Review Article Page 1 of 13 Use of thoracic ultrasound in acute respiratory distress syndrome Marina García-de-Acilu1^, Manel Santafé1, Oriol Roca1,2 1 Servei de Medicina Intensiva, Parc Taulí Hospital Universitari, Sabadell, Spain; 2Departament de Medicina, Universitat Autònoma de Barcelona, Bellaterra, Spain Contributions: (I) Conception and design: All authors; (II) Administrative support: None; (III) Provision of study materials or patients: None; (IV) Collection and assembly of data: None; (V) Data analysis and interpretation: None; (VI) Manuscript writing: All authors; (VII) Final approval of manuscript: All authors. Correspondence to: Marina García-de-Acilu, MD, PhD. Servei de Medicina Intensiva, ParcTaulí Hospital Universitari, Parc Taulí, 1, 08208 Sabadell, Spain. Email: mgarciaga03@tauli.cat. Abstract: Acute respiratory distress syndrome (ARDS) is a significant cause of morbidity and mortality in critically ill patients, yet it is often underrecognized. Current imaging techniques, such as CT scan and X-ray, have several limitations, including inter-observer reliability, limited accessibility, radiation and the need for transportation. Ultrasound has become an essential bedside tool in the critical care setting and the emergency room, offering several advantages over traditional imaging techniques. It is nowadays widely used for diagnosis and early management of acute respiratory and circulatory failure. Lung ultrasound (LUS) provides non-invasively valuable information regarding lung aeration, ventilation distribution and respiratory complications in ARDS patients at the bedside. Moreover, a holistic ultrasound approach, combining LUS, echocardiography, and diaphragm ultrasound offers physiological information that could help the clinician to personalize ventilator settings and guide fluid resuscitation in these patients. Ultrasound techniques could also inform about possible causes of weaning failure in difficult-to-wean patients. However, it is uncertain whether clinical decisions based on ultrasound assessment can improve outcomes in ARDS patients and this clinical approach requires further investigation. In this article, we review the use of thoracic ultrasound, including lung and diaphragm examination, for the clinical assessment of patients with ARDS, and discuss its limitations and future perspectives. Keywords: Lung ultrasound (LUS); acute respiratory distress syndrome (ARDS); lung monitoring Submitted Nov 03, 2022. Accepted for publication May 26, 2023. Published online Jun 30, 2023. doi: 10.21037/atm-22-4576 View this article at: https://dx.doi.org/10.21037/atm-22-4576 Introduction therapeutic strategies may vary, and it is poorly predictable. According to the 2012 Berlin definition, the diagnosis Acute respiratory distress syndrome (ARDS) is defined as of ARDS is based on the onset of hypoxemia and bilateral noncardiogenic pulmonary edema that leads to alveolar chest opacities within 1 week of a known clinical insult, shunt and gas-exchange impairment (1). According to recent which is not fully explained by cardiac failure or fluid findings, ARDS may represent up to 10% of intensive care overload (1). Bilateral infiltrates remain one of the hallmarks unit (ICU) admissions, but it is still underrecognized (2). of ARDS diagnosis and may be detected by computed Lung protective mechanical ventilation, positive end- tomography (CT) and X-ray. However, these imaging expiratory pressure (PEEP), prone positioning, and techniques have several limitations. Chest X-ray have only restrictive fluid therapy have become cornerstones of ARDS moderate inter-observer reliability (7). While CT scan has clinical management (3-6). However, the response to these proven to be more accurate than chest radiography, the risk ^ ORCID: 0000-0003-2353-9968. © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Page 2 of 13 García-de-Acilu et al. Thoracic ultrasound: a tool for ARDS management of transporting severely ill patients from and to the ICU Ultrasound examination must be considered. Furthermore, limited accessibility to Ideally, LUS examination should be conducted according these techniques in resource-constrained settings can hinder to a systematic protocol. Several approaches have been the identification of ARDS (8). proposed to explore the lung. First, the Bedside Lung Alternative imaging techniques have the potential to be Ultrasound in Emergency (BLUE) protocol was developed useful tools for the diagnosis and therapeutic management for the diagnosis of acute respiratory failure (ARF) in the of patients with ARDS. Among these techniques, Emergency setting (15). It defines 3 points of interest per ultrasound evaluation of the heart and lung has been widely lung (BLUE points): upper and lower BLUE points in implemented in the critical care setting (9). Bedside lung the anterior wall, and the posterolateral alveolar and/or ultrasound (LUS) offers several