B-lines in the lung: numbers to remember and scanning Zones
In routine clinical practice, 4 standard sonographic lung zones are evaluated on each side delineated using the parasternal line, anterior and posterior axillary lines and a horizontal line about the level of 3rd-4th intercostal space. Experts recommend scanning at least 1 posterior zone (posterior to posterior axillary line) on each side as well in supine patients when monitoring response to diuretic or ultrafiltration therapy.
Visualization of occasional B-lines, especially in the dependent zones of the lung is not abnormal. The presence of three or more B-lines per “rib interspace” in a longitudinal plane is abnormal and constitutes “B-pattern”. Generally, the observable distance between the lines should be no more than 7 mm.
B-pattern in two or more sonographic lung zones bilaterally is suggestive of “interstitial syndrome”, as demonstrated in Figure 1. This term encompasses pulmonary edema of various causes, interstitial pneumonia and diffuse parenchymal lung disease (e.g. fibrosis). In case of diffuse B-line pattern, whether you are dealing with pulmonary edema or fibrosis largely depends on the clinical context. However, findings such as irregular, fragmented pleural line, sub pleural abnormalities appearing as small hypoechoic areas and nonhomogeneous distribution of B-lines favor fibrosis. On the other hand, “focal” B-line pattern may be seen in pneumonia, atelectasis, pulmonary contusion, pulmonary embolism, pleural disease and malignancy.
There are several scoring systems based on various scanning approaches, primarily described for research purposes. Commonly described scanning zones are illustrated in the following 2-minute slideshow. When evaluating an acutely dyspneic patient, examining 2 anterior zones might be enough, while more comprehensive approach is needed in chronically volume overloaded patients (e.g. CHF patient in the clinic, patient on maintenance hemodialysis etc.). Also note that the studied chest zones could include more than 1 rib interspace with a number of possible probe positions. In the comprehensive 28-zone lung ultrasound (video below) however, each zone corresponds to one rib-interspace and the transducer position is transverse. Interestingly, most of the literature in dialysis patients was based on 28-zone method.