In the hands of a skilled clinician, lung ultrasound can diagnose most - but not all - pulmonary diseases with accuracy approaching a CT scan. And it can be done at the bedside, in under 3 minutes with zero ionizing radiation.
The basic physics of ultrasound are important here to help you remember why you see what you see. Ultrasound waves propagate well through liquids and solids, and poorly through air. Since lungs are normally filled with air with a thin pleural lining around it, you will be able to visualize the pleura well, but will not be able to see anything past it, unless there is disease like consolidations, fluid or masses.
Patient & probe positioning
Place the phased array or Linear probe (for a higher resolution pleural examination) in the first rib space just lateral to sternum. Probe is longitudinal with probe marker pointing to the head.
In a systematic fashion, slide the probe down, one rib space at a time. Then move laterally and slide up the lung. With each movement, you should redefine your barriers of ribs and identify the pleural line. It is important to remember that in order to see the lung tissue, your ultrasound probe has to be perpendicular to the lung at any given point.
To be thorough when looking for a pneumonia or focal consolidation, it can be helpful to divide each lung into 6 sections and complete the ultrasound exam anteriorly, laterally, and posteriorly. This method of detecting lung pathology is far more sensitive than a portable chest x-ray.
With the probe on the chest wall and probe marker pointing to the patient head, the first thing you see is subcutaneous tissue. Look for the pleural line, which is the first shimering, shiny line. It should have an "ants on a log" appearance as the patient breaths. Next you should orient yourself by looking for the superior and inferior rib shadows. In this image, since the probe marker on the screen is on the left, this is toward the patient's head and the left rib space is therefore superior.
In normal healthy lung, you should see a reflections of the pleural line at equidistant intervals. This is an ultrasound artifact that only appears in normal, well-aerated lung. Based on the ultrasound above you can conclude that in this rib space, the patient appears to have healthy lung tissue.
In contrast to A lines, B lines (also called comet tail artifacts) are laser-like artifacts that arise directly from the pleural line. In order to qualify as B lines they need to meet a few criteria:
Arise from the pleural line & move together with lung sliding
At least 3 B lines in one ribspace
Long, laser-like lines that obliterate the A lines
The reason why B lines are so important is their sensitivity and specificity are far better than the traditional method of listening with your stethoscope. In one trial involving 79 patients in which congestion was assessed with lung ultrasound, the vast majority of patients with moderate to severe congestion did not have crackles present on auscultation. In this same study, peripheral edema was absent in 80-87% of the patients with significant pulmonary edema.
The presence or absence of B-lines can help you narrow down the diagnosis in seconds:
3+ B-lines in multiple lung fields, especially bilaterally: Pulmonary edema
Focal B-lines: Consolidation, mass
A-lines + lung sliding: Asthma, COPD, PE
If B lines present:
3 or more B lines in multiple lung fields, especially if bilateral: pulmonary edema (or diffuse pulmonary fibrosis/scarring)
Focal B-lines: A-lines everywhere else with lung sliding, and focal B-lines: Consolidation, mass
If B lines absent:
A-lines present with lung sliding: think asthma, COPD, PE, etc
No lung sliding: think pneumothorax, pleurodesis
Pleural effusions are one of the most satisfying discoveries to make. It is especially important to make it using ultrasound given that it is nearly 100% sensitive for detecting effusions much smaller than a chest x-ray can detect. The characteristics and echogenicity of the fluid can also give you a good idea of what disease process is going on (transudative vs exudative processes).
Patient & probe positioning
Using the phased array probe, place the patient in a position that will allow the effusion to collect at the base of the lung, either in bed at 30-45 degree angle, or sitting up.
Place the probe along the midaxillary line with the probe marker to the head and look for identify your borders.
Once you have identified your borders, you should see the lung coming in and out of view. There are multiple things to note in this image that will confirm your diagnosis.
Spine sign - if this anechoic space between the liver and the lung were filled with air, then ultrasound waves would not pass through and posterior structures would not be seen. Here you can clearly see a hyperechoic area below the effusion corresponding with the spine.
Jellyfish sign - the atelectatic lung can often be seen on on the rostral portion of the image, compressed and floating in the fluid.
Curtain sign - with each inspiration you can see the lung filling with air and obscuring the pleural effusion.