Pleural effusion volume (ultrasound)
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Measurement of a pleural effusion volume with point-of-care ultrasonography may be a useful tool for intensivists and is an active area of research in critical care 7.
In controlled settings ultrasound may detect constitutive pleural fluid, can reliably detect effusions >20 mL in clinical settings, and may approach the sensitivity and specificity of computed tomography. Assessment of pleural effusions by ultrasound has historically been semi-quantitative, a gestalt of whether the effusion appears minimal, mild, moderate, or severe 6.
Some intensivists and emergency physicians also use ultrasonography to directly quantify the volume of a given pleural effusion, incorporating this measurement with an assessment of gas exchange in deciding whether to drain the fluid collection in question 1.
To this end, three formulae have enjoyed robust support in the literature, and are used commonly in clinical practice:
- Balik formula 4
- patient supine with mild (15°) trunk elevation, transducer perpendicular to the dorsolateral chest wall, measurements taken at end-expiration
- operator measures the maximum distance (in millimeters) between the visceral and parietal pleura
- Pleural effusion volume (mL) = (measured distance) x 20
- Eibenberger formula 3
- patient supine, transducer perpendicular to the chest wall, measurements taken at maximum inspiration
- operator measures the maximum distance (in millimeters) between the lung and posterior chest wall
- pleural effusion volume (mL) = (47.6 x distance) - 837
- Note: Due to the subtraction in the above equation the Eibenberger formula returns a spurious negative value if the distance between the pleural layers is small (<18 mm). For very thin effusions other (e.g. the Balik) formulae should be used.
- Goecke formula 5
- two popular variants exist, which are both performed in the erect position with the transducer on the dorsolateral chest wall
- the index marker is directed cephalad (a longitudinal orientation) with distance measurements (cm) taken at end-expiration
- the first Goecke formula uses the craniocaudal extent (lateral height) of the effusion
- one caliper is placed in the near field in the costophrenic angle
- the subsequent caliper is placed in the far-field at the lung base, constituting a maximum distance between lung and diaphragm
- pleural effusion volume (mL) = distance (cm) x 90
- the second Goecke formula measures the distance between the lung base and the mid-diaphragm (the subpulmonary height)
- height of the dome of the diaphragm is connected to the lung base, the line being perpendicular to one's field of insonation
There exists no consensus on a best or preferred formula; patient positioning factors and ease with calculation of a specific formula typically dictate one's choice.
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