Pleural fluid collections in critically ill patients (2024)

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Pleural effusion is defined as the excessive accumulation of fluid in the pleural space, indicating an imbalance between pleural fluid formation and removal. The presence of a pleural effusion may be a primary manifestation or a secondary complication of many disorders. A subsequent review will cover air leaks and pneumothorax.

Pathophysiology

The inner surface of the chest wall and the surface of the lungs are covered by the parietal and visceral pleura, respectively, with a 10–24µm separation normally between the two surfaces. This space is usually filled with a very small amount of fluid. However, large amounts (4–5 litres in an adult) of fluid can accumulate in the pleural space under pathological conditions.

The parietal pleura has sensory innervation. Both pleural surfaces are mainly supplied by systemic arterial vessels. Lymphatic vessels from the parietal pleura drain to lymph nodes along the anterior and posterior chest wall; lymphatics from the visceral surface drain to the mediastinal lymph nodes. The pleural space typically contains a small amount of a colourless alkaline fluid (0.1–0.2mlkg⁻¹, pH 7.62), which has a low amount of protein (<1.5gdl⁻¹). Approximately 90% of accumulated fluid in the pleural space is drained by the venous circulation; the other 10% is absorbed by the lymphatics.

A delicate balance between the oncotic and hydrostatic pressures of the pleural space regulates filtration and drainage of pleural fluid. Net absorption of pleural fluid is slightly greater than net filtration forces. In addition, lymphatic drainage from the parietal pleura can surpass the rate of fluid filtration in the pleural space. Chest wall and diaphragmatic movements also enhance absorption of pleural fluid by the vascular and lymphatic vessels. Excessive filtration of fluid can overwhelm these efficient absorptive mechanisms and lead to the formation of pleural effusion.

Types of fluid collections

Pleural effusions are traditionally classified as either transudates or exudates. Diseases that affect the filtration of pleural fluid result in transudate formation and often occur bilaterally. Inflammation or injury increases pleural capillary membrane permeability to proteins and various types of cells and lead to the formation of an exudative effusion.¹

Blood in the pleural space (haemothorax) is normally related to trauma or surgical interventions and requires early drainage, plus consideration for surgical exploration for control of bleeding. In chest trauma, an initial drainage of 1500ml or>200mlh⁻¹ is an indication for surgical exploration.² Early drainage is essential before clotting occurs as thereafter only serum will drain leaving residual clot. If extensive, this will cause mechanical problems and may become infected. Importantly, blood clot will not drain effectively through a fine bore drain; surgical decortication via thoracotomy may be required.

Collection of lymphatic fluid (chyle) in the chest from disruption of the major lymphatic trunks is a rare but recognized problem after surgical procedures such as oesophagogastrectomy and trauma. The drained fluid is characteristically milky if the patient is on enteral feeding. Management is initial drainage and institution of total parenteral nutrition to reduce the volume of losses and provide nutrition. Persistent large volume losses for >10 days are an indication for surgical correction of the leak.³

Chyliform or pseudochylous pleural effusions grossly resemble chylothorax. However, these effusions contain no chylomicrons and pathogenesis does not involve the thoracic duct. There is a high lipid content (cholesterol crystals or lecithin–globulin complexes) causing a milky white appearance. Pseudochylous pleural effusions occur commonly with longstanding pleural effusions and are associated with rheumatoid pleuritis.

Causes of pleural effusions

Each case of pleural effusion must be evaluated on an individual basis. Knowledge about its prevalence in underlying illnesses can be of help in developing the differential diagnosis (Table1). A flow chart for the diagnosis of pleural effusion is shown in Fig.1.

Symptoms and signs

Symptoms depend upon the amount of fluid and the underlying cause. These include pleuritic chest pain, dyspnoea, and non-productive cough. Physical findings are reduced tactile fremitus, dullness on percussion, and diminished or absent breath sounds. A pleural rub may also be heard during inspiration during the early phase of inflammation. Even large pleural effusions can often remain asymptomatic and present as incidental findings. Pulmonary function tests show a restrictive ventilatory defect and reduced functional residual capacity. In the intensive care unit (ICU) setting they are difficult to detect on clinical examination alone due to patient positioning and chest wall oedema.

