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.