The diaphragm is a thin, dome-shaped muscle that separates the thoracic and abdominal cavities (Figure 1). In a healthy adult, it is only 2–3 millimeters thick. Despite its small size, it is responsible for 60–80% of ventilation needs (1, 2). The diaphragm plays a crucial role in the respiratory muscle pump, aiding in coughing and the expulsion of secretions (3). It also reduces the risk of lung infections. Both mechanical ventilation and damage to the phrenic nerve can lead to diaphragmatic dysfunction (DD), characterized by an imbalance between the diaphragm’s ability to provide enough negative pressure for vital capacity and the workload imposed upon it. DD during mechanical ventilation (MV) is recognized as an important factor influencing clinical outcomes (4–9), and prolonged mechanical ventilation can result in decreased diaphragm thickness.

Studies have shown that for every 10% reduction in diaphragm thickness (DT) in critically ill patients, intensive care unit (ICU) mortality and hospitalization rates increase by 1.55- and 1.66-fold, respectively (10–12). More than 10 million patients worldwide require MV therapy each year, with approximately 30% of these patients needing extended ventilator use (13). Wasting atrophy of the diaphragm is a contributing factor to respiratory failure, and it is important to note that clinical symptoms may not occur until one’s diaphragmatic strength has decreased to 30% of its capacity (14). Early detection of DD is crucial, as early intervention can improve symptoms (15–17). However, there has been a gradual increase in the number of studies on non-critical patients in recent years, this suggests that diaphragmatic dysfunction is also common among perioperative non-critical patients. Therefore, the objective of this review was to analyze and summarize the indicators and criteria for ultrasound assessment of DD in non-critical patients. To achieve this, databases such as PubMed and Web of Science were searched with the aim of providing a reliable basis for clinical use.

Currently, there is no uniformity in the selection of ultrasound indicators and thresholds for diaphragmatic dysfunction, although there is an international expert consensus that a diaphragmatic excursion (DE) of less than 2 cm is the criterion for diagnosing diaphragmatic dysfunction (18), no article has been found to use this criterion. As a result, the main problem faced by clinicians is the lack of standardized criteria, while ultrasound is the preferred diagnostic tool for diaphragm dysfunction, a wide range of indicators and thresholds are summarized in the literature, which significantly affects clinicians’ judgment and delays early intervention.

The gold standard for the diagnosis of diaphragmatic dysfunction is phrenic nerve stimulation and transdiaphragmatic pressure assessment (7, 19), however, these methods are invasive and not clinically applicable (20). In recent years, ultrasound has become a widely-used noninvasive technology (21), it allows noninvasive, reproducible, and safe assessment of the diaphragm’s anatomy and function (22–26). Two commonly used ultrasound modes are B-Mode and M-Mode (27, 28), and the key indicators of diaphragm function assessed by ultrasound include DE, diaphragm thickening fraction (DTF), and DT.

Measurements of DT and DTF require the use of a high-frequency linear ultrasound transducer (3–12 MHz). The patient should be in a semi-recumbent position, and the probe should be placed in the midaxillary line at ribs 8–10, perpendicular to the intercostal space. In B-Mode, the diaphragm can be visualized as a three-layered structure, with the upper hyperechoic layer being the pleura, the lower layer being the peritoneum, and the middle layer being the diaphragm (29) (Figures 2A,B). In contrast, DE measurements are performed using a low-frequency abdominal convex probe (3–5 MHz), the patient should be positioned at a 45-degree semi-recumbent angle, and the ultrasound probe should be placed parallel to the right costal margin at the right midclavicular line, using the transverse section of the liver as an acoustic window. Alternatively, the probe can be placed perpendicular to the costal margin to obtain a longitudinal section of the liver (Figures 2C,D). It is also possible to obtain diaphragm images at different interfaces using liver vessels as markers, however, this method is not commonly used (Figure 3). In B-Mode, the high echo shadow covering the liver surface represents the diaphragm, switching to M-Mode allows for the observation of the diaphragm waveform synchronized with the respiratory cycle (Figure 4B). On the left side, the probe is placed at the 8–10th rib along the midaxillary line, parallel to the intercostal spaces, the other methods are the same as for the right side (Figure 4A) (29). Ultimately, ultrasound is clinically reproducible (32) and has become an essential tool for most clinicians, its overall measurement failure rate has decreased from 27% a decade ago to 0.7% today, demonstrating the effectiveness of ultrasound technology (33).

Table 1 provides a comparison of the materials and methods used for assessing diaphragmatic dysfunction across the literature. Diaphragmatic ultrasound is widely used to assess diaphragmatic dysfunction in various medical conditions, including neuromuscular diseases (46, 47), chronic respiratory diseases, and conditions requiring intensive care (18, 21, 48), it helps clinicians diagnose and monitor conditions such as diaphragmatic paralysis or weakness. Additionally, diaphragmatic dysfunction can significantly impact weaning outcomes, thus, ultrasound can provide essential insights to predict the success of extubation (40, 49, 50). Ultrasound can also be used to counsel patients with respiratory failure in making decisions about the necessity and potential success of noninvasive ventilation (51). Additionally, it plays a crucial role in enhancing the understanding of ventilator management among patients with coronavirus 2019 disease (52).

