Weaning is an important part of mechanical ventilation. There was no significant difference in the baseline data between the two groups. The mechanical ventilation time and ICU stay time of the successful weaning group were significantly shorter than those of the failed weaning group. Patients with failed weaning need to be intubated again, which increases the probability of repeated treatment and prolongs the time of mechanical ventilation and treatment. The greatest difficulty in the weaning process is the lack of one or more indicators to accurately predict whether mechanical ventilation can be successfully weaned. Ultrasound evaluation of diaphragm function has received increasing attention, including the observation of diaphragm thickening fraction (DTF), diaphragm displacement (DE) and lung ultrasound score (LUS). These indicators have certain predictive value for the weaning results of patients on mechanical ventilation. The function of diaphragmatic and respiratory muscles in elderly patients has changed due to age-related pathophysiological changes.
For the weaning of elderly patients off of mechanical ventilation, there is a lack of relevant research on the ultrasound indicators that can predict respiratory muscle function and the strength of their predictive value Previous studies have mostly focused on the evaluation of predictive indicators in patients over 18 years old, for example, Wafaa et al. [
20] investigated diaphragmatic thickness as a predictor index for weaning from mechanical ventilation. While this study focused on elderly patients over 65 years old, which further clarifies the predictive value of the various indicators.
Predictive value of the rapid shallow breathing index
One commonly used weaning indicator is RSBI, which is simple, noninvasive and repeatable. The results of this study showed that the RSBI of the successful weaning group was better than that of the failure group. ROC curve analysis showed that the failure rate of weaning was high when RBSI was greater than 102 times/(min * l). RBSI is a parameter reflecting the change in lung volume, which can reflect the contractile force of all respiratory muscle groups, and it is easy to detect clinically, so it can be used as an important reference index to evaluate the results of weaning.
Role of lung ultrasound in predicting weaning outcomes
During the weaning process, pulmonary inflammation and cardiac insufficiency may cause loss of ventilatory volume and even lead to weaning failure. Lung ultrasound can noninvasively assess the location and extent of aeration loss, thereby assisting in the diagnosis of respiratory disorders [
21]. In this study, we used LUS to quantify the degree of aeration loss before extubation. LUS was shown to be a reliable quantitative index of respiratory function and an independent predictor of weaning failure. Patients with high LUS were not suitable for weaning from mechanical ventilation. Summer et al. [
14] pointed out that monitoring changes in LUS after SBT could be used to adjust treatment and to decide on the timing of extubation.
Elderly patients more often have COPD, in which case the evaluation of diaphragm function is more effective and meaningful. The reasons are as follows: it can be seen that pulmonary ultrasound has high value in the evaluation of pulmonary consolidation, but it has low value in the evaluation of chronic obstructive pulmonary disease (COPD) and other pulmonary hyperinflation disorders, and the acute exacerbation of chronic obstructive pulmonary disease is often the common cause of respiratory failure in elderly patients, so other indicators need to be added to improve the accuracy. Alvisi et al. [
22] discussed the predictors of weaning outcome in chronic obstructive, pulmonary disease patients, and has shown how the failure of weaning in elderly patients with COPD is related to the decline in their lung function. Lung function is closely related to diaphragm function, so ultrasound to evaluate the diaphragm can be worthwhile.
Role of diaphragm ultrasound in predicting weaning outcomes
The age of elderly patients is an independent factor of extubation failure. In-depth analysis revealed a decline in pulmonary function, and the decline in pulmonary function was closely related to respiratory-driven diaphragm function. Therefore, it is important to evaluate the function of the diaphragm in elderly patients. Abbas et al. [
23] analyzed the role of diaphragmatic rapid shallow breathing index in predicting weaning outcome in patients with acute exacerbation of COPD, and age is an independent influencing factor of extubation failure. With increasing age, the lung organ reserve function of patients gradually decreases, and organ dysfunction is more likely to occur, leading to difficulty in extubation. At the same time, the respiratory drive reserve of the elderly is lower than that of the young, suggesting that the impairment of lung function in the elderly is related to their lower respiratory drive reserve, and the main driving force of respiration comes from the diaphragm, so the diaphragm function will affect the success rate of breathing and weaning.
The evaluation of diaphragmatic function is of great help to predict the outcome of weaning in elderly patients. The diaphragm is the most important respiratory muscle, playing a very important role in the process of spontaneous breathing. There is a correlation between mechanical ventilation time and diaphragm displacement. The more the diaphragm is displaced, the stronger the contractile function of the diaphragm, the stronger the respiratory muscle group, and the higher the weaning power are. In addition, long-term mechanical ventilation will also affect diaphragm function, resulting in diaphragm atrophy and dysfunction of diaphragm contraction, leading to weaning failure. Therefore, bedside ultrasound monitoring of diaphragm function can assess the risk of adverse events in elderly ICU patients on mechanical ventilation and predict the success rate of weaning. One traditional method for detecting diaphragm activity is transdiaphragmatic pressure measurement, but its operation is more complicated and tedious, and its clinical application is limited. Bedside ultrasound evaluation of diaphragm activity is noninvasive, safe, simple, convenient and fast, and it can also monitor the motion amplitude, thickness and contraction speed of the diaphragm in real time during breathing, making it more easily accepted by clinical practice.
