1.What’s the benefits of lung ultrasound?
In the past few years, pulmonary ultrasound imaging has been used more and more clinically. From the traditional way of judging the presence and amount of pleural effusion, it has revolutionized lung parenchymal imaging. We can diagnose the 5 most common severe causes of acute respiratory failure (pulmonary edema, pneumonia, pulmonary embolism, COPD, pneumothorax) in more than 90% of cases with a simple 3~5 minutes’ lung ultrasound diagnosis.
2.How to choose ultrasound probes?
The most commonly used probes for lung ultrasound are L10-5 (also called small organ probe, frequency range 5~10MHz, linear array transducer) and C5-2 (also called abdominal probe or large convex, 2~5MHz, convex array transducer), and some scenes are also using P4-2 (also called cardiac probe, 2~4MHz, phased array transducer)
The USB ultrasound probe L10-5 is convenient to obtain a clear pleural line and observe the echo of the subpleural tissue. The ribs can be used as a marker to observe the pleural line, which can be used as the first choice for pneumothorax assessment. The frequency of the abdomen probe is medium, which can inspect the entire chest and also observe the pleural line more clearly. The phased array probe is easy to pass through the space between the two ribs for imaging and has a deep detection depth. It is often used for the assessment of pleural effusion, but it is not good at detecting the condition of pneumothorax and pleural space.
3.Where to scan?
A modified bedside lung ultrasonography (mBLUE) scheme or a two-lung twelve-partition scheme and an eight-partition scheme are commonly used in lung ultrasound evaluation. There are a total of 10 check points on both sides of the lungs in the mBLUE program, which is suitable for situations that require rapid inspection. The 12-zone plan and the eight-zone plan are to slide the ultrasound probe in each area for more thorough scanning.
The location of each checkpoint of the mBLUE scheme is shown in the figures below:
check point
location
upper blue point
the point between the middle finger of the hand on the side of the head and the base of the ring finger
diagram point
the ultrasound probe finds the position of the diaphragm at the mid-axillary line
M point
the midpoint of the connection between the upper blue point and the diaphragm point
PLAPS point
M point extension line perpendicular to the intersection of the posterior axillary line
Back blue point
the area between the subscapular corner and spine
The twelve-division scheme is to divide the thorax into six areas, anterior, lateral, and posterior chest wall, by the patient’s parasternal line, anterior axillary line, posterior axillary line, and paraspine line. Each area is divided into two upper and lower areas, a total of 12 Districts. The eight-zone scheme does not include the 4 zones of the posterior chest wall, and is often used for the diagnosis and evaluation of ultrasound interstitial lung syndrome. The specific scanning method is to start from the midline in each area, the midline of the probe is completely perpendicular to the bony thorax (longitudinal axis section), first slide to the outside to the dividing line, return to the middle, then slide inside to the dividing line, and then back Midline.
4.How to analyze ultrasound images?
As we all know, air is the “enemy” of ultrasound because of ultrasound attenuating rapidly in the air, and the presence of air in the lungs makes it difficult to directly image the lung parenchyma. In a normally inflated lung, the only tissue that can be detected is the pleura, which is shown as a high echo horizontal line under ultrasound, called the pleural line (the one closest to the soft tissue layer). In addition, there are parallel, repetitive, high-echoic horizontal line artifacts below the pleural line, called A-line. The appearance of the A line means that there is gas below the pleural line. This gas can be normal lung gas or free gas from pneumothorax.
During the ultrasound examination of the lungs, the pleural line is first one to be located, unless a large amount of subcutaneous emphysema is generally visible. In normal lungs, the viscera and parietal pleura slide relative to each other with breathing, which is called lung sliding. As shown in the two pictures below, the upper picture shows lung sliding, and the lower picture shows no lung sliding.
In general, patients with pneumothorax, or have a large amount of pleural effusion to make the lungs far away from the chest wall, the lung sliding signs will disappear. Or pneumonia consolidates the lungs, and adhesions appear between the lungs and the chest wall, which can also make the lung sliding sign disappear. Chronic inflammation produces fibrous tissue and weakens lung mobility, and lung sliding cannot be seen in the chest drainage tube as in the late stage of COPD.
