So you get a patient with shortness of breath, and you have no idea what the reason is…. but they can’t lie flat and the Xray tech is busy with the trauma. Lung US can help you – but that’s weird, right? Air is supposed to be the enemy of ultrasound with gas scatter artifact making what you want to see very hard. Well, believe it or not, with the lung, ultrasound will turn into your go-to tool for quick evaluation. There has been a study that has described a methodical approach to this, the RADIUS study, and one of the key elements of this is evaluating artifact. Yup, that’s right, ARTIFACT….
To complete our Lung Ultrasound mini-module from last week, I wanted to speak a bit about some artifacts that help us in our diagnostic approach to the patient with shortness of breath: A lines and B lines. There have been several studies describing the utility of lung ultrasound B lines to diagnose alveolar interstitial syndrome (AIS), ARDS, pulmonary edema, and pneumonia (of course with the help of the gurus – Lichtenstein, Noble, Volpicelli, Blaivas).
The chest is broken up into 8 total anterolateral areas, 4 on each side:
The patient does not need to lie flat – thats what is great about it, as most short of breath patients prefer to sit up a bit. By using the low frequency phased array probe, allowing better visualizing in between ribs, setting the depth to 16-18cm and turning off the tissue harmonics knob that will limit your ability to see artifact (this is the usual presets anyway, so basically, just dont touch your knobs once you turn the machine on), you will be able to evaluate the patient for the above diagnoses. Place your transducer in each of the above areas, perpendicular to the chest wall, and proceed to fan (changing the angle of your transducer) and slide to different rib spaces.
Normal lung: The pleural surface acts as an acoustic reflector with resulting distal reverberation artifacts – semi-circular arches at varying distance from the top of the screen (A Lines):
Abnormal lung: When more fluid accumulates in the lung, additional artifacts develop from the sound waves coming across air and fluid, called B lines. Seen in ARDS, pulmonary edema, these bright laser-like rays coming from the pleural line and extending to the bottom of the screen. In the diagnosis of ARDS, AIS, pulmonary edema – there needs to be more than 2 rays on the screen, in more than 2 areas of the chest, bilaterallly.