The measures of diaphragmatic function include DE and DTF, which are used as ultrasound parameters to predict the success of extubation. Our study showed that sonographic indices were useful in evaluating diaphragm function to predict extubation success, supporting previous results. Diaphragm dysfunction or atrophy can be observed after mechanical ventilation for as little as 24 h [
24]. Diaphragm ultrasound can be used to determine diaphragm movement to evaluate diaphragm function. Boussuges et al. [
19] proposed a cut-off value for diaphragm movement of 1 cm for healthy men and 0.9 cm for healthy women. In our study, diaphragm movement < 1.3 cm was defined as hemidiaphragm dysfunction. Comparing diaphragm movement in the two groups, hemidiaphragm dysfunction was shown to be significantly associated with an increased risk of weaning failure. Therefore, US evaluation of diaphragm function may be useful to predict weaning or extubation success. Monitoring diaphragm function by US can help to detect diaphragm dysfunction earlier and aid in the decision whether to extubate.
DTF was previously explored to predict weaning from mechanical ventilation, DTF, also known as the “ejection fraction” of the diaphragm, refects active diaphragm contraction and inspiratory effort. It is associated with ICU length-of-stay, the duration of mechanical ventilation, and mortality, Ultrasonography is a widely available non-invasive tool that provides both structural and functional information about the muscle [
25]. DiNino et al. [
3] investigated the associations between DTF at end-expiration and end-inspiration and extubation success during either SB or PS weaning trials. They concluded that DTF measured in this way may be useful to predict extubation success or failure. Minas et al. reported a positive correlation between DTF and respiratory muscle pressure, though statistical significance was not reached.
Diaphragm indicators were better than RSBI according to the AUC and specific sensitivity. In our study, DTF was better than RSBI, LUS and DE with regard to sensitivity and specificity. The best cut-off values for predicting weaning success were DTF ≥ 30%, DE ≥ 1.3 cm, LUS ≤ 11, and RSBI ≤ 102. The specificity of DTF in predicting weaning outcome at 84% was higher than those of RBSI (53%), LUS (55%), and DE (62%), while the sensitivity at 94% was higher than those of RBSI (85%), LUS (71%), and DE (65%). The area under the DTF curve at 0.881) was significantly higher than that under the other curves. Therefore, a DTF of greater than or equal to 30% had the most accuracy for predicting weaning success, which is consistent with Ferrari et al., who reported that a cut-off value of 36% was associated with successful weaning. Samanta et al. reported that a DTF cut-off greater than or equal to 25.5% gave an AUC of 0.91 and sensitivity and specificity of 97% and 81%, respectively [
26].
Despite the different cut-off values in these studies, most authors agree that DTF is a better weaning predictor than RSBI. A systematic review and meta-analysis performed by Llamas-Álvarez et al. [
27] concluded that DTF is a modest predictor of weaning outcome in the general population of critically ill patients.
Combined model for predicting weaning outcomes
As discussed, none of the individual indices measured, RSBI, LUS DE or DTF (all AUCs < 0.9), accurately predicted weaning outcome independently. However, by using logistic regression to combine them, a significantly increased AUC of 0.919, with a sensitivity and specificity of 96% and 94%, respectively, was achieved. It takes 10 min to half an hour to combine the four indicators, but it is of great value. The combined indicator can better predict extubation and is of great help to clinical practice. Diaphragm. Ultrasonography (DUS) allows serial radiation-free bedside evaluations of diaphragmatic function in critically ill patients. A combined approach consisting of a theoretical module followed by a practical training is more effective in managing acoustic windows and performing accurate measurements [
28].
The present study has several limitation. Most of the studies including this study on the use of diaphragm activity and the diaphragm thickness change rate to make weaning decisions are observational studies, So, the observational design of the study has a limitation to describe the advantage of using chest ultrasound in elderly patients as compared to young patients with a considerable clinical advantage, and there is a lack of randomized controlled trials to evaluate whether weaning timing can reduce the rate of extubation failure. This study was conducted in a single centre, but multi-centre studies are needed to further confirm the results. A limitation of this study is that the sample size was small. In the future, more ICUs can be combined for multi-centre and larger-sample statistical analysis. Finally, ultrasound is an operator-dependent technique, So it depends on the technical proficiency of the operator.