Editor’s note: This is the first in a three-part series examining prehospital applications for point-of-care ultrasound. For a primer on using this technology in the field, see “It’s Time to Embrace Point-of-Care Ultrasound” from September 2016 at www.emsworld.com/article/12231975.
Though respiratory distress calls are one of the most common scenarios prehospital clinicians face, we historically struggle with differentiating the cause. The classic example is cardiogenic pulmonary edema vs. COPD. Many patients with a history of one also have a history of the other. A 2013 study found that Australian paramedics in a metropolitan 9-1-1 service diagnosed patients suffering from acute pulmonary edema with a sensitivity of only 29%.1 Interestingly, only 3% of these patients presented with the classic textbook finding of pink, frothy sputum. Given that the proper treatments for these conditions are distinctly different, it is important to be able to properly differentiate the pathology at hand—and that requires a more accurate assessment than a stethoscope can offer.
The biggest enemy of ultrasound is air. When the beam from the transducer meets air, the ultrasound waves scatter chaotically instead of reflecting nicely and providing information to create a usable image. This is why using ultrasound to image air-filled lung tissue has historically been considered futile. Lung ultrasound was first described in the 1980s and not widely adopted until much more recently. This is a modality that allows EMS the opportunity to be on the forefront of medicine.
As to fit the scope of a general overview, we’ll focus on five major findings: lung slide, A-lines, B-lines, consolidation and effusion.
Five Major Findings
The interface between the visceral and parietal pleura shows up as a bright line just past the ribs. Lung slide refers to the shimmering effect that sliding between the pleural layers creates. A lack of slide indicates either air between the layers (pneumothorax), adhesions that may be caused by pneumonia or scarring, or an unventilated lung—possibly from a bronchial intubation (more on that later). Finding the lung point (the exact spot where lung slide begins) next to nonsliding tissue increases the specificity for pneumothorax—though a fair argument could be made that finding the lung point is not necessary, depending on patient presentation. An otherwise healthy patient with acute chest trauma and no lung slide is unlikely to be suffering from a condition other than pneumothorax.
When we look for A- and B-lines, we are not looking for lines that physically exist. These lines are artifacts of the interaction of ultrasound waves with air, fluid and tissue.
A-lines appear as repetitive horizontal lines below the pleural line. The lines repeat at a perfect distance each time—the distance between the pleural line and the face of the transducer. The ultrasound machine uses the amount of time it takes a wave to return to the transducer to determine how deeply to draw a structure. In the case of lung ultrasound, some of these waves reflect a second time off the interface between the transducer and the skin. These particular waves will again reflect off of the pleural line before reaching the transducer face and being interpreted—taking exactly twice as long to make the journey as the first set of waves. This repeats several times, creating these “ghost” pleural lines down into the lung tissue.
B-lines are vertical lines that extend from the pleural line deep into the lung tissue—the reason a low-frequency transducer (phased array or curvilinear) is typically used when examining the thorax—and have a similar appearance to rays of light. They also decimate A-lines at the point where they intersect. These are seen as a result of widened interlobular septae from fluid accumulation (pulmonary edema, acute lung injury, pneumonia). It is important to note that patients with history of severe lung disease, such as pulmonary fibrosis, may have chronic B-lines even without any acute pathology.
Normal fissures in the lung tissue may also create single B-lines and are not pathologic. Typically B-lines are considered pathologic when three or more are present within one intercostal space, and the number of lines tends to correlate with the severity of the disease process. This can allow a provider to evaluate treatment efficacy, such as nitrates and positive pressure in the pulmonary edema patient, especially in extended patient contact scenarios.
Consolidation is seen when the alveoli are either collapsed or filled with fluid, emptying them of air and allowing visualization of the actual lung tissue. Typically consolidation is the result of pneumonia or atlectasis. Sometimes the tissue may take on a texture similar to that of the liver; this is referred to as hepatization.
Pleural effusions, like any other free fluid in the body, are seen as a black stripe within the pleural cavity. These may require draining to restore pulmonary function. Patients with consolidation or effusion may be positive for spine sign, where the spine is visible in the thoracic cavity. While the spine is always visible in the abdomen, the presence of air in the thorax prevents ultrasound waves from penetrating and reflecting deeply enough to image the spine, whereas fluid and/or consolidated tissue allows for deeper imaging. If you can see the spine above the diaphragm, something is wrong.
