Lung ultrasound: A and B-lines
Lung ultrasonography can be used alone or in conjunction with limited echocardiography and sonographic assessment of the inferior vena cava to determine a patient’s volume status in day-to-day nephrology practice. B-lines seen on lung ultrasound provide semi-quantitative estimation of extravascular lung water, which is particularly important in critically ill patients and those with cardiorenal syndrome.
In normal aerated lung, hyperechoic, horizontal lines arising at regular intervals from the pleural line can be seen, which are called A-lines. These are reverberation artifacts that arise when the ultrasound beam reflects off of the pleura and, instead of entering the probe, partially reflects off of the probe face back to the pleura again before getting back to the machine. This double-length pathway is interpreted and displayed as if the source of the echo lies at two times the distance between pleura and skin because, the distance at which a particular structure is displayed on the screen depends on how long the echoes returning from that structure take to reach the probe. Multiple reverberations result in multiple A-lines, at multiples of the pleural depth. In short, if you see A-lines, the lungs are filled with air.
B-lines are defined as discrete laser-like vertical hyperechoic artifacts that arise from the pleural line and extend to the bottom of the screen without fading, move synchronously with lung sliding and erase A-lines. They used to be called “comet tails” and “lung rockets” in the past, and this terminology is obsolete now. B-line formation is incompletely understood but they are believed to be a subtype of reverberation artifact, called the ring-down artifact.
Here is the full post on introduction to lung ultrasound on the Renal Fellow Network.
Here is a short video explaining why we should perform lung POCUS in dialysis patients.