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3. Lung Imaging
Chest X-ray radiography is a common auxiliary means in the diagnosis and treatment of critical care. It can provide information about the course of the disease and cardiopulmonary status. And in the critical care unit, patients often need a variety of life-supporting catheters, X-rays can help clinicians determine the location of these catheters and whether there are other complications during the placement of the catheters. Routine chest X-rays can help doctors determine if treatments are effective and manage potential complications.
Since the X-ray is such an important thing, how do we confirm its quality?Rubinowitz AN et al. summarized a series of judgment processes . The first step is to assess the quality of the image technically, then clinicians should focus on the location of patients’ catheters such as stomach tube, tracheal cannula, and all other support devices. The next step is to assess the patient’s condition. Clinicians should systematically evaluate patients’ cardiopulmonary status, size of heart border, look for the effusion of the lung and pleural. Then, clinicians should not forget to compare the current X-ray image with the prior studies and evaluate whether the patient is recovering or getting worse.
22.214.171.124 Monitoring and Support Devices
Endotracheal and Tracheostomy Tubes
Endotracheal intubation is common in patients requiring short periods of mechanical ventilation. Improper placement of the endotracheal tube is very common. The location of endotracheal intubation can be simply determined by auscultation to determine whether the respiratory tone of the left and right lung is symmetrical. However, for patients in the critical care unit, the lung is often damaged, so the stethoscope method may not provide accurate information. So, clinicians always choose bedside X-ray to evaluate the endotracheal tube’s location. With the patient’s head in a neutral position, the proper position of the endotracheal tube is its tip should be located 3–5 cm above the carina . Too deep or too shallow placement of the tube can lead to bad results. Endotracheal tube insertion into the right main bronchus is common because the right main bronchus is thicker and straighter. Deep insertion may lead to hyperventilation of the lung on this side or even lead to pneumothorax, and may lead to contralateral atelectasis. Shallow endotracheal intubation may increase the risk of catheter prolapse or laryngeal injury. Another poorly positioned catheter is that the catheter strays into the esophagus, which, if not detected in time, will lead to excessive accumulation of gas in the gastrointestinal tract, and even lead to gastrointestinal perforation.
Central venous catheters are commonly used for fluid rehydration, parenteral nutrition, or CVP monitoring. To reduce the risk of thrombosis, the central venous catheter tip should be located inside the superior vena cava and outside the venous flap . When the catheter is too deep, it can enter the right atrium, increasing the risk of arrhythmias and heart perforations.
Sometimes a catheter goes into an artery, which is often seen in the subclavian artery or common carotid artery. Bright red blood ejections from the catheter are usually observed when the catheter is inserted into the artery, but this may not be evident in ICU patients with heart failure or hypotension, so X-ray examination is still needed to confirm its position. Subclavian venipuncture is of particular concern because it may result in hemothorax, pneumothorax, or chylothorax.
126.96.36.199 Pulmonary Parenchymal Abnormalities
Atelectasis, or a decrease in the volume of air in the lungs, is the most common cause of chest X-ray opacity in ICU patients . The most common atelectasis is in the lower-left lobe, followed by the lower right lobe and the upper right lobe .
The X-ray manifestations of atelectasis can be classified into direct X-ray signs and indirect X-ray signs. Direct signs include reduced lung transparency in the atelectasis part, an increase in the density of uniformity. In the convalescence period or with bronchiectasis, X-ray may appear uneven density of the cystic transparent area.