advantages, including pleural syndrome (PLAPS), i.e., “PLAPS point”. A twelve- avoiding the need for transportation and radiation, as well region examination allows for a quantitative approach of as superior detection of lung pathologic abnormalities lung aeration and is generally used in ICUs (16-20). Each associated with ARDS when compared to auscultation chest wall can be divided into three areas using anterior and and chest radiography (10). Since its first description in posterior-axillary lines as anatomical landmarks (anterior, 1968 (11), the clinical utility of LUS has been questioned lateral, and posterior), wherein each area is divided in for many years because the impedance difference between two regions, namely, superior and inferior regions. In tissue and air was considered a limiting factor for each region of interest, the lung surface of all adjacent ultrasound examination. In recent decades, it has become an intercostal spaces must be explored by moving the probe indispensable bedside tool for managing critically ill patients. transversally (19). Dorsal lung segments of the upper lobes, International guidelines and consensus recommend the located behind the scapula, are the only regions that cannot use of LUS in ICUs and emergency departments (11-13). be explored by LUS. Echocardiography has also become an essential tool for diagnosis, monitoring and guiding clinical decisions in the critical care setting (9). It allows assessment of left and LUS findings right ventricular systolic function, filling pressures, preload A normal ultrasound pattern is defined by lung sliding and responsiveness and pulmonary hypertension. A combination regularly spaced horizontal lines (A-lines) (Figure 1). A-lines of LUS and echocardiography provide valuable information correspond to reverberation artifacts of the pleural line and for patients with ARDS at the bedside. Recently, ultrasound indicate the presence of a high gas-volume ratio. Thus, has been used to evaluate respiratory muscle function in they can be seen in normally aerated lungs, as well as in specific clinical scenarios, such as difficult weaning. hyperinflated lungs and pneumothorax. In this article, we review the use of thoracic ultrasound, As the lung loses aeration, the LUS pattern varies from including lung and diaphragm examination, for the clinical A-lines to B-lines. B-lines result from the widening of the assessment of patients with ARDS, and we discuss its pulmonary interlobular septa by either fluid accumulation limitations and future perspectives. (from increased hydrostatic pressure or altered capillary permeability), inflammation, or fibrosis. They have been Basic ultrasound examination described as a sign of alveolar-interstitial syndrome (20). The number of B-lines depends on the degree of aeration Probe selection loss. More than two B-lines per scan make a B-pattern and Different probes may be used for LUS. The choice depends correlate with thickened interlobular septa, whereas five or on the patient’s anatomy and the suspected abnormality to be more correlate with ground-glass areas (18,21). Coalescent assessed (14). Linear high-frequency probes are suitable for B-lines correspond to severe lung aeration loss resulting pleural examinations, particularly in patients with thin chest from partial filling of alveolar spaces. walls and during the examination of the anterior part of the Lung consolidation results from massive aeration loss. chest. Phased-array and convex low-frequency probes are It appears as a tissue-like echotexture, with a superficial more convenient for exploring lateral and posterior regions boundary at the pleural line and a deep irregular boundary and can be helpful in patients with a thick chest wall for with the aerated lung called the “shred sign” (22). When assessing abnormalities that occur in deeper areas of the lung. the hole lobe is involved, this boundary is regular. © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Annals of Translational Medicine, Vol 11, No 9 June 2023 Page 3 of 13 A B C D Figure 1 Lung aeration. (A) Normal aeration; (B) multiple and well-separated B-lines; (C) coalescent B-lines; (D) consolidation. A B Figure 2 Diaphragm evaluation. (A) Diaphragmatic excursion. (B) Thickening fraction. Tdi-ei, diaphragm thickness at end inspiration; Tdi- ee, diaphragm thickness at end expiration. Diaphragm ultrasound the percentage inspiratory increase in diaphragm thickness relative to end-expiratory thickness [TFdi = (end-inspiratory There are two methods for evaluating diaphragm function (23) (Figure 2). First, the intercostal approach thickness − end-expiratory thickness)/end-expiratory allows for assessing diaphragm thickness and thickened thickness × 100%]. It reflects the contractile activity of fraction. (23). Thickening fraction (TFdi) is calculated as the diaphragm (24). However, recent consensus statement © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Page 4 of 13 García-de-Acilu et al. Thoracic ultrasound: a tool for ARDS management remarks the controversy existing regarding cut-off value for standard imaging technique for evaluating lung morphology diaphragm dysfunction assessment and extubation failure and performing a quantitative analysis of lung tissue prediction (25). aeration (31). Moreover, it can identify other abnormalities, The diaphragmatic excursion is measured using a such as pleural effusion, atelectasis, and nodules (32), and subcostal approach (24). The excursion is quantified in can detect respiratory complications. Nevertheless, CT M-mode, with the patient in a semi-seated position, and has significant limitations in the critical care setting, as it should only be measured during unassisted breathing, as employs ionizing radiation and requires patient transfer. It active contraction of the diaphragm cannot be distinguished is a costly resource that may not be readily available at all from passive displacement due to ventilator inspiratory hospitals (33). pressures (26). LUS has emerged as a promising alternative imaging Ultrasound assessment of accessory inspiratory technique that could potentially aid in identifying the (parasternal intercostal muscle) and expiratory (rectus underlying cause in patients with ARF. It has shown high abdominis, external oblique, internal oblique, and sensitivity and specificity in detecting various lung and transversus abdominis) muscles could add useful pleural diseases, including pleural effusion, pneumothorax, information regarding the patient’s inspiratory effort, consolidation, and alveolar-interstitial syndrome patient-ventilator interaction, and expiratory muscle effort (15,22,34-40). (23,27). The parasternal intercostal thickening fraction and Several studies have addressed the ability of LUS to its ratio to the diaphragm thickening have been shown to be differentiate ARDS from acute pulmonary edema in good predictors of the need for reintubation (28). patients with alveolar-interstitial syndrome (41). When applied to particular clinical settings, LUS findings may help rapidly rule out the most frequent causes of ARF. In Echocardiography the BLUE protocol (15), for instance, LUS signs are used Echocardiography is frequently used in the critical care to build up different profiles, which may lead to the cause field. Echocardiographic examination technic description of ARF in patients presenting to the emergency department is beyond the aim of this review due to the complexity and with dyspnea. ARDS patients typically show multiple the amount of information that can be obtained. Briefly, non-homogeneous B lines with a non-gravity-dependent critical care echocardiography (CCE) can be divided into distribution that may coexist with spared areas, pleural basic and advanced skill sets, and it can be performed using thickening with decreased or abolished lung sliding, and either a transthoracic (TTE) or a transesophageal (TEE) subpleural or translobar consolidations (42). Demonstration approach (29). CCE allows the intensivist to define the of this dyshomogeneous sonographic pattern has been pathophysiology of respiratory failure and rule out the reported to strongly predict non-cardiogenic pulmonary involvement of any cardiogenic mechanism. It also provides edema in its early stages (41). Recently, an ultrasound-based a comprehensive assessment of hemodynamics when definition of ARDS has been proposed for its diagnosis in respiratory and circulatory failure co-exist and could be a resource-constrained setting (8). The Kigali modification used to guide fluid resuscitation (30) and modify ventilator considers LUS findings, rather than chest X-ray or CT. However, it should be noted that ultrasound signs are not settings. specific to ARDS. Patients with diffuse parenchymal lung diseases may also show a B-line pattern with thickened Initial approach in ARDS patients pleural line and reduced or abolished lung sliding (43). Ultrasound examination should, therefore, be integrated ARDS diagnosis with a clinical evaluation to optimize the diagnostic ARDS is not a specific disease, and its current definition accuracy (44). is based on clinical criteria (1). Its diagnosis requires a LUS could also become a reliable tool for early new or worsening respiratory distress within seven days detection of the most common complications of mechanical from a clinical insult, impaired oxygenation, and bilateral ventilation (20). It shows high specificity and sensitivity chest radiographical abnormalities. These radiographical for the diagnosis of pleural effusion, pneumothorax (36), abnormalities may be identified by chest-X-ray or CT scan. ventilator-associated pneumonia (45), and atelectasis (46). In ARDS patients, lung CT is considered to be the gold During the coronavirus disease (COVID-19) pandemic, © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Annals