
- LDH > 226 IU/L
- pH = > 7.4
- low neutrophil count (<30%)
- in the absence of trauma, a red blood cell count > 50000/µL is indicattive of neoplasia
What is the sign of pleural fibrosis seen on X-ray
Rounding of the borders of the lung lobes is consistent with pleural fibrosis seen with long-standing or irritant effusions
Fibrosing pleuritis is a cause of persistent dyspnea after pleural fluid drainage
What is the signification of the absence of a glide sign on TFAST
Absence of a glide sign dorsally is suggestive of pneumothorax
- if combined with absence of lung sounds on auscultation then this will confirm the diagnosis
What is the signification of A-lines
A-lines are below the pleural glide sign and are represented by a serie of parallel lines in the horizontal plane
They represent the resonance of ultrasound between pulmonary interlobular fissures
What is the signification of B-lines
B-lines are echogenic lines which extend from the pleural slide to the bottom of the ultrasound sector angle
- they represent an artefact caused by an air-fluid interface
The prsence of B-lines suggests increased lung fluid
- pulmonary edema
- pneumonia
- hemorrhage
- neoplasia
- any inflammatory process