3.2 Lung and Diaphragm Ultrasound
Ultrasound is a noninvasive technology and has been employed for the bedside assessment of the lung reliably. As a versatile imaging technique, ultrasound provides insights into the presence of lung consolidation, pleural effusion, or interstitial-alveolar syndrome, especially, it allows the possibility of gaining regional information. In recent years, ultrasound has played a crucial role in bedside assessment of the critically ill among imaging techniques. In 1942, a neuroscientist Karl Dussik first introduced the use of an ultrasound machine as a medical diagnostic tool  and André Dénier first described the diagnostic application of ultrasound . More importantly, in 1989, the Fraçois Jardins intensive care team introduced the application of lung ultrasound in emergency care. Since 1991, point-of-care ultrasound (POCUS) has been used by intensive care physicians for the diagnosis of intensive care medicine. In recent years, the International Consensus Conference on lung ultrasound has standardized nomenclature, technique, and indications to use lung ultrasound in critical care practice . Ultrasound evaluation of diaphragm function and structure is accurate, safe, and noninvasive. Bedside ultrasound was used to evaluate the diaphragm function by measuring the diaphragm activity, diaphragm thickness, and the change rate of diaphragm thickness. It was used to identify the diaphragm dysfunction and the loss of diaphragm function.
3.2.2 Lung Ultrasound
188.8.131.52 Selection of Lung Ultrasound Probe
Conventional examination modes are divided into B-mode and M-mode. B-mode ultrasound is a mode in which a linear or convex probe scans an anatomical plane and then converts it into a two-dimensional image. M-mode ultrasound is a mode to record the reciprocating motion of a structure toward or away from the probe. Different probes have their own characteristics. Generally, high-frequency probes have weak penetration capacity, but high resolution. On the contrary, low-frequency probes have strong penetration capacity, but low resolution. Firstly, according to the patient’s body shape and chest wall thickness, low-frequency probes (convex array or phased probe, frequency 1–5 MHz) which can detect a certain depth are usually selected; secondly, high-resolution high-frequency probes (linear array probe, frequency 5–10 MHz) can be selected for pleural lesions or pneumothorax according to the location of the lesions. For obese patients, low-frequency probe is recommended to detect pleural lesions.
184.108.40.206 Operational Skills
Ultrasound examination of the lung does not require a high level of ultrasound section, which can be scanned vertically or parallel to the coastal space. Generally, each side of the chest wall is divided into three parts [9, 10] by the front axillary line and the rear axillary line. Each part is further divided into upper and lower parts, that is, each side of the chest is divided into six areas, a total of 12 bilateral areas, corresponding to different parts of the lungs. Lung ultrasound should be performed by left-right contrast, and all intercostal spaces in each region should be examined in sequence. The probe should be perpendicular to the thorax and slide along the longitudinal and transverse direction. When the probe is placed in a sagittal position and the angle is adjusted perpendicular to the long axis of the rib space, the marker points generally point to the head side and slip from the head to the foot in the vertical rib space. Most of the pleura can be observed, but the ribs will cover it. Transversely, the probe is placed horizontally along the long axis of the intercostal space, with the marker point facing the sternum and sliding along the long axis of the intercostal space. The whole pleura of the intercostal space can be observed, but it is limited to the pleura of the intercostal space.
220.127.116.11 Image Interpretation
Understanding lung ultrasound images requires familiarity with basic lung ultrasound signs. Because ultrasound cannot penetrate the air, the lungs are the main air-containing organs, combined with the obstructive effect of thoracic bone structure, so the lung has been regarded as the forbidden area of ultrasound examination. However, the lung is an organ of liquid and gas blending. Any lung lesion is accompanied by the change of liquid–gas ratio, and the detection of liquid by ultrasound is sensitive, the change of gas and liquid in the alveoli and interstitium of damaged lungs will produce some ultrasound images and artifacts, which make it possible for lung ultrasound. With the decrease of the ratio of gas to liquid in the lung, the lung lesions, or changes from normal gasified lung tissue to mild interstitial edema, severe interstitial edema and alveolar edema, or from focal to diffuse, eventually develop into consolidation, and even pleural effusion and pneumothorax.
18.104.22.168 Bat Sign
22.214.171.124 Lung Glide Sign
The pleural line moves synchronously with respiratory motion and appears in normal lung tissue. The extent of lung glide reached its maximum in the lower part of the lung, when the lung was descending toward the abdomen. Under B-mode ultrasound, visceral and parietal pleura slipped relatively. and under M-mode ultrasound, hyperechoic pleura line moved to and fro with respiratory motion. Lung glide sign is not obvious in the condition of lung hyperinflation and emphysema. The disappearance of lung glide sign can be seen in pneumonia, atelectasis, pneumothorax, weak breathing, apnea, pleural adhesion, airway obstruction, or one-lung ventilation.