Imaging

Chest radiography

Standard posteroanterior and lateral chest radiography remains the most important technique for the initial diagnosis of pleural effusion. The classical signs of pleural effusion on erect chest X-ray are blunting of the costo-phrenic angle, hom*ogeneous opacification with no air bronchograms and a meniscus with the apex towards the axilla (Fig.2a). On supine chest radiography (commonly used in ICU), moderate-to-large pleural effusions may escape detection because the pleural fluid settles posteriorly and no change in the diaphragm or lateral pleural edges may be noted. In these cases, a pleural effusion must be suspected when there is increased opacity of the hemithorax without obscuring of the vascular markings (Fig.2b). In large pleural effusions there is a complete white out of the hemithorax. Mediastinal shift to the opposite side along with a visible lung margin will differentiate it from collapse of the lung due to endobronchial obstruction. If an effusion is suspected but not clear on plain chest X-ray, ultrasonography should be performed.

Ultrasonography

The classical appearance of a pleural effusion is an echo-free layer between the visceral and parietal portions of the pleura (Fig.3). The nature of the fluid and presence of loculations can be seen. The volume of an effusion can be estimated from measurements of dimensions of an effusion in various planes or by measuring the width between the lower lung margin and diaphagm.⁴

Computerized tomography scanning

Computerized tomography (CT) scans for pleural effusions should be performed with contrast enhancement. In simple uncomplicated pleural effusions, the CT scan shows crescent-shaped opacities in the posterior and basal portions of the hemi-thorax (Fig.4a). In cases of difficult drainage, CT scanning should be used to delineate the size and position of loculated effusions (Fig.4b). CT scanning may also be used to differentiate between benign and malignant pleural thickening.

Laboratory tests

After obtaining a sample of pleural fluid, the clinician should determine whether the effusion is a transudate or exudate. If the fluid is a transudate, the possible causes are relatively few and further diagnostic procedures are not necessary. In contrast, if the fluid is an exudate, further diagnostic tests are required.

Light's criteria

Transudative and exudative pleural effusions are differentiated by comparing protein and lactate dehydrogenase concentrations in the pleural fluid to those in the blood. Exudative pleural effusions meet at least one of the following criteria, whereas transudative pleural effusions meet none:The sensitivity for exudate is 98% and specificity 83% with the above criteria. Twenty-five per cent of patients with transudative pleural effusions are mistakenly identified as having exudative pleural effusions by the above criteria.⁵ Additional testing is therefore needed if a patient identified as having an exudative pleural effusion appears clinically to have a condition likely to produce a transudative effusion.

• the ratio of pleural fluid protein to serum protein is>0.5;

• the ratio of pleural fluid lactic dehydrogenase (LDH) and serum LDH is>0.6;

• pleural fluid LDH is more than two-thirds normal upper limit for serum.

Glucose concentration

The glucose concentration in transudates and most exudates is similar to that of serum. The conditions that cause low pleural fluid glucose concentrations are rheumatoid arthritis, tuberculosis, empyema, and malignancies with extensive pleural involvement.

pH

The pH of normal pleural fluid is approximately 7.62 owing to active transport of HCO₃⁻ into the pleural space. A low pH is seen in inflammatory and infiltrative processes such as infected para-pneumonic effusions, empyema, malignancies, collagen vascular disease, and oesophageal rupture. Urinothorax is the only transudative effusion that can present with a low pleural fluid pH. A para-pneumonic pleural fluid with pH<7.2 indicates a poor patient outcome and requires drainage.⁶

Amylase concentration

A high pleural amylase concentration (>200Udl⁻¹) occurs both in acute and chronic pancreatitis, malignancy, or oesophageal rupture. The clinical setting usually separates these entities and assay of isoenzymes can be helpful (salivary vs pancreatic source) (Table2).⁷

Management of pleural effusions

Diagnosis vs treatment

The management of pleural effusions in ICU patients with respiratory failure remains controversial. All pleural effusions associated with pneumonic illness should be aspirated for diagnostic investigation.⁸ Quantitative assessment of the volume of pleural effusions is crucial to select critically ill patients for thoracentesis.

