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Which patients with a parapneumonic effusion need a chest tube?

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Hospitalized patients with pneumonia who develop a complicated parapneumonic effusion or empyema need to undergo chest tube placement.

WHAT IS PARAPNEUMONIC EFFUSION?

Parapneumonic effusion is a pleural effusion that forms concurrently with bacterial or viral pneumonia. Up to 40% of patients hospitalized with pneumonia develop a parapneumonic effusion.1 The effusion progresses through a continuum of 3 stages: uncomplicated, complicated, and empyema.

Uncomplicated parapneumonic effusion is an exudative effusion without bacteria or pus that is caused by movement of fluid and neutrophils into the pleural space. Pneumonia itself causes an increase in interstitial fluid and capillary leakage. The effusion becomes complicated as a result of bacteria invading the pleural space, causing a further increase in neutrophils in the pleural fluid. Empyema is defined as the presence of frank pus in the pleural space.

CLINICAL SIGNIFICANCE

According to the US Centers for Disease Control and Prevention, pneumonia accounts for 674,000 emergency department visits each year; of the patients hospitalized, up to 40% develop a parapneumonic effusion.2 The only study done on rates of death associated with parapneumonic effusion showed that, compared with patients with no effusion, the risk of death was 3.7 times higher with a unilateral effusion and 6.5 times higher with bilateral effusions.3

INITIAL EVALUATION

The initial evaluation of suspected parapneumonic effusion should include chest radiography with lateral or decubitus views, followed by thoracentesis if indicated. If thoracentesis is performed, the fluid should be tested as follows:

  • Gram stain
  • Appropriate cultures based on clinical scenario (eg, aerobic, anaerobic, fungal)
  • Total protein in pleural fluid and serum
  • Lactate dehydrogenase (LDH) in pleural fluid and serum
  • Glucose
  • pH.

CLASSIFICATION OF EFFUSIONS

When pleural fluid is obtained, the total protein and LDH levels are used to categorize the effusion as either transudative or exudative based on the Light criteria.4 An effusion is confirmed as exudative when 1 of the following 3 criteria is met:

  • The ratio of pleural fluid protein to serum protein is greater than 0.5
  • The ratio of pleural fluid LDH to serum LDH is greater than 0.6
  • The pleural fluid LDH is greater than two-thirds the upper limit of normal for the serum LDH.
Parapneumonic effusion: Categories, management, and prognosis
Once an effusion is characterized as exudative, the American College of Chest Physicians consensus guidelines are used for further classification and prognostication.5 The guidelines estimate the risk of poor outcome by dividing the effusions based on their characteristics: the anatomy of the effusion, the bacteriology of the fluid, and the chemistry of the pleural fluid (Table 1). Based on these guidelines, effusions are divided into 4 categories. Category 1 and 2 effusions are considered uncomplicated, category 3 is considered complicated, and category 4 is empyema. The outcomes of interest are prolonged hospitalization, prolonged systemic toxicity, increased ventilator impairment, increased morbidity rate, and increased mortality rate.5

Category 1 effusions are defined as free- flowing fluid with a thickness of less than 10 mm on any imaging modality. Thoracentesis for pleural fluid analysis is not required. The prognosis is very good.

Category 2 effusions are defined as free- flowing fluid with a thickness greater than 10 mm and less than 50% of the hemithorax. Thoracentesis is typically done because of the size of the effusion, but Gram stain and culture of the pleural fluid are usually negative, and the pH is at least 7.2. The prognosis is good.

Category 3 effusions are considered complicated because the anatomy of the pleural space becomes altered or because bacteria have invaded the pleural space. The effusion is larger than 50% of the hemithorax or is loculated, or the parietal pleura is thickened. Since the bacteria have invaded the pleural space, Gram stain or culture of pleural fluid may be positive, the pleural fluid pH may be less than 7.2, or the glucose level of the fluid may be less than 60 mg/dL. The prognosis for category 3 is poor.

Category 4 effusions are defined as empyema. The only characteristic that separates this from category 3 is frank pus in the pleural space. The prognosis is very poor.

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