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PLEURAL EFFUSION

Lung US is significantly more accurate for the diagnosis of pleural effusion than CXR (1,2,3) and is as accurate as CT (4). US can detect effusions as small as 30ml when using the thoracic spine sign (5), compared to 200-500ml required to detect an effusion on PA erect CXR (6). Further, the beauty of US is that it allows differentiation of the contents of the effusion: sediment, septations etc. 

QUAD SIGN

A localised small effusion is seen as an anechoic space anterior to the visceral pleura. Because the parietal and visceral pleurae and the borders of the rib shadow create a quadrangle, this is called the quad sign. On inspiration, the lung will expand into this space making it smaller. 

The Ddx for a localised effusion is pleural thickening. Pleural thickening is typically hypoechoic. Pleural thickening >7mm is more likely to be malignant.

Localised pleural effusion: note how the quad is almost obliterated when the lung expands with inspiration is almost o in shape of the quad with respiration

Hypoechoic thickened pleura which maintains it's shape with respiration

Flash of colour is seen as the pleural fluid moves with respiration (note needs low Nyquist limit and high colour gain)

Vascularity within thickened pleura (note needs low Nyquist limit and high colour gain)

SPINE SIGN

Basal and larger effusion are seen as an extension of the thoracic spine above the diaphragm at R4/L4 in the supine patient. To get an accurate assessment for the pleural volume ensure your hand is as close to the bed as possible as pleural fluid is dependent. 

Spine sign: thoracic spine extends above the diaphragm

As a rule of thumb, for visual assessment:

small effusion (approx 250ml): 1-2 spines

moderate (approx 500ml): 2-4 spines

large (approx 1L): >4 spines

QUANTIFICATION OF AN EFFUSION

Multiple formulae exist to quantify an effusion. 

The easiest is Balik's formula (7) in which the anteroposterior interpleural distance of the largest fluid pocket is measured in mm and multiplied by 20, giving a volume in millilitres. To get the right view, with the patient supine, at L4/R4 find the largest pocket of fluid, turn the transducer transverse on the chest and measure the anteroposterior interpleural distance. 

Interpleural distance measurement with L4 with the transducer oriented transversely on the chest.

NATURE OF THE EFFUSION

Transudates are always anechoic. Exudates may be anechoic in the early stages, but are more frequently hypoechoic or contain fine sediment (8). This fine sediment has been called plankton sign by Lichtenstein (9). Sediment seen in a haemothorax tends to layer basally and is called haematocrit sign. An empyema may have fine hyperechoic dots which represent air suspended in fluid. 

from: Brogi, E., Gargani, L., Bignami, E. et al. Thoracic ultrasound for pleural effusion in the intensive care unit: a narrative review from diagnosis to treatment. Crit Care 21, 325 (2017).

empyema: note the hyperechoic air bubbles in the fluid are arranged in a haphazard way unlike in consolidation where the air bubbles follow the bronchiole branching pattern.

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REFERENCES

1. Kataoka H, Takada S. The role of thoracic ultrasonography for evaluation of patients with decompensated chronic heart failure. J Am Coll Cardiol. 2000; 35(6):1638-46.

2. Lichtenstein D, Goldstein I, Mourgeon E, Cluzel P, Grenier P, Rouby JJ. Comparative diagnostic performances of auscultation, chest radiography, and lung ultrasonography in acute respiratory distress syndrome. Anesthesiology. 2004;100(1):9-15.

3.Rocco M, Carbone I, Morelli A, Bertoletti L, Rossi S, Vitale M, Montini L, Passariello R, Pietropaoli P. Diagnostic accuracy of bedside ultrasonography in the ICU: feasibility of detecting pulmonary effusion and lung contusion in patients on respiratory support after severe blunt thoracic trauma. Acta Anaesthesiol Scand. 2008;52(6):776-84.

4. Volpicelli G, Elbarbary M, Blaivas M, Lichtenstein DA, Mathis G, Kirkpatrick AW, Melniker L, Gargani L, Noble VE, Via G, et al. International evidence- based recommendations for point-of-care lung ultrasound. Intensive Care Med. 2012;38(4):577-91.

5. Qureshi NR, Gleeson FV. Imaging of pleural disease. Clin Chest Med. 2006;27:193-213.

6.Blackmore CC, Black WC, Dallas RV, Crow HC. Pleural fluid volume estimation: a chest radiograph prediction rule. Acad Radiol. 1996;3(2):103-9

7.Balik M, Plasil P, Waldauf P, Pazout J, Fric M, Otahal M, Pachl J. Ultrasound estimation of volume of pleural fluid in mechanically ventilated patients. (2006) Intensive care medicine. 32 (2): 318

8.Yang PC, Luh KT, Chang DB, et al. Value of sonography in determining the nature of pleural effusion: analysis of 320 cases. AJR Am J Roentgenol. 1992;159(1):29-33.

9. Qureshi NR, Rahman NM, Gleeson FV. Thoracic ultrasound in the diagnosis of malignant pleural effusion. Thorax. 2009;64(2):139-43.

10. Lichtenstein D. Ultrasound examination of the lungs in the intensive care unit. Pediatr Crit Care Med. 2009;10(6):693-698.