Get to the Point: Lung Ultrasound
The incredible power this tool has! This must be exactly how Luke Skywalker felt when he was gifted his first lightsaber. No longer was I making an educated guess on if my patient was suffering from a pneumothorax or not, I now had the power to make an incredibly accurate assessment and diagnosis.
Soon after I was bestowed with my newfound tool, we were launched for an interfacility transfer, a septic patient requiring admission to an ICU. Drats! I was hoping and praying to the EMS gods for trauma, a way to utilize this device! As we entered the room, I was dumbfounded as my partner placed the probe on our patient's chest.
“Wow, I’m definitely seeing some B-lines. What do you think about slowing down that volume and starting some norepinephrine?”
“Sure thing!” My mind rapidly searched for the term B-lines. Frankly, I didn’t know where alphabetically based lines were, much less what in the world a B-line represented.
Lung ultrasound has been around for years, however, has seemed to gain increased popularity during the COVID19 pandemic due to its bedside diagnostic capabilities. While lung ultrasound is often associated with trauma, there is incredible potential for use in medically ill patients. Before we take a dive into some of its potential, let’s briefly review ultrasound.
Air is often considered the enemy of ultrasound. This is evident each time you pick up a bottle of ultrasound gel and place it on your probe. Your goal with the gel is to reduce air between the surface you are imaging and the probe, allowing for the travel of sound waves. These waves reflect off structures within the body and return to your probe, producing an image. Given that our lungs are primarily filled with air, how is ultrasound useful for lung imaging? Keep reading to learn! First, let's cover some basic anatomy of your probe and lung field.
Probes come in various shapes and sizes depending on the imaging goal. Probes, however, maintain consistency throughout in that they all utilize a probe marker. The probe marker is often a palpable dot or line on the side of the probe. This marker correlates with a mark or logo on the screen to ensure your probe is pointing in the direction you think it is. For lung imaging, we’ll be pointing our marker toward the patient's head.
Given that our first imaging will be looking for pneumothorax, and we expect air will “rise”, we’ll first inspect the anterior chest of our patient.
As we look through the anatomy of our displayed image, we see several key things. The top of our image is made up of skin, muscle, and tissue. This leads us down to two black columns with a bright white line between then some “static” appearing tissue below. The two black columns are defined as rib shadows. Given that bones are hard in adults, the ultrasound waves cannot travel through them causing a black, anechoic appearance. The white line between the two rib shadows is your pleural line.
This line often described as “ants marching” is where your Visceral and Parietal pleura meet. Why does this matter? Well, can you envision a disease process where your Visceral and Parietal lung tissue would not meet? A pneumothorax! While there are other causes of the absence of what is termed “lung sliding”, this is incredibly sensitive for pneumothorax in the proper clinical context. An additional tool to assist in the detection of pneumothorax is M-Mode.
M-Mode, or motion mode, places a line or “pick”, through the ultrasound. This line now graphs all motion that runs through it. Standard lungs will appear like so:
The motion graph at the top almost appears like water and a sandy beach, right? Well, the creatives of ultrasound imaging also thought so-calling this the “seashore sign”. In the event of a pneumothorax, the lung will appear like this:
This is called a bar code sign and is indicative of a lack of lung sliding. This is either because there is a disruption between the contact of your visceral and parietal layers or that lung isn’t being ventilated.
You can use this knowledge of imaging to find the end of the pneumothorax. After identification of a potential pneumothorax by a lack of lung sliding, move your probe down the chest. You may soon be able to find the location where you now see lung movement or sliding. This location, where the movement of the “good lung” is seen is called Lung Point and is pathognomonic for pneumothorax. Keep in mind, however, that the presence of a pneumothorax on ultrasound does not signify a tension pneumothorax, you must use additional signs and symptoms to correlate with your ultrasound findings. Interestingly enough, ultrasound maintains a higher sensitivity and specificity for determining pneumothorax when compared to chest X-ray.
What else can cause a similar appearance, where one lung appears with a bar code sign? Imagine you are intubating a difficult airway. You utilize all the tricks up your sleeve, and with your heart racing push the ET tube through the cords. “Looks a little deep”, your partner notes. Is it? Using the above knowledge we just learned, we can place our probe on the chest and actually see both lungs being ventilated. In the event of right mainstem intubation, we wouldn’t see lung sliding on the left side and would be presented with a bar code sign.
Alright, we’ve covered the low-hanging fruit in lung ultrasound, pneumothorax, but is POCUS useful for anything else in the lungs?
We mentioned before that air is the enemy of ultrasound and your probe will allow clear imaging utilizing liquid mediums. If this is all the case, then what would happen if your lungs were filled with liquid, say pulmonary edema? Well, you would develop B-Lines. B-Lines are artifacts that are often described as a “water fall” appearance within the lungs. These can be caused by pulmonary edema, pneumonia, or even trauma like pulmonary contusions.
B-lines must meet certain criteria, including at least 3 being visible (1). In the differential of pulmonary edema, they should be present in more than a single rib space (section of lung) and bilaterally. B-lines in a single lung can be often associated with pneumonia. Another rule for B-lines is that they must extend beyond A-lines. A-lines? What are those? I’m happy you asked! A-lines are artifact from your pleural line. They are seen in normal lung tissue and easily distinguished by their frequency on the image-you can measure these out and find them consistently spread apart on the image.
Where is this useful? Imagine you are dispatched for a female who is short of breath. You arrive on scene and she is visibly short of breath. You listen to her lungs, hearing wheezing throughout. Eyeing over her history, you note she is being treated for COPD, asthma and has a history of CHF. Is this asthma? COPD? Is this a “cardiac wheeze” where pulmonary edema is causing turbulent airflow? Imagine you’re able to pull out your ultrasound probe, place it on her chest and see B-lines in multiple lung fields bilaterally. You can now paint your treatment toward a specific disease process!
As you can see, lung ultrasound isn’t just limited to trauma. While we’ve just scratched the surface of the potential uses, the value of this simple physical exam tool is quite obvious. Why guess when you can actually see?