Introduction

Thoracentesis is a common procedure that is used for both diagnostic evaluation and symptomatic treatment of large pleural effusions and suspected empyemas in the emergent setting. Pleural effusions are the most common sequelae of pleural disease, usually due to cardiogenic, inflammatory or infectious disorders. A procedure performed by emergency physicians, critical care intensivists, and hospitalists, it can be done safely and successfully with the use of bedside ultrasound (US) and proper training.¹ In fact, ultrasound guided thoracentesis has been shown to decrease hospital stay, cost, and complications such as hemorrhage and pneumothorax when used appropriately².

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Learning Objectives

After reading this article, the physician should be able to:

• Understand the benefits and limitations of using ultrasound for thoracentesis
• Identify structures with the use of ultrasound to efficiently perform thoracentesis and limit its complications

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In addition to identifying and quantifying the amount of pleural fluid present, there have been a few studies utilizing ultrasound to distinguish transudative versus exudative pleural effusions. Hirsch, et al found that complex and septated fluid was correctly identified as an exudative process via ultrasonography in 74% of cases³.

However, Yang, et al determined that complex effusions (either septated or non-septated) or homogeneously echoic effusions are always exudative⁴. The converse, however, may not be true and thus anechoic fluid, although usually transudative, can be either transudative or exudative. In order to correctly correlate ultrasonographic characteristics of effusions, more experience is needed beyond the ability to identify the presence of pleural fluid and thus is beyond the scope of this article.

Needle or catheter drainage of pleural effusions can be performed using fluoroscopy, ultrasound, or computed tomography (CT) scanning. Ultrasound’s advantage over CT includes its easier accessibility and non-ionizing radiation. It’s also less expensive than CT and much quicker to perform than CT guided drainage⁵.

It is also more logistically feasible to perform US at the bedside in critically ill patients who might be mechanically ventilated or too ill to be moved from the emergency department or intensive care unit^5,6. Free-flowing pleural effusions are also much easier to target while using US, as they trek to the most dependent areas of the thorax – a much more difficult location to target using CT⁵.

Ultrasound guided thoracentesis has been proven to reduce the incidence of complications such as pneumothorax, which has been reported to be as high as 20 to 39%⁷. To date there have been no randomized, controlled trials comparing ultrasound-guided versus physical examination-guided thoracentesis. Grogan, et al found a 29% reduction in the pneumothorax rate when ultrasound was utilized for identification of needle placement⁷. Even when a pneumothorax does occur, the incidence of pneumothorax requiring tube thoracostomy is also significantly reduced with ultrasound guidance⁸. Ultrasound also reduces the number of unsuccessful clinical attempts at thoracentesis (“dry tap”), because more than 50% of needle insertions are below the diaphragm⁹. Ultrasound increased the accuracy of site selection by approximately 26% and decreased the number of near misses in a study by Diacon et al.¹⁰.

Studies have shown that critical care, internal medicine and emergency medicine physicians are capable of using this modality to diagnose and treat patients with symptomatic pleural effusions with few complications while improving patient outcomes^1,11,12.

Indications

Thoracentesis is indicated for patients who present to the emergency department with dyspnea due to large, non-traumatic pleural effusions that require symptomatic relief. It is also indicated for patients with pleural effusions of any size that may require diagnostic analysis of the pleural fluid, such as when the etiology of the effusion is unclear or the condition has not responded to medical therapy as expected.

Effusions can be categorized as transudative or exudative. The most common reason for transudative effusion is heart failure, followed by liver cirrhosis and nephrotic syndrome. Exudative effusions result from increased capillary permeability and consequent movement of intravascular constituents into the pleural space. Usually this is due to an infectious or inflammatory cause such as pneumonia, malignancy, empyema, or other infections.

Relative contraindications include volumes of fluid insufficient to collect, usually < 1 cm thick on a lateral decubitus chest X-ray; bleeding diathesis and systemic anticoagulation; and cellulitis over the proposed puncture site.

Performing the Study

After informed consent, prepare the site and use standard sterile technique. Given that this procedure may be uncomfortable, consider adding anxiolysis to local anesthesia. If the patient requires mild sedation, this should be done before the procedure. As with all procedures, proper positioning is the key to success. – Thoracentesis can be performed while the patient is either sitting upright or leaning over a Mayo stand or with the patient supine.

