The human respiratory system is roughly divided into two parts. The first part represents the airways that conduce air from the outside into the body, through the nose or mouth, and then through the pharynx and larynx into the trachea. The trachea divides into two main bronchi; each of them goes to one side into the left and right lung. In both lungs, bronchi divide further into smaller and smaller airways. The cartilage is no longer present in the bronchial wall at the approximately 10th generation of bronchi, and then bronchi become bronchioles. They usually divide seven more times. When they divide for six more times, first alveoli appear. There are 23 divisions of airways altogether.

The second part is the lung parenchyma, which is a place where gas exchange occurs. It is mainly composed of a large number of thin-walled alveoli in which gas exchange takes place. There are also capillaries for suppling alveoli with blood that needs to be oxygenated and small airways (respiratory bronchioles and alveolar ducts).

Bronchial tree

Available at: https://i.pinimg.com/736x/9f/2d/78/9f2d78b6adf41100c8e5f0e71c5ede93.jpg (Accessed: 13 November 2019).

MECHANICS OF BREATHING

The lungs are full of small air-filled spaces. Consequently, when we are investigating the healthy lung parenchyma with ultrasound, we cannot see the usual structure of the tissue as when performing the ultrasound of parenchymal organs. The ultrasound waves are completely dispersed when traveling into lung tissue. That is why lung ultrasound is frequently called “the ultrasound of artifacts”.

The reason for that is the presence of air. Air has a much lower acoustic impedance than lung tissue, which means that it opposes ultrasound waves less than the tissue. The acoustic impedance (Z) is defined as the density of the medium (ρ) multiplied by the speed of sound in the medium (v) (Z=ρv). The higher the difference between the acoustic impedance of two different media (in our case: air and tissue) at the interface, the greater the reflection of ultrasound. Almost all ultrasound waves reflect at the interface between air and tissue. Where ultrasound waves are reflected and scattered, this can mostly be observed as specific artifacts (A-lines, B-lines, etc.), which will be explained later The reasons for these artifacts are the interfaces between air and tissue and the lung parenchyma that is filled with alveoli.

A-LINES

You can see an artifact called A-lines between the two acoustic shadows of the ribs or “bats’ wings.” These are a normal finding in healthy lungs but can also be present in some pathologies. The A-lines are hyperechoic lines that are parallel to the pleural line and arranged at an equal distance. The distance among adjacent A-lines (first A-line, and the pleural line) is the same as the distance between the pleural line and skin.

A-lines are the so-called reverberation artifacts. They appear because ultrasound waves reflect from the pleura. When these waves travel to the ultrasound probe, some of them are recorded, and the remaining waves are reflected from the probe back to the pleura. When these secondary reflected waves reflect from pleura, the same phenomenon occurs, and for the waves that are recorded now, it takes precisely two times longer to be recorded. The ultrasound machine calculates as if they travel two times more distant than the first ones (pleural line). Consequently, the first A-line is shown on the screen. When waves reflect multiple times, other A-lines appear.

Scheme of A-lines

Available at: https://media.springernature.com/lw785/springer-static/image/art%3A10.1007%2Fs13665-017-0164-1/MediaObjects/13665_2017_164_Fig1_HTML.gif (Accessed: 13 November 2019).

Acoustic shadows of the ribs

Available at: https://accessemergencymedicine.mhmedical.com/data/books/ma3/ma3_c007f002ab.png (Accessed: 13 november 2019).

Anatomical drawing fused with a corresponding ultrasound image demonstrating the superficial chest wall structures

Available at: http://viewer.zmags.com/publication/1f3688e9#/1f3688e9/1 (Accessed: 13 November 2019)

Intercotal space with one A-line, PAME Maribor

B-LINES

B-lines (also known as “comet tails,” “ultrasound lung comets,” or “lung rockets”) are hyperechoic artifacts that origin at the pleural line and spread like comets or rays to the end of the ultrasound visual field. Where there is fluid in the lungs, B-lines tend to replace A-lines and move synchronously with pleural sliding. Scientists do not completely agree with how B-lines are formed. They could be formed as a consequence of thickened interlobular septa, or because of fluid filled alveoli. What matters is that we know that they are produced because of fluid in the lung parenchyma. We can observe B-lines, especially when a patient has pulmonary edema. More than three B-lines in a single view between two ribs have a specificity of 95% and a sensitivity of 97% for diagnosing pulmonary edema. Keep in mind that few B-lines can also be present in a healthy person.