of Translational Medicine, Vol 11, No 9 June 2023 Page 5 of 13 LUS emerged as a valuable diagnostic tool, aiding in early with a longitudinal orientation. Second, coalescent B lines diagnosis, therapeutic decision-making, and follow-up indicate severe loss of aeration and have been associated monitoring of patients with COVID-19 pneumonia (47), with increased lung density in homogeneous diseases (51). thereby potentially reducing the use of conventional However, non-homogenous diseases, such as ARDS, diagnostic imaging resources (CT scan and chest X-ray). may also present focal coalescent B lines and subpleural Notable LUS findings in COVID-19 patients include consolidations. Additionally, complete loss of aeration is various forms of B-lines, an irregular or fragmented pleural attributed to a lung region whenever a tissue-like pattern is line, consolidations, pleural effusions, and absence of lung observed, independently of its size, which may result in an sliding (48-50). overestimation of loss of aeration when this pattern involves only a small portion of the visualized lung region (17). Recently, Mongodi et al. proposed a modified score to Evaluation of lung aeration further improve the accuracy of ultrasound assessment of LUS provides a noninvasive and reproducible imaging lung aeration (55). This score considers the percentage of method of assessing lung aeration at the bedside. The involved pleura to establish the severity of loss of aeration. different degrees of aeration may be semiquantitatively The use of a high-frequency linear probe in a traverse evaluated according to the LUS findings (Figure 1). orientation aligned with the intercostal space enables Alternative rating systems have been proposed. Bouhemad visualization of a longer pleura length. Furthermore, the et al. (20) described a score based on four steps in six visualization of LUS signs, such as subpleural consolidation, regions per hemithorax: (I) A-lines correspond to the may improve diagnostic accuracy. This score seems to normal aeration of the lung parenchyma, (II) multiple and perform better than the traditional one, but further research well-separated B-lines correspond to a moderate decrease is needed to confirm its efficacy. in lung aeration resulting from interstitial syndrome, (III) The use of LUS monitoring may be of special interest coalescent B-lines correspond to a more severe decrease in severe ARDS patients on extracorporeal membrane in aeration resulting from alveolar involvement, and (IV) oxygenation, in which a chest X-ray may be poorly consolidation corresponds to complete loss of aeration. informative (56). LUS score monitoring can provide The global LUS score corresponds to the sum of six valuable information regarding potential lung recovery, regions’ score in each hemithorax and ranges from 0 to 36. with a persistent high score indicating a negative outcome. Variations of these patterns may be used to express both Additionally, LUS can not only quantify, but can also loss of aeration and re-aeration in different clinical settings assess the distribution of aeration loss, which can guide (16,20,51). mechanical ventilation settings and other possible strategies. A correlation between LUS score and extravascular lung In patients with focal ARDS, LUS shows a heterogeneous water assessed by transpulmonary thermodilution (52), pattern with consolidation or B-lines, mainly in lower as well as overall lung tissue density assessed by posterior regions, while anterior lung regions remain more quantitative CT imaging (53), has been observed in aerated (14). In such cases, high PEEP levels may lead ARDS patients. However, recent studies reported only a to overdistension of normal parenchyma (57), but prone moderate correlation with transpulmonary thermodilution positioning may yield adequate responses (58). Conversely, techniques in an unselected population of critically ill in patients with diffuse ARDS, aeration loss involves all patients. The heterogeneity of the findings may be related regions homogeneously, and increasing levels of PEEP may to the varying distribution pattern of lung water for lead to improvements. LUS can identify changes in aeration different underlying pathologies, which may be influenced patterns induced by increases in PEEP (20). by different factors, such as hemodynamics, capillary integrity, and renal function (54). Notably, determining Circulatory failure and resuscitation and interpretating B-lines at the bedside can be difficult and highly variable (49). Patients with ARDS may benefit from conservative fluid The application of LUS for assessing lung aeration management (6), as fluid overload can contribute to worse in ARDS patients has some limitations (55). First, the outcomes (59). However, most ARDS cases are associated width of the intercostal space limits pleura visualization with hemodynamic instability, making