The BLUE Protocol
While these may seem like a lot of variables and interpretations to remember, a useful tool exists to expedite the diagnosis of pulmonary pathology. The Bedside Lung Ultrasound in Emergency (BLUE) protocol, as described by ultrasound experts Daniel Lichtenstein, MD, and Gilbert Mezière, MD, in 2008, is a simple algorithm to determine the cause of a patient’s respiratory distress. All diagnostic outcomes in the protocol have a specificity of over 90%—extremely high for any prehospital test.2 The full protocol takes less than three minutes to perform and utilizes sites familiar to anyone experienced in using a stethoscope: the upper and lower chest at the anterior, anterolateral and posterior aspects. The only big difference is scanning the legs for deep vein thrombosis if indicated. While that is beyond the scope of this article, it is another simple test that takes only a few seconds.
For a BLUE protocol algorithm flow chart, see Figure 1. As demonstrated by the algorithm, the example of pulmonary edema vs. COPD can be differentiated very rapidly: Assuming lung slide is intact, the patient with B-lines is suffering from pulmonary edema, not COPD. If A-lines are present, the diagnosis of pulmonary edema is excluded and leaves the diagnostic choices of pulmonary embolism vs. COPD. At this point the legs may be scanned for presence of DVT, or other data gathered from the assessment may be used to stratify the risk of pulmonary embolism: EtCO2 values, patient history or possibly even scanning the heart—the right ventricle may be enlarged in the case of massive pulmonary embolism. Once again it must be emphasized that POCUS is not magic, nor does it replace a thorough assessment. A quality assessment and proper knowledge of physiology are required to make the most of this tool.
Aside from patient pathology, lung ultrasound provides protection from provider liability and patient harm in the case of the intubated patient. While waveform EtCO2 can reliably demonstrate that the distal end of the endotracheal tube is past the glottic opening, that’s the extent of the data it provides—the tube may be properly placed superior to the carina, or it may be buried deep into the bronchus. Nearly 6% of emergently placed endotracheal tubes, both in the emergency department and prehospital, may be placed in a bronchus.3 Auscultation of equal lung sounds is unreliable, especially in children and in noisy environments such as a moving ambulance or aircraft.
A quick scan will indicate if both lungs are being ventilated faster and more reliably than auscultation and in any environment. A diagnostic accuracy study published in 2009 and conducted with anesthesiologists showed that using auscultation to identify tracheal vs. bronchial intubation yielded a sensitivity of 66% and a specificity of 59%. By checking for lung slide and visualizing the endotracheal tube balloon via ultrasound, both the sensitivity and specificity rose significantly, to 93% and 96% respectively. Overall participants were able to correctly identify where the endotracheal tube was placed 62% of the time by auscultation and 95% via POCUS.4
Many applications of lung ultrasound yield a very high return on a relatively low training investment. Through these simple assessments, prehospital providers will be able to narrow down their differential diagnosis accurately and provide the right treatment immediately to their patient instead of deferring it to the hospital or, worse, providing the incorrect treatment and further harming the patient.
1. Williams TA, Finn J, Celenza A, Teng TH, Jacobs IG. Paramedic identification of acute pulmonary edema in a metropolitan ambulance service. Prehosp Emerg Care, 2013 Jul–Sep; 17(3): 339–47.
2. Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. Chest, 2008 Jul; 134(1): 117–25.
3. Geisser W, Maybauer DM, Wolff H, Pfenninger E, Maybauer MO. Radiological validation of tracheal tube insertion depth in out-of-hospital and in-hospital emergency patients. Anaesthesia, 2009 Sep; 64(9): 973–7.
4. Ramsingh D, Frank E, Haughton R, et al. Auscultation versus point-of-care ultrasound to determine endotracheal versus bronchial intubation: a diagnostic accuracy study. Anesthesiology, 2016 May; 124(5): 1,012–20.
Branden Miesemer, NRP, FP-C, is a flight paramedic in the Midwestern United States and an adjunct paramedicine instructor for several local colleges. He is an advocate for leveraging technology and social media to provide low-cost, cutting-edge medical education and training. Follow him online at EMSPOCUS.com, Facebook.com/emspocus and on Twitter at @emspocus.