TOC | Pulm

Parapneumonic effusion

It is a common complication of bacterial pneumonia (about 40%). Most parapneumonic effusions are small and resolve with appropriate antibiotic therapy. Thoracentesis should be done if the effusion layers out to a thickness equal to or greater than 10 mm on the lateral decubitus chest radiograph to determine as early as possible if a chest tube should be placed.

Pathophysiology:

The spectrum of parapneumonic effusions has been divided into three stages, although they are not sharply defined and represent a point on a continuous spectrum.

1. The exudative stage which results from a focus of parenchymal infection leading to increased pulmonary interstitial fluid. Some of this fluid crosses the visceral pleura and accumulates as a small sterile pleural effusion. The pleural fluid is an exudate with primarily PMNs, a normal glucose level, and a normal pH level. Antibiotics begun in this stage will affect resolution of both the pneumonic and pleural process. The initial sterile free flowing exudative parapneumonic effusions may rapidly progress (within a day) to the second stage.
2. The fibropurulent stage characterized by infection of sterile pleural fluid. Pleural fluid further accumulates and contains many PMNs, bacteria, and cellular debris. Fibrin deposits cover both the visceral and parietal pleura. Fibrin membrane partitions result in loculated effusion. This loculation makes complete pleural space drainage difficult. During this stage, the pleural fluid pH and glucose levels become low, LDH levels increase, and bacterial organisms or frank pus may be present.
3. The organization stage. Fibroblasts grow into the exudate from both the visceral and parietal pleural surface to produce an inelastic membrane called the pleural peel. These untreated effusions may also drain spontaneously through the chest wall (empyema necessitans) or into the lung to produce a bronchopleural fistula.

Bacteriology:

Anaerobic organisms are responsible for the majority of culture-positive empyemas. In a study by Bartlett et al. (Lancet 1:338-340, 1974), in patients with positive pleural fluid cultures, 35% had only anaerobic organisms, 41% had both anaerobic and aerobic organisms, and 24% had only aerobic organisms. Therefore anaerobic cultures should always be obtained.

Approximately 35% of patients with anaerobic pneumonia will have a culture-positive pleural effusion, whereas fewer than 5% of patients with parapneumonic effusions secondary to pneumococcal pneumonia will have culture-positive pleural effusions (Light et al. Am. J. Med. 69:507-511, 1980).

Clinical Manifestations

Aerobic bacterial pneumonia with pleural effusion usually presents with an acute febrile illness consisting of chest pain, sputum production, and elevated WBC count. By contrast, anaerobic bacterial infections involving the pleural space usually present with a subacute illness. Anaerobic infections should be considered in patients with a history of alcoholism, an episode of unconsciousness, or other factors that predispose them to aspiration. The majority of patients with anaerobic pleuropulmonary infections will have significant weight loss, leukocytosis and mild anemia.

Diagnosis

In order to state a patient has a parapneumonic effusion, a bacterial pneumonia needs to be diagnosed. Other possibilities for pulmonary infiltrates and pleural effusions need to be considered and may include pulmonary embolism, acute pancreatitis, Dressler's syndrome, and other diseases.

With the involved side dependent, if the thickness exceeds 10 mm, a diagnostic thoracentesis should be performed immediately. An empyema or complicated pleural effusion can be identified only by examination of the pleural fluid. Clinical evaluation is inadequate to predict simple sympathetic effusion from complicated effusion.

Pleural fluid should be examined grossly for color, turbidity, and odor. If pus is present, the pleural space should be drained via tube thoracostomy. In the absence of gross pus, pleural fluid is sent for other studies to assist in determining if immediate chest tube placement is required. These pleural fluid studies should include glucose, LDH, amylase, protein, pH, WBC count plus differential, Gram stain, aerobic and anaerobic bacterial cultures. Depending on the clinical setting, fungal and mycobacterial smears and cultures and cytological studies should be done. Accurate pH measurements requires that the pleural fluid be collected anaerobically in a heparinized syringe (0.2 ml of 1:1000 heparin is drawn up into the syringe and then flushed to eliminate any artifact due to low pH of heparin). The syringe is then placed on ice, and the pH should be measured expeditiously.

Treatment:

Antibiotics after sputum, blood, or pleural fluid cultures obtained.

Thoracentesis is fluid over 1 cm thick.

Chest tube drainage for grossly purulent pleural fluid/empyema. It is to be removed when drainage becomes serous and < 50 ml per day.

[In the Textbook of Respiratory Medicine by Murray and Nadel (2nd edition), Richard Light offers the following recommendations for implementing chest tube drainage: 1) purulent pleural fluid, 2) positive pleural fluid Gram stain, 3) pleural fluid glucose less than 50 mg/dl, 4) pleural fluid pH below 7.00 and 0.15 units lower than arterial pH. If these four criteria are not met, a chest tube should still be considered if pleural fluid pH is below 7.20 or if pleural fluid LDH is above 1000 IU/L. ]

Nonetheless, with the guidelines, if the pleural fluid characteristics are borderline for chest tube placement, repeating a thoracentesis at 12- to 24- hours may be helpful. If the pleural fluid LDH decreases and the pH and glucose increases, the patient is likely improving. But if the LDH is increasing and the pH and glucose are decreasing a chest tube should be placed.

Intrapleural Fibrinolytics

One major reason for failed drainage is tube obstruction by organized empyema and multiple pleural space loculations. If the decision is made to use fibrinolytics, the usual dose of streptokinase is 250,000 U, in 30 to 60 ml normal saline given intrapleurally via chest tube. The usual dose of urokinase is 100,000 U diluted in the same manner. It appears that streptokinase and urokinase are equally effective. The chest tube is clamped for 1 to 2 hours. This treatment can be repeated for up to 14 days. The decision regarding fibrinolytics versus surgery should be made in consultation with a thoracic surgeon.

Open Drainage and Empyemectomy/Decortication

Patients in the late fibropurulent or organizational stage may have inadequate pleural drainage after tube thoracostomy and intrapleural fibrinolytic therapy. An open drainage procedure can be performed under local anesthesia. Segments of one to three ribs overlying the lower part of the empyema cavity are resected, and one or more short large-bore tubes are inserted into the cavity. The cavity is irrigated daily with mild antiseptic solution, and drainage from the tubes can be collected in a colostomy bag. Open drainage is preferred to decortication only in those patients who are thought to be too ill to tolerate decortication. With decortication a full thoracotomy is performed and all fibrous tissue is removed from the pleural surfaces and all pus is evacuated from the pleural space.

References: April 17, 1996 The Virtual Hospital Home Page of the University of Iowa