They have a high negative predictive value for ruling out pneumothorax: 98–100%. The visualization of even one B-line essentially rules out the diagnosis of a pneumothorax.

B-lines PAME Maribor

Pneumonia

Pneumonia is an infection that causes inflammation of the alveoli in one or both lungs. Alveoli may fill with fluid or pus. A variety of microorganisms, such as bacteria, viruses, and fungi can cause pneumonia.

Signs and symptoms of pneumonia can vary from mild to severe, depending on the health and age of the patient and the type of microorganism that causes the disease. Such signs and symptoms are chest pain when coughing or breathing, cough with phlegm or pus, fatigue, fever, sweating, shaking chills, nausea, vomiting, diarrhea, or shortness of breath. Mild signs and symptoms are often similar to those of a cold or flu, but they usually last longer.

Pneumonia has a characteristic appearance under ultrasound. In the early stages of pneumonia, where only some alveoli are filled with fluids, we can see B-lines where fluid-filled alveoli are surrounded by the air-filled lungs. When we see localized focuses of numerous B-lines in connection with the appropriate clinical signs and symptoms, the diagnosis is early pneumonia. Another characteristic feature is the consolidation of the lungs. It occurs when the inflammatory fluid fills the alveoli, and the lungs appear solid and have a homogenous hypoechogenic or isoehogenic  echotexture like the liver. If the consolidation is not translobar (i. e. does not involve whole lobulus of the lung) and appears near the pleural line the shred sign is formed. Which is formed as a result of interface between consolidation and deeper lying normally aerated tissue. This is accompanied by air bronchograms that appear when the air remains within small bronchi, and it is seen by the ultrasound like small air bubbles all lined up within a bronchus. When the small bubbles are seen as they bubble in and out with each breath, we use the term dynamic air bronchogram. The air is replaced with fluid with time. Fluid-filled bronchi have a hypoechoic appearance with echogenic walls (fluid bronchograms). A dynamic air bronchogram has a 97% positive predictive value for pneumonia. It is important to differentiate pneumonia from atelectasis. In pneumonia, the volume of the lungs remains the same or increases, and airspaces are filled with inflammatory fluid.

 

Dynamic air bronchogram, PAME Maribor
Lung consolidation PAME Maribor
Air bronchogram, PAME Maribor

Pulmonary edema

Pulmonary edema is a condition caused by excess fluid in the lungs. The fluid collects in the alveoli, making it difficult to breathe. Hearth problems are usually the cause of pulmonary edema, but it can be caused by pneumonia, certain toxins and medications, trauma to the chest wall, and high altitude. It is called an acute cardiogenic pulmonary edema or ACPE when its cause is the heart.

Signs and symptoms can appear suddenly or develop over time. It all depends on the cause of pulmonary edema. The acute onset of pulmonary edema can cause dyspnea that worsens when active or even when lying down, a feeling of suffocating when lying down, wheezing or gasping for air, cold skin, blue-tinged lips, palpitations, productive cough (sputum that may contain blood), anxiety. In long-term or chronic pulmonary edema, we can observe more shortness of breath, difficulty breathing with exertion and when lying down, wheezing, gaining weight rapidly, edema of the lower extremities, and fatigue. Traveling to or exercising at very high altitudes can cause the so-called high-altitude pulmonary edema (HAPE). Signs and symptoms are similar to acute onset, but they also include headache, chest discomfort, and fever.

When using ultrasound for diagnosing pulmonary edema, we are looking for B-profile (anterior interstitial syndrome with lung sliding). The B-profile will have two separate, positive zones for B-lines, bilaterally. Positive zones are determined when there are at least 3 B-lines in one intercostal space during the respiratory cycle. The sensitivity of POCUS for the alveolar-interstitial syndrome (AIS) is 97%, with a specificity of 95%.

