What is pleural tapping (thoracentesis) and how is it performed in patients with pleural effusion, particularly those with underlying conditions such as lung disease, heart failure, or cancer?

Pleural Tapping (Thoracentesis): Comprehensive Clinical Guide

Definition and Purpose

Thoracentesis is an invasive procedure to remove pleural fluid for diagnostic evaluation or therapeutic relief of dyspnea, and should always be performed with ultrasound guidance to minimize complications. 1

The procedure serves two primary functions:

• Diagnostic: Determining etiology of undiagnosed unilateral pleural effusion or bilateral effusion with normal heart size, particularly when malignancy is suspected requiring cytological examination 1
• Therapeutic: Relief of dyspnea in symptomatic patients, especially those with malignant pleural effusions causing respiratory compromise 1

Pre-Procedure Assessment

Imaging Evaluation

Ultrasound examination immediately before the procedure is essential to accurately locate fluid, identify loculations or septations, and mark the optimal insertion site 1. This approach reduces pneumothorax risk from 8.9% to 1.0% in malignant effusions 2.

Key imaging steps:

• Chest radiography to determine size, laterality, and presence of mediastinal shift 1
• Ultrasound to locate fluid and identify potential complications 1
• Assessment of mediastinal shift direction before starting—ipsilateral shift suggests trapped lung or bronchial obstruction, making fluid removal unlikely to relieve dyspnea 3

Patient Selection Considerations

Asymptomatic patients with malignant pleural effusion should not undergo routine drainage, as observational data shows these patients rarely require intervention during follow-up 2. The risks of invasive procedures outweigh benefits when symptoms are absent 2.

Relative contraindications include:

• Minimal effusion volume 1
• Bleeding diathesis or anticoagulation 1
• Mechanical ventilation (though morbidity is not necessarily increased) 4
• Severe renal failure 1

Procedure Technique

Site Selection and Needle Insertion

Use ultrasound to identify the insertion site in real-time, typically in the mid-scapular or posterior axillary line, one to two intercostal spaces below the upper border of the effusion 1.

• Small-gauge needles (21 or 22 gauge) minimize pneumothorax risk when removing small volumes (35-50 mL) for diagnostic purposes 4
• Larger needles may be necessary for therapeutic drainage but increase morbidity 4
• Ultrasound can identify intercostal vessels to decrease hemorrhagic complications 2

Volume Removal Guidelines

Limit initial fluid removal to 1-1.5 L unless pleural pressure monitoring is available to minimize re-expansion pulmonary edema risk 1, 3. This recommendation is critical for patient safety.

If pleural manometry is used:

• Continue drainage if pleural pressure remains above -20 cm H₂O 3
• Stop immediately if pressure drops below -20 cm H₂O or if patient develops dyspnea, chest pain, or severe cough 3
• Pressure >19 cm H₂O with removal of 500 mL or >20 cm H₂O with removal of 1 L predicts trapped lung 1

When contralateral mediastinal shift is present and the patient tolerates the procedure without chest tightness, cough, or dyspnea, removal of several liters is probably safe 3.

Fluid Collection for Analysis

Obtain at least 25 mL (ideally 50 mL) of pleural fluid for cytological examination when malignancy is suspected 1.

Critical Complications and Prevention

Pneumothorax

Ultrasound guidance reduces pneumothorax risk by 90% (from 8.9% to 1.0%) in malignant effusions 2. A meta-analysis of 6,605 thoracenteses showed overall pneumothorax risk of 6.0%, reduced by 19% with ultrasound guidance 2.

Avoid blind thoracentesis without ultrasound guidance, which significantly increases pneumothorax risk 1, 3.

Re-Expansion Pulmonary Edema

Re-expansion pulmonary edema (RPE) occurs in 0.5-2.2% of large-volume thoracentesis and is related to excessively negative pleural pressure generation rather than volume removed 3.

Prevention algorithm:

1. Assess chest radiograph for mediastinal shift before starting 3
2. Limit removal to 1-1.5 L without manometry 3
3. Monitor continuously for warning symptoms: chest tightness, cough, or dyspnea 3
4. Stop immediately if symptoms develop 3
5. Obtain post-procedure chest radiograph to evaluate lung re-expansion 3

Neither patient nor operator may be aware of precipitous pressure decrease, making clinical symptoms an unreliable sole indicator 3. This is a critical pitfall.

Other Complications to Monitor

• Bleeding, infection, and organ laceration require vigilant monitoring 1
• Hemothorax and solid organ puncture are reduced with ultrasound guidance 2
• Chest tube placement was required in 2.2% of non-ultrasound-guided procedures versus 0% with ultrasound 2

Post-Procedure Management

Immediate Assessment

Obtain chest radiograph after the procedure to evaluate lung re-expansion and identify trapped lung 3. However, radiographic lung re-expansion is a poor surrogate for normal pleural elastance—71% of patients with successful radiographic expansion still have abnormal visceral pleural recoil 5.

When Dyspnea Persists

If dyspnea is not relieved after thoracentesis, investigate alternative causes:

• Lymphangitic carcinomatosis 1, 6
• Atelectasis 1, 6
• Thromboembolism 1, 6
• Tumor embolism 1, 6
• Endobronchial obstruction 1, 6

Recurrent Effusions

For recurrent malignant effusions, consider definitive interventions such as chemical pleurodesis or indwelling pleural catheter placement 1. Nonexpandable lung occurs in at least 30% of patients with malignant pleural effusions and may contraindicate pleurodesis 2.

Palliative therapy with periodic outpatient thoracentesis is appropriate for patients with far advanced disease and poor performance status 1.

Special Populations

Malignant Pleural Effusion

Ultrasound evaluation before thoracentesis can identify nonexpandable lung, which aids in guiding definitive management 2. Patients with nonexpandable lung have significantly shorter median survival (7.5 vs. 12.7 months) 2.

Patients on Mechanical Ventilation

Ventilator-dependent patients have no greater morbidity with thoracentesis than non-ventilated patients when performed by experienced operators 4.

Small or Loculated Effusions

Ultrasound is particularly valuable to decrease morbidity when small or loculated volumes of fluid are present 4.

Operator Experience Requirements

Thoracentesis should be performed by experienced operators, and when attempted by physicians-in-training, close supervision by credentialed individuals is necessary 4. This is essential for minimizing complications and maximizing diagnostic yield.