Background and Indications for Examination
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.
Probe Selection and Technical Considerations for Thoracentesis
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.
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.
Color-flow Doppler can identify surrounding blood vessels. This may help to avoid inadvertent injury to vessels while performing the procedure.
Performing Ultrasound-Guided Thoracentesis
Performing thoracentesis without ultrasound guidance requires a very careful and thorough physical examination. When a large effusion is present, the trachea may be shifted to the contralateral side. Auscultation can reveal an area of decreased or absent breath sounds corresponding to an effusion. This finding can be confirmed with percussion in the area of decreased auscultation, which should reveal dullness. A decrease in tactile fremitus, however, is the key physical exam finding that distinguishes effusion from consolidation. Except in the case of a complicated and loculated effusion, the area of decreased breath sounds and dullness to percussion should lie in the inferior aspect of the chest and should be dependent on gravity. Pitfalls to performing thoracentesis guided by physical exam alone include the following:
Difficulty in accurately locating small effusions.
The diaphragm and lung are dynamic and can move into the plane of the effusion being evaluated.
Difficulty in distinguishing a pleural effusion from an elevated hemidiaphragm.
It cannot assess for loculations.
Physical exam is suboptimal in the obese patient, especially if the ribs cannot be palpated and used as landmarks.
Ultrasound-guided thoracentesis can be performed by either a static or dynamic technique. Static ultrasound guidance uses the probe to identify the ribs, the intercostal spaces, the pleura, and the effusion itself. It also helps to identify the depth of the effusion, the best angle of entry, and other surrounding structures to be avoided. The skin is marked (based on identifying the best place to drain the effusion), the probe is removed, and the procedure is then carried out in a fashion similar to landmark-based thoracentesis. The main advantage of this technique is that the clinician will have two free hands to perform the procedure. In large effusions, the static technique is usually sufficient as the amount of fluid is easily accessible.
Dynamic ultrasound guidance uses real-time imaging to perform the thoracentesis. The advantage of this technique is being able to visualize the needle entering the intercostal space toward the effusion and the ability to redirect the needle as needed. It is advantageous in smaller effusions that may be more difficult to reach using the static method.
The first step in performing an ultrasound-guided thoracentesis is recognizing the normal sonographic anatomy of the ribs, the pleura, and the lung parenchyma. The visceral and parietal pleura can be observed between two ribs and approximately 0.5 cm below the surface. The pleural line, which consists of both the parietal and visceral pleura, is 0.2–0.4 mm thick, and is not differentiated on ultrasound unless a pleural effusion is present separating the two. Therefore, these two structures are in close apposition and will appear as a single bright echogenic line. The normal pleura will exhibit lung sliding, which is viewed as the pleural line between the chest wall and the aerated lung moving up and down in time with respiration. The lung pulse may also be observed, which is described as a subtle shimmering movement of the pleural line in time with the cardiac cycle. The ribs will appear as hyperechoic structures just proximal to the pleural space and will exhibit posterior acoustic shadowing due to an attenuation of the ultrasound beam. The lung parenchyma is visualized as a series of repetitive horizontal lines beneath the pleural line. These are formed by air artifact, and are referred to as “A-lines,” which are normal (see Chap. 7).
The boundaries of the thoracic cavity should be carefully determined by examining the diaphragm, the liver, and the spleen. The liver and spleen both appear as isoechoic structures just below the diaphragm and biliary ducts are often seen within the liver structure.