In anesthesiology, diaphragm ultrasound helps determine the residual muscle relaxation in patients under general anesthesia, addressing the complexities and interferences associated with the gold standard train-of-four ratio procedure (53), it is possible to predict and prevent postoperative pulmonary complications in surgical patients, including those undergoing heart surgery, thoracic surgery, and upper abdominal surgery (34, 54–58). In rehabilitation medicine, diaphragm thickness is positively correlated with patients’ functional scores and functional independence scores before and after rehabilitation, suggesting that diaphragm thickness can influence patients’ rehabilitation progress (59). Overall, diaphragm ultrasound has become a valuable tool in routine clinical practice, particularly for assessing diaphragmatic function in various medical conditions.

There are few studies related to ultrasound in nonsurgical patients, such as outpatients with chronic obstructive pulmonary disease (COPD) and interstitial lung disease (ILD). However, evidence suggests that ultrasound monitoring of diaphragm function is useful in assessing a range of lung diseases (48, 93). In patients with COPD, lung hyperinflation causes the diaphragm to shift caudally, negatively affecting its function (94). The clinical presentation of COPD patients is shown in Figure 5. In the past, the assessment of patients with COPD mainly involved using the 6-min walking test and FEV1/FVC evaluation (forced expiratory volume in the first second/forced vital capacity). In recent years, ultrasound has also played an important role in analyzing patients with COPD, and DE is an essential indicator of decreased exercise tolerance and dyspnea, which are related to lung function and respiratory muscle strength (96, 97). The lower normal limit value of DE in healthy subjects is 3.3 cm for men and 3.2 cm for women during deep breathing (98). This corresponds to the fact that diaphragmatic mobility is greater in men than in women. However, the diagnostic threshold for DD is significantly higher than that used in most clinical trials.

The occurrence of DD in COPD leads to a significant decrease in DTF, TF, and DE (99). Notably, ultrasound monitoring of the diaphragm, both in outpatients and hospitalized COPD patients, can effectively assess the disease status of patients (100–105). Moreover, early detection of diaphragmatic dysfunction can help in formulating relevant strategies to reduce the occurrence of adverse clinical outcomes. However, the lack of uniformity in the ultrasound criteria for diagnosing DD will result in a much higher rate of leakage and misdiagnosis. Additionally, a large number of sample sizes and experiments are needed to further validate the accuracy of the diagnostic criteria.

Bernardinello et al. conducted TF ultrasound measurements on outpatients with ILD for several months, they found that a TF <30% was the diagnostic criterion for DD. Of the 82 ILD patients followed up, 24 experienced DD, resulting in an incidence rate of 29%. Furthermore, DD was more likely to occur in patients with connective tissue disease (CTD-ILD) than in healthy subjects. In their study, TF <30% was found to be an independent predictor of moderate/severe dyspnea (OR: 3.8, p = 0.009 and OR: 6.3, p = 0.021, respectively) (41). On the other hand, idiopathic pulmonary fibrosis (IPF) patients did not exhibit similar results, this led to the conclusion that CTD-ILD, a systemic disease that may decrease muscle strength, is different from IPF, a chronic lung disease in which muscle strength is better maintained. These findings are in line with previous research (106). Additionally, CTD-ILD patients who developed DD were more likely to experience severe dyspnea. Therefore, identifying risk factors for DD in CTD-ILD patients could help prevent poor clinical outcomes. Meanwhile, another study by Santana et al. demonstrated that DE correlates with ILD severity in ILD patients, they also found that FVC% <60 is highly accurate for predicting DD (107). In clinical practice, diaphragmatic ultrasound imaging has a high sensitivity and specificity for identifying reduced DE in ILD patients with FVC% <60. By combining ultrasound with lung function indices, it becomes easier to monitor ILD patients after surgery and can also serve as a prompt for physicians to reduce the use of medications, such as corticosteroids, that may lead to myopathy.

The assessment of diaphragmatic dysfunction using ultrasound in patients with neuromuscular diseases is a critical area of study due to the essential role the diaphragm plays in respiration. Neuromuscular diseases, such as amyotrophic lateral sclerosis (ALS), Duchenne muscular dystrophy (DMD), stroke, myasthenia gravis (108), and Lambert-Eaton syndrome (46), can lead to significant diaphragmatic weakness or paralysis, severely affecting respiratory function. Consequently, in cases of acute myasthenic crisis, patients may experience acute respiratory failure requiring invasive ventilation. Transitioning to ALS, early detection poses challenges, with low survival rates primarily attributed to respiratory muscle involvement. Timely intervention is critical, as diaphragmatic ultrasound can effectively predict FVC <50% by measuring parameters such as DE (<5.5 cm) during deep breathing. This comprehensive assessment facilitates prompt intervention for respiratory failure, potentially improving patient prognosis (109).