126.96.36.199 Signs of Interstitial Syndrome
Alveolar interstitial syndrome is defined as the presence of three or more adjacent B-lines in a coastal space, which may be limited or diffuse. Linear and convex probes can measure the average distance between B-lines. B-line interval is about 7 mm, suggesting interlobular septal thickening, interstitial pulmonary edema, or lesion; Multiple B-lines with spacing less than 3 mm indicate alveolar pulmonary edema or lesion. The more severe pulmonary edema presents as diffuse B-line. Line B has seven characteristics: comet tail sign; from a pleural line; high echo; laser sample; no attenuation, direct to the edge of the screen; erase line A; and move with the lung sliding.
188.8.131.52 Tissue-Like Sign
184.108.40.206 Debris Sign
Short-line, debris-like, strong echo spots appear at the junction of consolidated and air-filled alveoli, which are called debris signs (Fig. 3.11). Ultrasound images of lung consolidation are varied at different stages, if the gas in the consolidated lung tissue is not fully absorbed, ultrasound shows high echo when it meets gas. In particular, inflammatory lung consolidation occurs in the stage of severe insufficient ventilation and incomplete absorption of air in the lung tissue, ultrasound images can show inhomogeneous bright/dark echoes, similar to debris, so it is called debris sign.
220.127.116.11 Bronchial Inflation Sign
Bronchial inflation sign is a heterogeneous, tissue-like (similar to liver echoes) ultrasound image with a punctate or linear hyperechoic sign. It is also an ultrasound sign in the process of pulmonary consolidation. The reason for this is that the air in the bronchus, which is inside consolidated lung tissue, is not fully absorbed; ultrasound produces a bright echo when it meets gas (Fig. 3.11). According to the dynamic changes of bronchial gas with respiratory motion, there are static bronchial inflation signs and dynamic bronchial inflation signs. Dynamic bronchial inflation sign can rule out the diagnosis of obstructive atelectasis. A small sample study found that dynamic bronchial inflation sign was more common in inflammatory lung consolidation and static bronchial sign was more common in atelectasis. For some inflammatory lung consolidation, bronchial fluid filling signs can also be seen.
18.104.22.168 Pulmonary Pulsation
Pulmonary pulsation is an early and dynamic diagnostic sign of complete atelectasis. Under normal conditions, the slippage of two pleura layers hinders the vibration of pleural line caused by cardiac activity observed by M-mode ultrasound. When the main bronchial obstruction or one-lung ventilation leads to complete atelectasis, visceral and parietal pleura slide disappears. Under these conditions, pleural line vibration caused by heart beating can be recorded by M-mode ultrasound. The disappearance of lung glide sign under B-mode ultrasound and the pulsation of pleural line with the beating of heart under M-mode ultrasound is called pulmonary pulsation.
22.214.171.124 Stratospheric Sign (Bar Code Sign)
126.96.36.199 Pulmonary Point
The pulmonary point is a special ultrasound sign in the diagnosis of pneumothorax. In the same image, one side of the lung tissue has the phenomenon of lung sliding and pulsation, while the other side does not exist. This critical point is called a pulmonary point. This phenomenon is a specific manifestation of pneumothorax. Under B-mode ultrasound, B-line disappeared, A-line and lung glide disappeared, while under M-mode ultrasound, the critical point of ultrasound sign replacing coastal sign was lung point. The principle is that when pneumothorax occurs, ultrasound detects the boundary of lung tissue compressed by gas. Ultrasound detects the side of pneumothorax, showing that lung glide disappears, and the side of normal lung tissue, showing that lung glide exists. But not all pneumothorax has pulmonary point, when the scope of pneumothorax is large or the examination is incomplete, it may not be able to find the pulmonary point, and pulmonary bullae can also be shown as pulmonary point.
