Pre-brief

Lung ultrasound (LUS) has become an invaluable component of assessing patients in respiratory distress. B-lines have become one of the more useful findings in lung ultrasonography allowing for rapid detection of interstitial edema and initiation of appropriate therapies before a chest X-ray is even obtained. Is it that simple? Do B-lines equal pulmonary edema? No, of course not, and clinicians need to be aware of the limitations of lung ultrasound and B-line patterns.

Definitions:

• B-line: Vertical hyperechoic line emanating from the pleura extending to the bottom of the screen and moving with the pleura during lung sliding. Variable clinical significance.
• Z-line: An echogenic vertical line emanating from the pleural line, not moving with lung sliding, not erasing A-lines, and ending after 2-4 cm. No clinical significance.

The Brief

A 2019 meta-analysis comparing LUS and CXR in the diagnosis of pulmonary edema found LUS had a sensitivity of 0.88 and a specificity of 0.90, which was superior to CXR. LUS is therefore a ‘good’ tool in the detection of pulmonary edema, but there will certainly be false positives and false negatives, and clinicians need to incorporate clinical context when using LUS to diagnose pulmonary edema. The detection of B-lines can be affected by transducer size, shape, and frequency, pre-and post-processing parameters, gain and depth, focus setting, examiner expertise, etc.. B-lines could be missed if not enough lung zones are examined thoroughly. Z-lines could be misinterpreted as B-lines. Multiple other acute and chronic conditions can cause diffuse B-line patterns (pneumonitis, interstitial pneumonia, pulmonary fibrosis) or focal B-line patterns (pneumonia, atelectasis, pulmonary contusion, pulmonary infarction, malignancy, pleural disease).  B-lines can be found in the last intercostal space above the diaphragm in up to 28% of healthy subjects. Lastly, patients with signs and symptoms of pulmonary edema may not display the characteristic B-line pattern.

The point of this post is not to demonstrate how to detect B-lines, which can be easily referenced in some of the articles below. The point is to caution providers in interpreting B-lines as equivalent to pulmonary edema. 

Key Steps to Diagnose Pulmonary Edema with LUS

1. The International Liaison Committee on Lung Ultrasound recommends that each hemithorax should be split into four regions. A LUS scan positive for interstitial edema must have three or more B-lines in an intercostal space in at least two regions bilaterally. Utilizing a low-frequency abdominal or micro convex probe is preferred and setting the probe depth to at least 4-8 cm is recommended, with some experts recommending a depth of 16 cm.
2. Identify the B-line pattern as diffuse or focal. This narrows the diagnosis to disease patterns that cause relatively diffuse lung disease (pulmonary edema, ARDS, lung fibrosis, pneumonitis, interstitial pneumonia).
3. Identify lung sliding and ensure the B-lines move with the pleura. Lung sliding may be absent in ARDS lungs but is present in pulmonary edema.
4. Identify homogeneous distribution of B-lines. The B-lines may not be as evenly distributed in ARDS and pulmonary fibrosis, while they are in pulmonary edema
5. Switch to the superficial transducer to examine the pleura. ARDS and fibrosis will likely have a thick and irregular pleural line.
6. Incorporate the patient’s clinical presentation, exam findings, labs, and echocardiogram.