Tap vs drain

Pleural fluid characteristics remain the most reliable diagnostic test to guide management. The presence of frankly purulent or turbid/cloudy fluid on pleural aspiration indicates the need for prompt chest-tube drainage, as is the presence of organisms identified by positive Gram staining. Thereafter, pleural fluid pH is the most useful index predicting the need for chest-tube drainage and the pleural LDH and glucose concentration do not further improve diagnostic clarity in parapneumonic effusions. Routine pre-procedure checks of platelet count, prothrombin time, or both are only required in those patients with known risk factors. Fluid collections greater than a litre associated with respiratory compromise should be considered for drainage, irrespective of their nature.

Which drain?

Studies have shown that the small catheters (10–14Fr) are often as effective as larger bore tubes and are more comfortable and better tolerated by the patient.⁹ There are no large randomized control trials directly comparing small and large bore tubes. In the event of failure to drain a pneumothorax due to excessive air leakage, it is recommended that a larger bore tube is inserted. On the basis of clinical experience, large bore tubes are more effective for draining thick pus and blood.

Which bottle?

The chest tube is attached to a drainage system that allows one direction of flow only. This is the closed underwater seal bottle in which a tube is placed under water at a depth of approximately 3cm with a side vent which allows escape of air, or it may be connected to a suction pump. The respiratory swing in the fluid in the chest tube is useful for assessing tube patency and confirms the position of the tube in the pleural cavity. The use of integral Heimlich flutter valves are advocated in patients with pneumothorax as they permit ambulatory or even outpatient management and have been associated with a 85–95% success rate. A drainage bag with an incorporated flutter valve and vented outlet has been successfully used postoperatively.

Suction

High-volume, low-pressure suction pumps are used in persistent pneumothorax and following chemical pleurodesis. However, there is no evidence to support their routine use in the initial treatment of spontaneous pneumothorax. If suction is required, this should be performed via the underwater seal at a level of 10–20cm of water. Wall suction is effective but chest drains must not be connected directly to the high negative pressure available from wall suction.

Removing the drain

The timing of drain removal is dependent on the original reason for insertion and clinical progress. In cases of haemothorax, the drain should not be removed until it completely ceases to drain; however, for transudates, it should be left until it reduces to insignificant quantities (approx. 50–100ml in 24h). Clamping of the drain before removal is unnecessary. The chest tube should be removed either while the patient performs a Valsalva's manoeuvre or during expiration. A brisk firm movement is required with an assistant tying the previously placed closure suture.

Further management issues

Blocked chest tubes should be flushed with 20–50ml normal saline to ensure patency. Contrast-enhanced CT scanning is the most useful imaging modality in patients for whom chest-tube drainage has proved unsuccessful; it provides anatomical details such as loculations and ensures accurate tube placement. All patients with pleural infections should have an assessment of the effectiveness of the pleural fluid drainage along with resolution of sepsis 5–8 days after chest-tube insertion and commencement of antibiotics. For an empyema, there are no objective criteria to define the point at which a patient should be referred for surgery. Patients with purulent fluid and loculations are more likely to require surgical drainage. Failure of sepsis to resolve within 7 days has been suggested as an appropriate period after which a surgical opinion should be sought. A number of surgical approaches are available including video-assisted thoracoscopic surgery, open thoracic drainage, or thoracotomy and decortication. The type of procedure performed will depend on many factors including patient age, co-morbidity and surgical preference.

Re-expansion pulmonary oedema

Re-expansion pulmonary oedema occurs following rapid evacuation of large pleural effusions and during decompression of spontaneous pneumothorax.¹⁰ The two main factors involved are alteration of capillary permeability and increase of hydrostatic pressure.¹¹ Mild symptoms suggestive of re-expansion oedema are common after large volume thoracentesis in pleural effusion with patients experiencing discomfort and cough. It is recommended that no more than approximately 1.5 litres should be drained at any one time or drainage should be slowed to<500mlh⁻¹.

References

© The Board of Management and Trustees of the British Journal of Anaesthesia [2007]. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

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