188.8.131.52 Quadrilateral Sign/Sinusoidal Sign
184.108.40.206 Curtain Sign
During examination at the base of lung, images of the diaphragm, liver/spleen, or spine disappear as the lung expands during inspiration and appear as the lung volume decreases during expiration. This phenomenon occurs under normal ventilation; when there is pleural effusion or consolidation of pulmonary tissue, the phenomenon weakens or disappears.
3.2.3 Application of Lung Ultrasound
Ultrasound is a fast, noninvasive, and real-time imaging method. Lung ultrasound has been gradually improved and standardized. It plays an important role in the diagnosis, treatment, and judgment of disease changes.
3.2.4 Diagnostic Value of Pulmonary Ultrasound in Respiratory Diseases
POCUS for diagnostic assessment is of extensive use in intensive care units . POCUS in the emergency department alongside standard diagnostic tests is superior to standard diagnostic tests alone for establishing a correct diagnosis within 4 h .
220.127.116.11 Community-Acquired Pneumonia (CAP) and Ventilator-Associated Pneumonia (VAP)
According to the type of pneumonia, the imaging manifestations are different. Ultrasound signs of pneumonia are heterogeneous B-line; pleural abnormalities, mostly small pulmonary consolidation under pleura; debris sign; or bronchial inflation sign. Inflammatory consolidation manifests differently in different stages. Gas is not fully absorbed in the lung tissues of the lesions, which can be manifested as debris sign, accompanied by bronchial inflation sign, mostly dynamic bronchial inflation sign, or can be manifested as liver-like pulmonary tissue when the gas is fully absorbed. The application of lung ultrasound in screening or diagnosing of CAP or VAP has attracted much attention. 70–97% of CAP patients can see lung consolidation accompanied by bronchial inflation sign [16–18]. Consolidation has 93% sensitivity and 98% specificity in the diagnosis of CAP . However, for ICU patients, pulmonary consolidation and bronchial inflation sign can also occur in patients without pneumonia, which is related to long-term bedridden, controlled ventilation, and systemic inflammatory response. It seems that pulmonary ultrasound has limited value in the diagnosis of VAP. However, some scholars regard linear or tree-like dynamic bronchial images as one of the signs of VAP diagnosis, and can easily calculate the clinical ultrasound score at the bedside for early VAP diagnosis. It may even be better than the traditional clinical pulmonary infection score (CPIS) .
18.104.22.168 Pulmonary Edema
B-line is the most important ultrasound sign of pulmonary edema. It may be related to alveolar or interstitial exudation. According to the distribution and shape of B-line, it can be divided into homogeneous B-line, heterogeneous B-line, and converged B-line. The B-line distribution of pulmonary edema due to elevated hydrostatic pressure was more homogeneous, and there was no change of pleural line, and pleural sliding was not affected . The distribution of B-line in osmotic pulmonary edema shows that the non-dependent area is lighter, the dependent area is heavier, and even the signs of pulmonary consolidation appear. In addition, because of the high viscosity of the exudated fluid, the pleural sliding sign usually weakens or even disappears. Pulmonary edema due to elevated hydrostatic pressure is usually secondary to cardiac insufficiency and volume overload. Cardiac echocardiography can show a significant decrease in systolic function and an increase in the diameter of inferior vena cava. Osmotic pulmonary edema is usually secondary to severe infections and other factors, with normal cardiac function or enhanced systolic function without volume overload.
The pulmonary lesions of ARDS are heterogeneous. Qualitative imaging evaluation of pulmonary exudative lesions and consolidation by lung ultrasound can assist the diagnosis of ARDS [21–23]. The international consensus on lung ultrasound also suggests the diagnosis of ARDS if there are the following signs: (1) inhomogeneous B-line; (2) abnormal pleural line signs; (3) subpleural consolidation of anterior chest wall; (4) normal pulmonary parenchyma; and (5) weakening or disappearing of pulmonary glide sign . The Berlin definition of ARDS  suggests a rapid differential diagnosis of pulmonary edema for suspected ARDS patients without risk factors by echocardiography. Therefore, combined cardiopulmonary ultrasound is helpful for real-time diagnosis of ARDS at bedsides, and can differentiate pulmonary edema, atelectasis, pleural effusion, chronic heart failure, pulmonary interstitial fibrosis, and other pulmonary conditions leading to hypoxemia.