Stroke also affects respiratory function to some extent, resulting in a notable reduction in diaphragm mobility and lung function among affected patients (110). This diminished respiratory capacity can heighten the vulnerability of stroke patients to pulmonary infections. Similarly, patients with DMD exhibit lower DE and DTF compared to healthy adults (111, 112). Although there is no cure, diaphragmatic ultrasound can provide a clinical basis for assessing diaphragmatic function. In other words, ultrasound is the preferred tool for identifying patients who may have experienced diaphragmatic dysfunction before they display clinical symptoms, enabling early intervention.

Although ultrasound has become a commonly used tool for diagnosing diaphragmatic dysfunction in recent years due to its noninvasiveness and reusability, there is still confusion regarding the use of diagnostic indicators and criteria. Some literature suggests that diaphragmatic involvement is bilateral (66). In contrast, some studies have demonstrated that DD may be unilateral and associated with specific surgical procedures, such as lung resection (34). During quiet breathing in healthy individuals, the lower limit of normal DE is 0.9 cm and 1 cm in women and men, respectively. Meanwhile, during deep breathing, the lower limit of normal DE is 3.3 cm and 3.2 cm for women and men, respectively (98). While both lower values have been used in different articles to diagnose DD, in a randomized controlled study of patients treated with nerve blocks, DD was categorized into complete, partial, and no diaphragmatic dysfunction categories based on decreases in DE from baseline of >70%, 25–70, and < 25% (113), respectively. Overall, the metrics used to diagnose DD through ultrasound have also not been standardized and include DE (9, 34–40), DT, and DTF (18, 41–45).

By analyzing and summarizing the literature, this review found that DE is the most commonly used index. A DE measurement of <1 cm is often used to diagnose diaphragmatic dysfunction. In clinical practice, it has been observed that most normal patients have a DE between 1–2 cm, with a few exceeding 2 cm. These findings are consistent with the data obtained by Boussuges et al. regarding DE in normal subjects (32). However, there is an international consensus among experts on ultrasound diagnosis of diaphragmatic dysfunction in critically ill patients. According to this consensus, a DE <2 cm from baseline can be considered as the critical value for diagnosis of DD (18), this conclusion contradicts the findings of our review. However, it should be noted that this consensus is specifically for ICU patients, who often have additional conditions such as diaphragmatic edema, inflammation, and pulmonary atelectasis. These conditions require a greater DE to maintain normal tidal volume. Additionally, the thickness of the diaphragm is not standardized due to diaphragmatic edema and other factors, therefore, a DE <2 cm cannot be used as a diagnostic criterion for non-critical patients. In conclusion, diaphragmatic ultrasound plays an important role in clinical practice, but there is no consensus on the diagnostic criteria for non-critical patients. Currently, a DE <1 cm is the most reasonable criterion, but more clinical studies are needed in the future to confirm and supplement this criterion. Secondly, through literature review, we also found that the incidence of diaphragmatic dysfunction caused by brachial plexus block is very high. However, brachial plexus block has now become a common anesthesia method in orthopedic surgery, which can avoid the adverse effects of general anesthesia. In the future, further research can be conducted on another approach or concentration to reduce diaphragmatic dysfunction caused by brachial plexus block. Furthermore, the use of ultrasound at the bedside is limited by the presence of poor acoustic windows in some outpatients and ICU patients (93, 114, 115) and unfavorable imaging environments in obese patients (25).

In recent years, computed tomography (CT) has emerged as a new tool for characterizing diaphragmatic function, it enables visual assessment of diaphragm density, thickness, and height (116), facilitating the prediction of reintubation rates in patients in the ICU (117). Additionally, CT allows for the static assessment of the diaphragm and observation of morphological changes over time. Looking ahead, alongside ultrasound, CT is poised to become an indispensable tool for comprehensive assessment of diaphragm function.

In clinical practice, ultrasound remains a commonly used tool for assessing DD, it is not only noninvasive but can also be performed at the bedside, ensuring good patient compliance. Perioperative ultrasound assessment of diaphragm function can help in preoperative preparation, intraoperative monitoring, and postoperative evaluation. It allows clinicians to promptly and accurately assess diaphragm function and guide subsequent treatment strategies. However, more clinical data are required in the future to complement and support this review, with the ultimate goal of reaching a consensus on ultrasound assessment in non-critical patients.

X-YY: Writing – original draft. H-ML: Writing – review & editing. B-WS: Writing – review & editing. Y-YZ: Writing – review & editing, Investigation. J-GF: Writing – original draft, Resources. JJ: Writing – review & editing, Methodology. LL: Writing – review & editing, Conceptualization.