appropriate and may vary among patients when performing the study resuscitation of patients with shock is a crucial factor in © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Page 6 of 13 García-de-Acilu et al. Thoracic ultrasound: a tool for ARDS management determining outcomes (60), especially in patients with both assessment in real-time at the bedside and could be ARDS and circulatory failure. considered as an alternative to CT scan. It can identify CCE is a valuable bedside tool for hemodynamic re-aeration as a sequential change in the pattern from monitoring. It is used in shock patients to evaluate cardiac consolidation to B-lines and ultimately A-lines. However, function and guide fluid resuscitation, and it can be used consolidation may transform directly into a normal to obtain repeated measurements to assess changes of pattern (20). Similarly, LUS can detect lung collapse with different therapies on cardiac output (CO) (61). Briefly, high specificity and sensitivity (18). using the Doppler technique, stroke volume is estimated by Ultrasound assessment of aeration changes in ARDS multiplying the velocity of time integral (VTI) of the blood patients was first reported using TEE and LUS (70-72). flow in the left ventricle outflow track (LVOT) by the cross- Although these studies were limited to non-aerated sectional area of the aortic valve [0.785× (diameter)2]. Since dependent regions, they demonstrated significant the aortic diameter does not change, VTILVOT is a reliable correlations between the reduction of the consolidated parameter and can easily be used to track changes in stroke areas and the improvement of oxygenation. Bouhemad et al. volume (62). reported a significant correlation between PEEP-induced Fluid responsiveness may be assessed by CCE using lung recruitment based on ultrasound reaeration score and dynamic parameters based on heart-lung interactions in pressure-volume (PV) curve (20). The LUS score is based mechanically ventilated patients (63). The magnitude of on the examination of all intercostal spaces of 12 regions the effect depends mainly on the transmission of airway of interest: upper and lower parts of anterior, lateral, and pressure variations to the heart. Doppler measurement of posterior regions of both the left and right chest. The ascending aortic flow and VTILVOT may be obtained in the worst ultrasound abnormality detected in each region apical five-chamber view (TTE) and the deep gastric view characterizes the region examined. Four ultrasound aeration (TEE). The variation of VTILVOT(DVTI) can be calculated patterns were defined: (I) normal aeration (N), presence of as follows: DVTI (%) = (VTImax – VTImin)/((VTImax lung sliding with A lines or fewer than two isolated B lines; + VTImin)/2) × 100%, where VTImax and VTImin (II) moderate loss of lung aeration, multiple well-defined B represent the maximum and minimum VTI in 10 cardiac lines (B1 lines); (III) severe loss of lung aeration, multiple cycles. Variations in aortic flow velocity greater than 12% coalescent B lines (B2 lines); and lung consolidation (C), throughout the respiratory cycle accurately predict fluid the presence of a tissue pattern characterized by dynamic responsiveness (64). Similarly, variations in VTI also predict air bronchograms. The LUS score ranges from 0 to 36 and responsiveness (65). is calculated as the sum of points. An ultrasound reaeration TEE and TTE also allow measures of inferior and score can also be calculated from changes in the ultrasound superior vena cava diameter and variation, and cardiac pattern in each region, allowing regional analysis. output changes in response to passive leg raising. Recruitment mainly results from the reaeration of poorly Clinicians with advanced skills in transthoracic aerated lung regions (coalescent and separated B-lines) echocardiography can also obtain a Doppler measurement rather than from consolidations. of the maximal ascending aortic flow velocity (just distal to As has already been proposed (73), a practical approach based on LUS could be used to personalized recruitment at the aortic valve) immediately before and after performing a the bedside. First, LUS can be used to assess the degree and PLR. distribution of loss of aeration, to consider the potential for recruitment. Second, it may help to guide and individualize How LUS can be used to personalize MV settings PEEP settings based on lung morphology. LUS may in the acute phase of ARDS? identify closing pressure during a step-wise decremental PEEP trial. Changes in lung aeration Although LUS cannot identify lung overinflation, Computed tomography is considered the gold standard severely decreased or abolished lung sliding may be for assessing lung morphology and recruitment in ARDS associated with the impaired regional ventilation associated patients; however, it has several limitations, including high with high PEEP settings (74), suggesting PEEP-induced irradiation exposure and patient transportation (66-69). hyperinflation in non-dependent lung regions, especially if LUS may be a safe and feasible tool for lung recruitment lung sliding reappears when PEEP is decreased (42). © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Annals of Translational Medicine, Vol 11, No 9 June 2023 Page 7 of 13 Similarly, ultrasonography may be used to follow- would be important to best tailor clinical management up ventilator-associated pneumonia (VAP). Changes in and prevent further injury to the RV (81). The diagnosis aeration pattern (disappearance of B-lines and subpleural of ACP and PFO relies on echocardiography. It facilitates consolidation) have been correlated with CT scan non-invasive assessment of RV preload, contractility, improvements and may be associated with antibiotic and afterload (86), as well as monitoring of the response efficacy (16). to different strategies at the bedside. A RV protective approach, based on echocardiographic monitoring, has been suggested by Vieillard-Baron et al. (87), involving Diaphragm-protective ventilation adapting the therapeutic lung protective ventilation strategy Diaphragm atrophy and injury caused by mechanical according to the function of the right ventricle. However, ventilation have been associated with poor clinical it remains unclear whether this “RV protective approach” outcomes (75). The concept of diaphragm-protective would be associated with better outcomes compared to a ventilation has recently emerged and encourages physicians more conventional approach (80). to monitor and optimize respiratory effort and synchrony with the aim of preventing diaphragm atrophy and muscle How may LUS be helpful during the weaning injury while maintaining lung protection (76). Diaphragm from mechanical ventilation? ultrasound may be used to assess respiratory effort. It has been suggested that a diaphragm thickening fraction in Weaning from mechanical ventilation can be considered the range of 15–30% may be associated with the shortest as an exercise test and induces significant changes in lung duration of mechanical ventilation compared to lower aeration and cardiac function (88). The cause of weaning or higher thickening fraction values (77). However, failure is often multifactorial. Spontaneous breathing may controversy remains regarding cut-off values for assessing lead to an increase in aeration of posterior and dependent respiratory effort and regarding the effect that positive lung regions due to the active contraction of the diaphragm, pressure ventilation may have on patient effort (26). whereas nondependent lung regions may decrease their aeration (89). Lung aeration score changes during spontaneous breathing trial (SBT) may predict extubation Right ventricle assessment success or failure. A LUS score >17 at the end of the SBT ARDS is one of the most common causes of acute right has been reported to predict post-extubation distress (89). ventricular failure (RVF) in the critical care setting (78). LUS can also be used to assess pleural effusion, which may Acute cor pulmonale (ACP) is the most severe presentation also play a role in weaning failure. Ferré et al. also reported of RVF due to an acute increase in RV afterload. It occurs that an increase in the number of B-lines allows the in 25% of ARDS patients under protective mechanical diagnosis of weaning-induced pulmonary edema (90). ventilation (79,80) and is associated with poor outcomes CCE may be helpful for evaluating left ventricle systolic (78,81). Underlying mechanisms include lung inflammation, and diastolic function, which are associated with weaning pulmonary artery injury and the effects of positive pressure failure (91). It may accurately identify patients at high risk ventilation (82). Pneumonia (as the cause of ARDS), driving of weaning failure, but the ability to predict failure remains pressure ≤18 cmH2O, arterial oxygen partial pressure to uncertain. fractional inspired oxygen ratio (PaO2/FiO2) ≤150 mmHg, Diaphragmatic dysfunction is common in critically and arterial carbon dioxide partial pressure (P a CO 2 ) ill patients and is associated with prolonged mechanical ≥48 mmHg have been reported to be associated with ventilation and length of stay. Ultrasound measurement ACP (81). Acute right ventricular (RV) after loading may of excursion and thickening fraction seems to strongly lead to patent foramen oval (PFO). The prevalence of PFO correlate with diaphragm strength (27). However, in ARDS ranges between 16% and 19% (83,84). Although conflicting results have been reported regarding its PFO may reduce the harmful effects of elevated pulmonary ability to predict successful weaning (92-95). A significant vascular resistance on RV systolic function, it may worsen proportion of patients can be successfully extubated hypoxemia and limit the beneficial effects of recruitment