Atelectasis can be divided into compressive atelectasis and absorptive atelectasis. The former is usually caused by a large amount of pleural effusion, with sharp and smooth edges, moderate echo, and jellyfish-like shape with the beating of heart and movement of breath, the compressive atelectasis can be reduced or even disappeared after puncture and drainage. The latter is caused by airway obstruction, such as secretions or tumors. Lung ultrasound shows alveolar consolidation and homogeneous hypoechoic structures resembling liver-like structures. Compared with pneumonia, absorbable atelectasis has no dynamic bronchial inflation sign. Bronchial inflation sign is static (initial stage) or nonexistent (total air is reabsorbed in small airways). If bronchial inflation sign is dynamic, obstructive atelectasis can be excluded . Sometimes bronchial fluids can be found, which suggests that secretions or fluids fill the airway.
22.214.171.124 Pleural Effusion
Ultrasound can directly identify pleural effusion and consolidation .
Compared with bedside chest radiographs, ultrasound for detecting pleural effusion showed higher accuracy (93% vs. 47%) . Pleural effusion manifests as an echo-free area in dependent region [27, 28].
The appearance of pleural effusion under ultrasound can indicate the nature of the fluid. Pleural effusion can be characterized by anechoic, complex non-encapsulated, complex encapsulated, or homogeneous echo . Generally, complex pleural effusion suggests exudation, whereas anechoic effusion may be transudate. However, transudate can also be manifested as complex non-encapsulated effusion . This is because the transudate is not only water, but also has different components (such as cells, proteins, and fats). Transudate can also behave as an echo-free liquid. Homogeneous echo effusion is a manifestation of hemorrhage or empyema. In some cases, ultrasound images can help to judge the nature of the effusion. For example, thickened pleural or pulmonary consolidation with bronchial inflation sign (suggesting the site of infection) usually indicates exudate. Diffuse pulmonary congestion (B-line) indicates transudate because of heart failure. In supine position, in transverse scanning, the pleural space at the bottom of the lung is 5 cm or larger, which can predict 500 mL or more pleural effusion. The linear relationship between them was also determined. The pleural space per centimeter corresponded to 200 mL effusion [30, 31]. The application of ultrasound in chest puncture can reduce the number of pneumothorax cases and improve operation efficiency, even for more experienced operators .
The sensitivity, accuracy, and negative predictive value of lung ultrasound in the diagnosis of pneumothorax are much higher than those of chest X-ray and are close to those of CT , especially for traumatic patients [33, 34]. When diagnosing pneumothorax by lung ultrasound, we need to recognize the pleural sliding sign, pulmonary pulsation sign, B-line, consolidation, and pulmonary point. Pneumothorax can be diagnosed when the pleural slip sign disappears, the stratospheric sign and the pulmonary point are found by pulmonary ultrasound. If the pulmonary point appears below the midaxillary line, it indicates that at least 30% of the pulmonary parenchyma collapses .
Although the specificity with the signs described above for diagnosis of pneumothorax is almost 100%, in most cases, it is difficult to determine the pulmonary point because of the different degrees of pulmonary compression and sometimes focal pneumothorax in severe patients. Therefore, when pneumothorax is suspected in clinical practice, one by one lung tissue should be examined, if pleural sliding sign, pulmonary pulsation sign, B-line, consolidation, and pleural effusion can be found, the presence of pneumothorax in the examination site can be excluded.
