Pleural effusion is a commonly encountered condition, which may arise as a consequence of a number of disease states including malignancy, infections, and inflammatory conditions. Patients may develop complaints of shortness of breath, dyspnea on exertion, or pleuritic chest pain as part of their initial presentation. Sampling and drainage of pleural effusions is important to adequately diagnose the patient's condition and to alleviate their respiratory symptoms. Thoracic ultrasound is an invaluable tool for characterizing the quantity and quality of the effusion as well as facilitating safe sampling or drainage of the fluid. Thoracic ultrasound is also useful for the rapid visualization of the parietal and visceral pleural surfaces, evaluation for pneumothorax, and to some extent the evaluation of the lung parenchyma.
A pleural effusion is easily identified using ultrasound owing to the relatively echo-free nature of fluid. Small pockets of fluid can be easily visualized by ultrasound with the patient in the upright position. In contrast, 150 mL of fluid is typically necessary in order to visualize an effusion on a standard upright posteroanterior and lateral chest radiograph. Ultrasound can be particularly useful in differentiating pleural fluid from lung parenchyma, atelectasis, infiltrate, or an elevated hemidiaphragm identified on chest radiograph in the acutely ill patient.
Thoracentesis is typically a safe and relatively simple procedure. However, the incidence of pneumothorax has been reported to be as high as 20%–39%. No randomized controlled clinical trial has compared the use of ultrasound or physical examination alone to guide thoracentesis. However, several studies have shown that ultrasound decreases the complication rate of the procedure.
Bedside ultrasound-guided thoracentesis should be performed in the following patients:
- The diagnostic evaluation of a patient with new pleural effusion
- The patient with a pleural effusion who presents with respiratory compromise or dyspnea
- The patient who presents with a fever, chest pain and/or suspected empyema
Curvilinear Probe with a Frequency of 3.5–5.0 MHz
A curvilinear probe with a frequency of 3.5–5.0 MHz is best suited for performing a thoracentesis. This allows for better visualization of deeper structures and is more than adequate for visualizing more superficial structures adjacent to the chest wall. A higher-frequency probe between 5.0 and 7.0 MHz is effective for visualizing chest wall structures and the parietal pleura.
The sonographer should initially start off deep and survey the chest, the effusion, and surrounding structures. The depth should then be reduced so that the area where the needle will be introduced takes up most of the screen.
Gain or Time-Gain Compensation
The total gain can be increased to brighten the image as necessary. In obese patients, where imaging may be more difficult, the far gain can be adjusted in order to compensate for signal attenuation as the ultrasound beam travels to deeper structures.