If A line can be observed, it means that there is air under the pleural line and the sliding signs of the lungs disappear, it is likely to be a pneumothorax, and you need to find the lung points for confirmation. Lung point is the transition point from no lung sliding to normal lung sliding in pneumothorax, and is the gold standard for ultrasound diagnosis of pneumothorax.
Under M-mode, multiple parallel lines formed by the relatively fixed chest wall can be seen. In normal lung parenchymal images, due to lung sliding back and forth, sand-like echoes are formed below them, called beach sign. There is air below the pneumothorax and no lung sliding, so multiple parallel lines are formed, which is called a barcode sign. The dividing point between the beach sign and the bar code sign is the lung point.
If A line is not visible in the ultrasound image, it means that some tissue structures in the lung have changed, allowing it to transmit ultrasound. When blood, fluid, infection, contusion caused by clotting blood or tumors and other tissues fill the original pleural space, A line artifacts will disappear. Then you need to pay attention to the B line. Line B is also called the “comet tail” sign, which is a laser beam-like hyperechoic strip that emits vertically from the pleural line (visceral pleura) and reaches the bottom of the screen without attenuation. It will cover the A line and move with breathing. For example, in the picture below, we can’t see the existence of A line, instead of B line.
If there are several B lines in an ultrasound image you get, no worries, 27% of normal people have a limited B line in the 11th to 12th intercostal space (above the diaphragm). Under normal physiological conditions, less than 3 B lines are normal. But when you encounter a large number of diffuse B-line is abnormal, this is the manifestation of pulmonary edema.
After observing the pleural line, A line or B line, let’s talk about pleural effusion and lung consolidation. In the posterolateral area of the chest, pleural effusion and lung consolidation can be better assessed. The following picture is an ultrasound image examined at the diaphragm point. The black anechoic area is the pleural effusion, which is located in the pleural cavity above the diaphragm.
So how to distinguish between pleural effusion and hemorrhage? Sometimes fibrous exudates can be seen in pleural hemorrhage, and the effusion is usually a black homogeneous anechoic area, sometimes divided into small chambers, and floating objects with varying echo intensities can be seen around.
Ultrasound can visually assess the majority (90%) of patients with lung consolidation. The most basic definition of lung consolidation is the loss of ventilation. The magic of using ultrasound to diagnose lung consolidation is that when the patient’s lungs undergo consolidation, ultrasound can pass through the deep thoracic cavity of the lung where consolidation occurs. The lung tissue is hypoechoic, with wedge-shaped edges and unclear edges. Sometimes you may also see a sign of bronchial inflation, which is hyperechoic and moves with breathing. The ultrasound image with specific diagnostic significance for lung consolidation is liver tissue-like sign, which is a solid tissue-like echo similar to liver parenchyma that appears after the alveoli are filled with exudate. As shown in the figure below, this is an ultrasound image of lung consolidation caused by pneumonia. Some parts of the image can be seen as hypoechoic, which looks a bit like liver, and the A-line is not visible.
Under normal circumstances, the lungs are full of air, and nothing will be shown under color doppler mode. But when the lungs are consolidated, especially when there is pneumonia near the blood vessels, you can even see the blood flow image, as shown in the figure.
The sound image of pneumonia is the basic skill of lung ultrasound. It is necessary to move back and forth between the ribs, carefully check whether there is a hypoechoic area, whether there is a sign of bronchial inflation, whether there is a sign of liver tissue, and whether there is normal A line Ultrasound image of lungs.
5.How to decide the result of ultrasound examination?
Through a simple ultrasound scan (mBLUE scheme or 12-zone scheme), the characteristic data can be classified to determine the severe cause of acute respiratory failure. Completing the diagnosis quickly can alleviate the patient’s breathing difficulties faster and reduce the use of complex examinations such as CT and UCG. These characteristic data include: lung sliding, A performance (A line on both sides of the chest cavity), B performance (B lines appear in both chest cavities, and there are no less than 3 B lines or adjacent B lines adhesion), A/B performance (A performance on one side of the chest, B performance on the other side), lung points, lung consolidation and pleural effusion.