126.96.36.199 Pulmonary Embolism
Studies have shown that combined cardiopulmonary and vascular ultrasonography can diagnose pulmonary embolism more accurately [13, 36]. Other studies have shown that the specificity and sensitivity of combined cardiac, pulmonary, and vascular ultrasound in the diagnosis of pulmonary embolism are significantly higher than that of single cardiac, pulmonary, or vascular ultrasound, which can significantly improve the diagnosis of suspected pulmonary embolism and reduce the examination rate of CT pulmonary angiography [37, 38]. Because pulmonary embolism mainly affects the oxygenation of patients by affecting the ratio of ventilation to blood flow, there is usually no obvious lung lesion in patients. Ultrasound signs of pulmonary embolism are mainly line A, sometimes wedge consolidation caused by pulmonary infarction, but rarely accompanied by line B or large areas of pulmonary consolidation. In addition, the presence of deep venous thrombosis in lower extremities can be determined by ultrasound screening of lower extremity vessels thus providing indirect evidence for the diagnosis of pulmonary embolism.
3.2.5 Differential Value of Pulmonary Ultrasound in Etiology of Respiratory Diseases
The bedside lung ultrasound in emergency (BLUE) protocol provides a simple method to analyze and diagnose diseases, using the characteristics of lung ultrasound. This diagnostic scheme can diagnose five common causes of respiratory failure in 90.5% of cases of acute respiratory failure . Traditional medical examination methods include medical history and physical examination. The combination of electrocardiogram and echocardiography with BLUE improves the diagnostic accuracy. The first objective of the BLUE program is to quickly diagnose and treat dyspnea symptoms. The second goal is to reduce the use of computed tomography and X-ray avoiding radiation hazards to special patients such as pregnant women [40, 41]. At the same time, the program allows accurate diagnosis of acute respiratory distress in resource-poor clinics.
The BLUE protocol standardized the examination sites of the chest, including the upper blue spot, the lower blue spot, the posterior alveolar, and/or pleural syndrome (PLAPS) points on the left and right sides.
First, looking for lung glide sign. If the lung glide sign exists and there is a clear line A, it is called A-profile. Next venous system examination needs to be done. The specificity of diagnosing pulmonary embolism with lower extremity venous thrombosis was 99%. So before other lung areas are examined, veins need to be analyzed. Posterolateral alveolar pleural syndrome (PLAPS) includes posterior chest wall lung consolidations and pleural effusions. PLAPS can be seen for many reasons, and veins need to be prioritized before PLAPS can be found. It has important clinical significance to find lower extremity venous thrombosis after finding A-profile. If no lower extremity venous thrombosis is found, then look for PLAPS at the PLAPS examination area. If PLAPS exists, it is called A-no-V-PLAPS image, suggesting pneumonia. Without PLAPS, it is called nude profile (bare image features) (all items are normal), suggesting COPD or asthma. The A′ profile is defined by the presence of A-lines without lung sliding. This profile is seen in pneumothorax. A′ profile requires finding the pulmonary point. If there is a pulmonary point, it strongly suggests pneumothorax. The diagnosis of hemodynamic pulmonary edema is preferred when B features are found. B′ profile (anterior lung B-line without lung sliding), A/B profile (A-line in one hemithorax and the B-line in another hemithorax), and C profile (consolidation of the anterior chest wall) strongly suggest pneumonia. 86% of ARDS patients had one of four pneumonia images.
