Pleural effusion volume (ultrasound)

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 .

In controlled settings ultrasound may detect constitutive pleural fluid, can reliably detect effusions >20mL 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 .

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 .

To this end, three formulae have enjoyed robust support in the literature, and are used commonly in clinical practice:

• the Balik formula
  • patient supine, transducer perpendicular to the dorsolateral chest wall, measurements taken at maximum inspiration
  • operator measures the maximum distance (in millimeters) between the visceral and parietal pleura
  • Pleural effusion volume (mL) = (measured distance) x 20
• the Eibenberger formula
  • patient supine, transducer perpendicular to the chest wall, measurements taken at maximum inspiration
  • operator measures the maximum distance (in centimeters) between the lung and posterior chest wall (in practice, the same measurement as above) 
  • Pleural effusion volume (mL) = (47.6 x distance) - 837
• the Goecke formula
  • 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.