despite diaphragm dysfunction (94,96). Recently, the maneuvers and PEEP (83,85). predictive value of diaphragm ultrasound was also tested It has been proposed that early assessment of RV function in mechanically ventilated COVID-19 patients, but TFdi © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Page 8 of 13 García-de-Acilu et al. Thoracic ultrasound: a tool for ARDS management was not predictive of weaning failure (97). Lung aeration of training methods to assess competence is imperative for loss appears more frequently in patients with diaphragm the safe and effective use of such systems (104). dysfunction; thus, even when diaphragm dysfunction is Ultrasound examination can be difficult due to detected, physicians should be encouraged to investigate anatomical characteristics or artificial devices, such as other causes of failure (98). Nevertheless, it remains obesity, subcutaneous emphysema, and large thoracic uncertain whether diaphragm assessment adds to clinical dressings. If the lung is aerated, the examination only decision-making. allows analysis of the lung surface. Thus, results of LUS A multimodal approach, integrating LUS, diaphragm examination in diseases that may have no or minimal ultrasound, and echocardiography findings, may help extension to peripheral fields should be interpreted with identify patients at risk and predict SBT failure (88,99). caution. Although a holistic ultrasound approach has previously Finally, it should be also noted that LUS cannot been determined to be a weak predictor for extubation detect lung overinflation resulting from an increase in failure (100), understanding the pathophysiology of intrathoracic pressures (42). This is a major limitation when weaning failure can help physicians to optimize the clinical assessing lung recruitment in ARDS patients. status and physiological function before proceeding to mechanical ventilation discontinuation. A weaning trial Future perspectives can be considered as a cardio-pulmonary stress test. Therefore, failure may be the result of a combination Critical care ultrasound has greatly developed in the last few of cardiac dysfunction, impaired gas exchange, and/or years and has become essential to the modern practice of diaphragmatic dysfunction. Tuinman et al. proposed a critical care medicine. Assessing competency is, therefore, a standardized sonographic approach to weaning, including key point in the training process. Despite the increased use LUS aeration score, pleural effusion and ascites assessment, of LUS, training methods to acquire appropriate skills vary CCE, and evaluation of diaphragm and extra-diaphragmatic among centers and are not standardized (103). Specific and respiratory muscles (23). The ultrasound examination may standardized learning programs and examinations should be be performed before the SBT for the identification of part of the basic knowledge of all intensivists. patients at high risk of weaning failure and during the SBT LUS allows a repeatable semiquantitative assessment of for prediction of weaning outcome or to diagnose the cause lung aeration at the bedside. As stated before, the current of failure. scoring system shows some limitations and leaves room for improvement (53,55,105). Automation of lung aeration based on computer-assisted gray-scale analysis has also Limitations been described (106), with the advantage that it would be Ultrasound examination has several limitations. It is an operator-independent. Further studies are needed to assess operator-dependent technique that requires training for the clinical utility of this automatic assessment of lung image acquisition and interpretation, as well as appropriate aeration. integration with the clinical scenario. Simple LUS Progress has been made in ultrasound technology, such as examination can be easily acquired with short training and three-dimensional ultrasound and miniaturization. Pocket- is useful for detecting some abnormalities, such as pleural sized ultrasound devices have been recently developed and effusion and ruling out pneumothorax (101,102). Moreover, have a relatively low cost, but image quality is lower than ultrasonographic aeration assessment has shown a strong that of using traditional devices. Nevertheless, they are interobserver agreement (53,55). For more advanced suited for bedside use in emergency settings. skills, such as lung ultrasound score computation and The diagnostic accuracy of ultrasonography has been diaphragm function assessment, longer training may be extensively reported. However, the influence on clinical required (103). CCE ranges from basic practice, which is decision-making and outcomes remains uncertain and needs used to rapidly characterize causes of shock and respiratory further investigation. failure, to advanced CCE, which entails the performance Finally, whether personalizing mechanical ventilation— of comprehensive diagnostic