3.2.6 Monitoring and Guiding Therapeutic Value of Pulmonary Ultrasound
188.8.131.52 Ventilation Score
Since the number and type of ultrasound artifacts (A-line and B-line) seen in the intercostal space of lung ultrasound vary with the loss of pulmonary ventilation function , lung collapse or re-expansion can be assessed by tracking the changes in lung ultrasound. In vitro  studies have shown that progressive homogeneous ventilation loss determines the transition from A-line to B-line, and the number of B-line gradually increases and fuses. Tissue-like features are present when ventilation is completely lost. Lung ultrasound score (LUS)  is a useful tool to quantify the degree of pulmonary ventilation reduction and to compare the degree of improvement of pulmonary ventilation before and after treatment. It realizes the transformation from ultrasound image vectorization to numerical value. According to the axillary front line and the axillary posterior line, the chest wall of the patients was divided into three zones: anterior, lateral, and posterior. The three zones were divided into upper and lower zones, respectively. Thus, one side of the chest wall of the patients was divided into six zones and twelve zones on both sides. Lung ultrasound was performed in each area and scored according to the following criteria: normal pulmonary ventilation (0 points): the presence of lung sliding and horizontal A-lines or no more than two B-lines; moderate pulmonary ventilation reduction (1 point): the presence of multiple uniformly or unevenly segregated B-lines; severe pulmonary ventilation reduction (2 points): multiple rib spaces with combined B-lines; and pulmonary consolidation (3 points): The lungs show tissue-like echoes accompanied by dynamic or static bronchial inflation signs. According to the above criteria, the total score of 12 regions ranges from 0 to 36. Dynamic assessment of LUS can accurately reflect the changes of pulmonary ventilation before and after the implementation of any treatment measures that may affect pulmonary ventilation function thus guiding the next decision-making. Several studies have shown that the clinical value of pulmonary ultrasound in assessing the degree of pulmonary inflation is in good agreement with chest CT. In patients with ARDS, regional pulmonary ultrasound scores were closely related to tissue density assessed by computed tomography, and the gradual increase in scores from 0 to 3 was significantly related to the increase in density .
184.108.40.206 Monitoring the Effectiveness of Antimicrobial Therapy
Lung ultrasound can also be used to evaluate the efficacy of pulmonary infection and can assist in adjusting and stopping antibiotic therapy. The LUS can be calculated by observing regional changes before and after treatment aimed at improving pulmonary ventilation. It has been successfully applied to evaluate antibiotic-induced alveolar reaeration in VAP .
220.127.116.11 Monitoring the Extravascular Lung Water (EVLW)
The ultrasound manifestations of EVLW has increased B-line. For acute alveolar or interstitial exudation, increased diffuse B-line can be observed by lung ultrasound. At present, lung ultrasound score can be used for clinical monitoring of EVLW. Baldi et al.  evaluated EVLW with B-line score, B-line score correlated well with quantitative CT. The overall LUS was directly correlated with EVLW assessed by pulmonary thermodilution , and with the overall lung tissue density assessed by quantitative CT . Increased LUS is an early warning of harmful side effects of fluid resuscitation in sepsis patients, transthoracic lung ultrasound may serve as a safeguard against excessive fluid loading . The LUS is independently related to the 28-day mortality, as well as the APACHE II score and lactate level, in intensive care unit shock patients. A higher elevated LUS on admission is associated with a worse outcome .
18.104.22.168 Monitoring Lung Recruitment
Lung reaeration after lung recruitment maneuvers can be monitored by direct and real-time visualization . Bouhemad et al.  used LUS to measure the reaeration of PEEP 0–15 cm H2O in 40 patients with ARDS undergoing mechanical ventilation. The level of pulmonary alveoli (or collapse) was found to be positively correlated with the pressure-volume curve (P–V curve). Therefore, ultrasound can assess the potential of lung recruitment, and dynamically monitor and guide the parameter setting of mechanical ventilation. Lung recruitment combined with appropriate PEEP may improve oxygenation and some physiological indexes in ARDS patients, but not in all ARDS patients. The setting of PEEP according to the recruitability of lung can more effectively reopen the collapsed alveoli and reduce the side effects caused by PEEP. Lung ultrasound can synthetically judge the recruitability from the homogeneity, severity, airway patency (dynamic bronchial gas phase), and the presence or absence of tidal recruitment in the examination area. In the course of recruitment, the response of the lung to different recruitment maneuvers and recruitment time can be assessed by qualitative or semi-quantitative ultrasound scores, and the causes of non-recruitment can be comprehensively analyzed and better treatment strategies can be found . It can also detect the possible barotrauma caused by lung recruitment in time and adjust the treatment in time. It should be noted that pulmonary ultrasound could not detect lung hyperinflation.
22.214.171.124 Prone Position
Ultrasound is helpful in evaluating and managing prone position therapy. Pulmonary tissue in gravity-dependent area of ARDS patients in supine position is not easy to reopen due to the influence of gravity, abdominal pressure, and chest motion amplitude. It has been proved that prone position can improve the degree of pulmonary tissue expansion in gravity-dependent areas either alone or in combination with recruitment maneuver thus improve oxygenation and reduce mortality .
