B-lines come from the lungs. Specifically, they are vertical artifacts that appear on lung ultrasound when sound waves get trapped and bounce back and forth between fluid-filled or thickened tissue in the lung. They originate at or just below the pleural line (the thin membrane covering the lung surface) and shoot straight down to the bottom of the ultrasound screen without fading. In a healthy, air-filled lung, you won’t see them. They show up when something replaces or displaces the air in your lung tissue.
The Physics Behind B-Lines
Healthy lungs are full of air, and air creates a massive acoustic mismatch with the surrounding tissue. Ultrasound waves bounce off that air-tissue boundary almost entirely, producing horizontal lines called A-lines. These are the normal finding on a lung ultrasound scan.
B-lines appear when something changes inside the lung. When fluid, blood, collagen, or inflammatory material starts filling the tiny air spaces or thickening the tissue between them, the acoustic mismatch drops. The ultrasound pulse can now partially penetrate past the pleural surface, but it gets trapped between small pockets of remaining air. That trapped energy bounces back and forth and gradually leaks back toward the probe, creating a bright vertical streak that extends all the way down the screen. These streaks move in sync with breathing, which confirms they’re coming from lung tissue rather than from the chest wall.
One defining feature of true B-lines is that they erase A-lines wherever they cross them. If you see a vertical artifact that doesn’t obscure the horizontal A-lines beneath it, it’s likely a different type of artifact (such as a Z-line or I-line), which are shorter, fainter, and not considered clinically meaningful.
What Creates Them Inside the Lung
Two main anatomical changes produce B-lines. The first is excess fluid in the lung’s interstitial space, the thin scaffolding of tissue that separates the air sacs. Under normal conditions, lymphatic vessels drain this space continuously. When the drainage system gets overwhelmed, or when the barrier between blood vessels and air sacs breaks down, fluid accumulates and thickens the interlobular septa (the walls between lung segments). Each thickened septum can act as an acoustic trap that generates a B-line.
The second is scarring or fibrosis. In conditions like interstitial lung disease, collagen deposits replace normal tissue in those same walls. The result looks similar on ultrasound, though the distribution pattern can differ. Fluid-related B-lines tend to appear on both sides and concentrate in the lower, gravity-dependent parts of the lungs. Fibrosis-related B-lines may be more prominent in the lower back portions of the lungs and are often accompanied by irregularities in the pleural line itself.
Conditions That Cause B-Lines
The most common cause is pulmonary edema, particularly from heart failure. When the heart can’t pump efficiently, pressure builds in the lung’s blood vessels, pushing fluid into the interstitial space and eventually into the air sacs themselves. In animal studies, small amounts of excess fluid (under 6 mL per kilogram of body weight) produce only scattered B-lines with no obvious breathing difficulty. At moderate volumes (above 10 mL/kg), B-lines begin merging together and breathing becomes labored. At very high volumes (around 20 mL/kg), the B-lines fuse into what’s called a “white lung” pattern, with severe respiratory failure.
Cardiogenic pulmonary edema typically starts as interstitial swelling and, if it worsens, progresses to fluid flooding the air sacs directly. The spacing between B-lines on the screen reflects how much fluid is present, not which compartment it’s in.
Beyond heart failure, B-lines show up in viral pneumonia (which primarily affects the interstitial tissue), pulmonary fibrosis from autoimmune diseases like rheumatoid arthritis, acute respiratory distress syndrome, and even in newborns with “wet lung” after delivery.
How Many B-Lines Are Normal
Seeing one or two B-lines between a pair of ribs is considered normal and can occur in healthy people, especially at the lung bases. The threshold for concern is three or more B-lines in a single rib interspace. When that pattern appears in two or more lung zones on both sides of the chest, it’s labeled “interstitial syndrome,” a broad term meaning the lung tissue is abnormally waterlogged or thickened.
As a diagnostic tool for pulmonary edema, B-line assessment has a sensitivity around 83% when performed manually by physicians, meaning it catches roughly 8 out of 10 cases. Automated detection software pushes that to about 96%. Specificity is lower (63% for manual, 77% for automated), so B-lines can sometimes appear in conditions other than edema, which is why the clinical picture matters alongside the ultrasound findings.
B-Lines vs. Similar Artifacts
Not every vertical streak on a lung ultrasound is a B-line. The World Federation for Ultrasound in Medicine and Biology distinguishes several types. Z-lines and I-lines also originate near the pleural line but are shorter and fade before reaching the bottom of the screen. They have no established clinical significance. C-lines originate from the lower edge of a consolidated (solidified) area of lung rather than from the pleural surface.
Technically, true B-lines don’t originate exactly at the pleural line. Because of the size of the acoustic trap and the channel linking it to the surface, a B-line often starts slightly below the pleural plane. In practice, this distinction is minor, but it explains why some B-lines appear to begin a millimeter or two beneath the bright pleural line on screen.
How B-Lines Guide Treatment
Because B-lines directly reflect how much extra fluid or abnormal tissue is in the lungs, they’re used to monitor treatment in real time. In heart failure patients receiving diuretics (medications that help the body shed excess fluid), clinicians look for a decreasing B-line count as a sign that the lungs are drying out. Resolution of B-lines, along with clearing of fluid collections around the lungs on imaging, indicates the patient is approaching a normal fluid balance.
This makes lung ultrasound especially practical in emergency and critical care settings. Unlike a chest X-ray, it’s portable, radiation-free, and repeatable at the bedside as often as needed. A curved ultrasound probe at medium-to-high gain settings with the focal point placed at the pleural line provides the clearest B-line images. In comparative testing, the curved probe outperformed the linear probe for B-line visualization (63% vs. 37% detection rate), and higher gain settings improved detection by a factor of seven to nine compared to low gain.