Lung contusion

Lung contusion is a condition caused by blunt force trauma to the chest. It most commonly occurs as a consequence of motorcycle and car crashes. Nevertheless, it can also be seen in blast trauma.

When a sudden blunt force hits the chest, a big amount of energy is transduced to the lung parenchyma. The lungs are damaged because their structure cannot withstand and absorb the energy from the force that hits them. Acceleration and deceleration forces that are transferred to lung parenchyma in a traumatic event cause damage to the alveolocapillary membrane. These forces can simply tear apart small blood vessels and alveolar walls, which can cause bleeding. More fluid leaks in the alveolar space because of the accompanying tissue inflammation, and together with micro-hemorrhages, it causes pulmonary edema.

Clinically, patients with lung contusion are dyspnoeic, tachypnoeic, and cyanotic. Hypotension, reduced cardiac output, and compensatory tachycardia can also occur. Auscultatory findings can include rales and decreased breath sounds, especially in a severe contusion. In about half of the cases, patients wheeze, have bronchorrhea, and cough with blood present in the sputum .

When assessing the pulmonary contusion with the ultrasound, we are looking for the signs of traumatic pulmonary edema. Multiple B-lines, which start at the pleural line and spread to the bottom of the ultrasound screen, are present as a sign of interstitial syndrome or edema. Edema is present because of the inflammation of the damaged lung parenchyma. The inflammation leads to increased capillary permeability. Consequently, the interstitium becomes filled with fluid, and B-lines appear (discussed in the previous section in more detail).

Alveoli become filled with leaking fluid and proteins from inflamed blood vessels and tissue approximately 24 hours after the injury. When fluid leaks into the alveoli, it flushes out the surfactant. The surfactant is a fluid that is normally spread on the alveolar walls and decreases their surface tension. It allows them to expand during breathing easily. When the surfactant is flushed away, the compliance of the alveolar tissue is decreased. Consequently, a patient uses more energy to take a breath, and pulmonary parenchyma becomes condensed and filled with less air. We can see the second sign of pulmonary contusion, the so-called C-lines, or lung consolidation with ultrasound. It is a sign of unventilated and condensed pulmonary parenchyma that appears on the ultrasound like liver parenchyma. Do not confuse the pulmonary hepatization with the so-called mirror artifact, which is a normal finding in a healthy patient.

The sensitivity of ultrasound in detecting pulmonary contusion is 97.5%, and the specificity is 90%.

B-lines above the right diaphragm PAME Maribor

Right basal pneumonia with pleural effusion and air bronchogram PAME Maribor
C line

Available at: https://med.emory.edu/departments/emergency-medicine/sections/ultrasound/image-of-the-fortnight/lung/pneumonia.html (Accessed: 19 November 2019).

Pulmonary fibrosis

Pulmonary fibrosis is a progressive process in which normal pulmonary tissue is replaced with fibroblasts and collagen. Changes lead to progressively worsening ability to transfer gases to the blood, resulting in dyspnoea. Patients die because of tissue hypoxia. The most common cause of pulmonary fibrosis is unknown. We refer to this type of fibrosis as idiopathic pulmonary fibrosis. Diseases in which pulmonary fibrosis can be seen are connective tissue disorders (systemic sclerosis, Sjögren’s syndrome, polymyositis, dermatomyositis, rheumatoid arthritis).

Features of pulmonary fibrosis

When assessing pulmonary fibrosis with ultrasound, we can observe multiple B-lines. They appear as a consequence of the thickened interlobular septa and interstitium, thickened pleural line (more than 3 mm), absence of lung sliding and subpleural cysts. When searching for signs of pulmonary fibrosis, we need to examine the whole lungs (from the apex to the base on both sides at midclavicular, anterior axillary, medial axillary, posterior axillary, subscapular and paravertebral line).

The sensitivity of transthoracic ultrasound in diagnosing pulmonary fibrosis is around 89%. The specificity is around 50%.

Pleura line appearance: (A) Thickened (.3 mm) irregular pleura line (B) normal thickness (2 mm).

Available at: https://www.researchgate.net/figure/Pleura-line-appearance-Notes-A-Thickened-3-mm-irregular-pleura-line-due-to_fig3_323940330 (Accessed: 19 November 2019)

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