In the prone position of ARDS patients, ultrasound can assess the lesions or homogeneity of the lungs, and evaluate the lung recruitment in gravity-dependent areas (PLAPS point and posterior blue point in supine position). The degree of dorsal lung recruitment assessed by lung ultrasound after 3 h in prone position was correlated with clinical positive reaction . Therefore, the effectiveness of prone position can be predicted by semi-quantitative ultrasound score, which also can help to determine the time and frequency of prone position.
When mechanical ventilation is disconnected, it will cause significant changes in pulmonary ventilation volume. Ultrasound changes of pulmonary ventilation can predict the success or failure of extubation in patients who successfully passed the 1 h spontaneous breathing test (SBT). There was no significant change in overall pulmonary ventilation during the spontaneous breathing test in patients who succeeded in extubation. However, in patients with post-extubation distress, pulmonary ventilation decreased during the SBT .
3.2.7 Diaphragm Ultrasound
The diaphragm is an important respiratory muscle. In spontaneous breathing, diaphragm plays an important role in generating tidal volume . Many factors in ICU such as phrenic nerve injury after abdominal or cardiac surgery, neuromuscular disease, mechanical ventilation, sepsis can lead to diaphragm dysfunction, and thus increase the risk of weaning failure and prolong the time of mechanical ventilation . There are some methods of examining diaphragm function, including electromyography, transdiaphragmatic pressure, X-ray, magnetic resonance imaging, and so on. Most of them are invasive or radioactive examination. Bedside, ultrasound has the advantages of noninvasive, real-time, and highly repeatable. It can not only observe the shape of diaphragm, but also evaluate the function of diaphragm. It has been widely used in clinical diagnosis and treatment of critical care patients.
There was no significant difference in the thickness and the change of thickness between the left and right diaphragms. Compared with the left diaphragms, ultrasound could measure the mobility of the right diaphragms more intuitively, and the repeatability of the measurement of the right diaphragms was higher than the left one . Therefore, the liver is often used as an acoustic window to measure the thickness and movement of the right hemidiaphragm. However, when it is suspected that the patient has unilateral diaphragm injury, it is necessary to evaluate the bilateral diaphragm function. For example, in patients undergoing heart surgery, the phrenic nerve injury may cause complete paralysis in half of the diaphragm, while the other part of the diaphragm is not affected, which usually does not cause dysfunction of the whole diaphragm. Therefore, it is necessary to evaluate the function of the two diaphragms separately .
3.2.9 Diaphragm Thickness and Change Rate of Diaphragm Thickness
Diaphragmatic thickness refers to the distance between the pleura and peritoneum of the diaphragms at the thoracic involution. When inhaled, the diaphragm contracted and its thickness increased. The change rate of diaphragmatic thickness refers to the change degree of diaphragmatic thickness during respiration, which reflects the contractility of diaphragm . Change rate of diaphragmatic thickness = (maximum end inspiratory diaphragmatic thickness—end expiratory diaphragmatic thickness)/end expiratory diaphragmatic thickness × 100% [62, 63]. In normal conditions, when the lung volume increases from functional residual volume to total lung capacity, the average thickness of diaphragm increases by 54% (range: 42% ~ 78%) [64, 65].
3.2.10 Diaphragm Excursion
Diaphragm excursion refers to the displacement between the inspiratory and expiratory ends of the diaphragm. During the measurement, 3–5 MHz probe is selected, and the patient takes a half-lying position (the head of the bed is raised by 30°–45°). The excursion of different parts of the diaphragm is not exactly the same during the breathing process. The excursion of the middle and rear parts is greater than that of the front part. The measurement of the excursion of the diaphragm is mainly to measure the rear part . Since the amplitude of each breath is different in patients with spontaneous breath, it is necessary to avoid recording very deep or very shallow breath as the evaluation result. It is necessary to measure five respiratory cycles and take their average value as the evaluation result .