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Figure 1. Pleural fluid, which is anechoic on ultrasound, provides great contrast between the pleural and the hyperechoic air-filled lung and other structures such as the liver, diaphragm, kidney and spleen.

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Chest ultrasonography uses ultrasound waves reflected from media in the thoracic cavity, limiting the acoustic window to the intercostal space^6,13. As such, pleural fluid, which is anechoic on ultrasound, provides great contrast between the pleura and the hyperechoic, air-filled lung and other structures such as the liver, diaphragm, kidney and spleen (Figure 1)⁶.

Examination of the pleural space can be done with a low-frequency curvilinear or convex array 5-1 Megahertz (MHz) probe or a high-frequency linear 12-6 MHz transducer¹⁴. High frequency probes offer high resolution but at the cost of decreased depth; in patients who have larger amounts of fluid that are deeper within the pleural cavity, a low frequency convex array probe may be a better option.

By convention, the probe marker is pointed cephalad (towards the patient’s head) at the optimal puncture site, which is usually located between the seventh and ninth ribs and between the posterior axillary line and midline¹⁵. The diaphragm is a brightly echogenic structure that should be delineated clearly, as it is important to select an intercostal space into which the diaphragm does not rise up at the end of exhalation, to reduce the risk of injury to the diaphragm and liver^6,15. A mark should be placed on the chest wall where the deepest pocket of anechoic fluid is found, with no visualization of the diaphragm throughout the entire respiratory cycle.

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Figure 2. The transducer should be directly perpendicular to the chest as an oblique angle will overestimate or underestimate pleural volume. Once the optimal puncture site is marked the patient can then be draped; using standard aseptic technique the rest of the procedure can then be performed.

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It is important to note that the transducer should be directly perpendicular to the chest, as an oblique angle will overestimate or underestimate pleural volume. Once the optimal puncture site is marked, the patient can be draped and, using standard aseptic technique, the rest of the procedure can then be performed (Figure 2). Most studies that have been performed do not use real-time guidance for needle insertion and simply approximate the area with an “X marks the spot” method¹⁶.

By using motion-mode (M-mode) ultrasonography, it is possible to determine the depth of the lung and the amount of fluid between the chest wall and the lung parenchyma or visceral pleura¹⁵. While in M-mode, placing the line over the area with the largest amount of fluid, the image will show anechoic fluid as well as a sinusoidal wave pattern, which is consistent with the lung freely floating in the surrounding fluid (Figure 3). The M-mode tracing approximates how deep the fluid is from the chest wall, thus allowing the physician to determine how deep the inserting needle or catheter should be placed.

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Figure 3. While in M-mode, placing the line over the area with the largest amount of fluid will show anechoic fluid as well as a sinusoidal wave pattern.

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Limitations

Although studies have shown that physicians of different specialties are able to perform US-guided thoracentesis, its use is still operator dependent. Experience plays an important role. Both didactic and real-time education are crucial to assure high-quality care and reduce the number of bad outcomes such as pneumothorax, hemorrhage, and damage to adjacent structures such as the liver and diaphragm.

Pitfalls

The practitioner must be aware that the lung is a moving structure and that with respiration, both spontaneous and mechanically ventilated, the depth of fluid might change while the needle is being placed. One must allow enough room for the lung to move freely – in larger pleural effusions this is less of a concern. It also important to scan through the entire pleural space, as failure to select the largest pocket of fluid can increase the risk of injury to the lung¹⁷. The amount of fluid drained depends on the amount of fluid present and the hemodynamics of the patient. Given the concern of re-expansion pulmonary edema, physicians tend to remove 1000-1500 mL at one time; however, studies have shown that re-expansion pulmonary edema is uncommon and occurs in certain clinical circumstances such as large spontaneous pneumothorax that has been present for a long time, but not with drainage of large pleural effusions^18,19. Physicians should be wary of how much fluid is taken out, but the exact amount should be determined on a case by case basis.

Conclusion

Ultrasound guided thoracentesis is an easily learned procedure that can be used in a variety of settings because of its portability and non-ionizing radiation. It is most useful in the critical care setting when a patient cannot be moved easily or is hemodynamically unstable. Its use has been associated with decreased number of attempts and reduced rate of complications, and it is quickly becoming the standard approach^2,6-9,16,17.

References

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