Anatomy of the pleura
- is a serous membrane that covers the lungs
- made up of 2 layers: visceral, parietal
a) viseral: covers lungs and extends into fissures
b) parietal: lines inside of the thoracic cavity
Pleural space -characteristics/comopsition/function
- 10-20 u thick
- thin layer of fluid
- lubricates movement between the lung and chest wall
Visceral pleura -composition
- primarily connective tissue
- covers lung
Parietal pleura -composition
- thin layer of loose connective tissue
- contains blood vessels and lymphatic lacunae
- covered by a thin layer of mesothelial cells
- below it is enothoracic fascia
Lymphatic stroma - where found
Only within the parietal pleura
Lymphatic stroma -function
Remove fluid from the pleural space
Visceral pleura -function
Contributes to the elastic recoil of the lung
Parietal pleura - function
Fluid is produced and re-absorbed here
Normal pleural fluid - volume
- 8 ml per side
- forms at 0.01 ml/kg/hr
- about 15-20 ml per day in 70 kg adult
Normal steady state of pleural fluid
-absorption = production
Visualization of pleural fluid
- not normally visible on chest x-ray or CT scan
- about 150 ml per side are necessary to see pleural effusion on plain chest x-ray
Normal pH of pleural fluid
-about 7.6 normally
How does pleural fluid form
- a result of differences in hydrostatic and osmotic pressure between vessels and the pleural space
- parietal pleura is the most important surface for fluid formation + the synthetic vessels that supply pleural surfaces (not from pulmonary circulation)
Hydrostatic pressure
- pressure exerted by liquid (i.e. column of fluid) at equilibrium
- in the lungs this reflects the pulmonary venous pressures
Oncotic pressure
- osmotic pressure due to proteins and osmoles in the plasma
- draws fluid into the capillaries
Balance of oncotic and hydrostatic pressures (contributing to net pressure and gradient in the pleural space)
Flow = change in hydrostatic pressure (parietal pleura vs. visceral pleura) - change in oncotic pressure (parietal pleura vs. visceral pleura)
i.e. the Starling Equation
Because parietal and visceral pleura exterting opposing forces across the pleural space (for both oncotic and hydrostatic)
2 main causes of accumulation of pleural fluid in disease states
- Increased formation
2. Impaired absorption
Cause of increased fluid entry
- Increase in permeability due to disruption of endothelial cell layer (changing K constant -capillary filtration coefficient)
- Increase in microvascular pressure (i.e CHF)
- Decreased pleural pressure (ie in atelectasis)
- Decreased plasma oncotic pressure (ie in nephrotic syndrome, hypoalbuminemia)
Other causes of increased fluid entry - from outside the thoracic cavity
- diaphragm permeable excess fluid from other sources can enter the pleura (i.e. communication from other organs)
a) hepatic hydrothorax - sucked into chest across diaphragm from abdomen
b) urinothorax - abnormal communication from renal collecting system
c) chylothorax - abnomal communication with thoracic duct
Mechanisms of fluid accumulation due to increase permeability i.e. what is source of increased permeability
- infection
- malignancy
- inflammation
Mechanism of pleural fluid accumulation due to shift of starling forces
1) Things which decrease the oncotic gradient
- hypoalbuminemia
2) Things which increase the hydrostatic gradient
- CHF
- Atelectasis
Causes of decreased fluid exit
- factors that impair lymphatic drainage
- in many disease states the accumulation of fluid is multi-factorial
Common Hx for patient with pleural effusion
- SOB
- pleuritic chest pain
- asymptomatic
Physical exam for patient with pleural effusion
- percussion: dullness over area of effusion with decreased tactile fremitus (how does this differ from consolidation in the lung)
- auscultation: decreased air entry
Imaging of pleural effusion -modalities
- Ultrasound
- close to 100% sensitive for detecting pleural fluid
- good to identify loculated fluid vs. free-flowing - CT contrast
- especially good for visualizing pleural surface (thickening, tumors, etc)
Treatment pleural effusion
Thoracentesis
Thoracentesis -how?
- Insert needle through chest wall into the pleural space
- Drain pleural fluid
- Diagnostic and therapeutic
- can be done with U/S guidance at bedside
Analysis of pleural fluid
A) Observation of fluid at bedside 1) Note color 2) Note smell B) Chemistry (protein, LDH and albumin) C) WBC count and % differential D) Cytology (examine for cancer cells) E) pH and glucose F) Gram stain and cultures (can include AFB and TB culture if indicated)
Classification of pleural effusions
- Transudates
- Exuates
- based on the mechansm of fluid formation and pleural flui chemistry
Transudate -general mechanism
-result from an imbalance in oncotic and hydrostatic pressure
Exudate - general mechanism
-result of inflammation of the pleura or decreased lymphatic drainage
Differentiating transudate vs exudate
Use light’s criteria (analysis of pleural fluid)
- is 85% sensitive for exudate
a) fluid protein/serum protein > 05
b) fluid LDH/serum LDH > 0.6
c) Fluid LDH > 2/3 upper limit of normal
Cell count and differential for pleural fluid analysis
> 85% lymphocytes -TB, cancer, rheumatoid > 10% eosinophils -pneumothorax, hemothorax Neutrophils -very high in infections
Characteristics of trasudate
- low protein and LDH in pleural fluid
- implies intact endothelial membrane
- fluid accumulation from increased hydrostatic pressure or decreased oncotic pressure
Characteristics of exudate
- high protein and LDH in pleural fluid
- implies disruption of endothelial membrane
Differential diagnosis Transudate
LUCKI ME L-liver (hepatic hydrothorax) U - urinothorax C- CHF K- Kidney (low protein state - nephrotic) I - iatrogenic M-myxedema E -embolic
Exudate
-pretty much everything else (including malignancy, infection, PE, serositis due to connective tissue disease etc)
Approach to managing plural effusion
- Look for underlying cause
- Thoracentesis for fluid analysis
- Transudate
- work up for differential diagnosis (i.e. CHF, urinalysis, urea, creatinine, liver function, TSH, etc) - Exudate
- evaluate for clues to underlying cause
a) infection
b) malignant
c) pulmonary embolism
Why would check albumin in pleural effusion
Check gradient between pleural fluid and serum
-help to confirm transudate
Amylase
- not routinely indicated
- order if suspect esophageal rupture into pleural space or pancreatic diseases (rupture of a pseudocyst)