studies similar to the ones according to the lung morphology assessment performed expected of a cardiologist, often with additional advanced during LUS—improves clinical outcomes in ARDS patients hemodynamic assessment. Nevertheless, the development is currently under evaluation (NCT05492344). © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Annals of Translational Medicine, Vol 11, No 9 June 2023 Page 9 of 13 Conclusions License (CC BY-NC-ND 4.0), which permits the non- commercial replication and distribution of the article with Ultrasound is a non-invasive bedside radiation-free the strict proviso that no changes or edits are made and the technique that has become widely available and highly original work is properly cited (including links to both the feasible, and it can be easily applied in the critical care formal publication through the relevant DOI and the license). setting. A combination of LUS, echocardiography, and See: https://creativecommons.org/licenses/by-nc-nd/4.0/. diaphragm ultrasound provides valuable physiological information in ARDS patients. This information could be used to personalize ventilator settings at the bedside. References Moreover, ultrasound-based hemodynamic variables could 1. Acute respiratory distress syndrome: the Berlin Definition, improve fluid resuscitation. Finally, ultrasound techniques et al. Acute respiratory distress syndrome: the Berlin could inform about the possible causes of weaning failure. Definition. JAMA 2012;307:2526-33. However, despite being physiologically attractive, whether 2. Bellani G, Laffey JG, Pham T, et al. Epidemiology, clinical decisions based on ultrasound assessment may Patterns of Care, and Mortality for Patients With Acute improve outcomes in ARDS patients remains uncertain and Respiratory Distress Syndrome in Intensive Care Units in needs further investigation. 50 Countries. JAMA 2016;315:788-800. 3. Brower RG, Matthay MA, Morris A, et al. Ventilation Acknowledgments with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory Funding: None. distress syndrome. N Engl J Med 2000;342:1301-8. 4. Briel M, Meade M, Mercat A, et al. Higher vs lower Footnote positive end-expiratory pressure in patients with acute lung injury and acute respiratory distress syndrome: systematic Provenance and Peer Review: This article was commissioned review and meta-analysis. JAMA 2010;303:865-73. by the Guest Editors (Tobias Eckle and Dr. Benjamin Scott) 5. Guérin C, Reignier J, Richard JC, et al. Prone positioning for the series “Highlights in Anesthesia and Critical Care in severe acute respiratory distress syndrome. N Engl J Medicine” published in Annals of Translational Medicine. The Med 2013;368:2159-68. article has undergone external peer review. 6. Wiedemann HP, Wheeler AP, Bernard GR, et al. Comparison of two fluid-management strategies in acute Peer Review File: Available at https://atm.amegroups.com/ lung injury. N Engl J Med 2006;354:2564-75. article/view/10.21037/atm-22-4576/prf 7. Meade MO, Cook RJ, Guyatt GH, et al. Interobserver variation in interpreting chest radiographs for the Conflicts of Interest: All authors have completed the diagnosis of acute respiratory distress syndrome. Am J ICMJE uniform disclosure form (available at https://atm. Respir Crit Care Med 2000;161:85-90. amegroups.com/article/view/10.21037/atm-22-4576/coif). 8. Riviello ED, Kiviri W, Twagirumugabe T, et al. Hospital The series “Highlights in Anesthesia and Critical Care Incidence and Outcomes of the Acute Respiratory Distress Medicine” was commissioned by the editorial office without Syndrome Using the Kigali Modification of the Berlin any funding or sponsorship. The authors have no other Definition. Am J Respir Crit Care Med 2016;193:52-9. conflicts of interest to declare. 9. Vieillard-Baron A, Millington SJ, Sanfilippo F, et al. A decade of progress in critical care echocardiography: a Ethical Statement: The authors are accountable for all narrative review. Intensive Care Med 2019;45:770-88. aspects of the work in ensuring that questions related 10. Lichtenstein D, Goldstein I, Mourgeon E, et al. to the accuracy or integrity of any part of the work are Comparative diagnostic performances of auscultation, appropriately investigated and resolved. chest radiography, and lung ultrasonography in acute respiratory distress syndrome. Anesthesiology Open Access Statement: This is an Open Access article 2004;100:9-15. distributed in accordance with the Creative Commons 11. Ross AM, Genton E, Holmes JH. Ultrasonic examination Attribution-NonCommercial-NoDerivs 4.0 International of the lung. J Lab Clin Med 1968;72:556-64. © Annals of Translational Medicine. All rights reserved. Ann Transl Med 2023;11(9):320 | https://dx.doi.org/10.21037/atm-22-4576 Page 10 of 13 García-de-Acilu et al. Thoracic ultrasound: a tool for ARDS management 12. Mayo PH, Beaulieu Y, Doelken P, et al. 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