|Year : 2020 | Volume
| Issue : 2 | Page : 211-219
Application of lung ultrasound for predicting outcome of weaning from mechanical ventilation
Mona R Hosny, Randa A Shoukry
Department of Anesthesiology, Intensive Care, and Pain Management, Faculty of Medicine, Ain Shams University, Abbasia, Cairo, Egypt
|Date of Submission||17-Feb-2019|
|Date of Acceptance||09-Feb-2020|
|Date of Web Publication||27-Jun-2020|
Mona R Hosny
Department of Anesthesiology, Intensive Care, and Pain Management, Faculty of Medicine, Ain Shams University, Abbasia, Cairo 11566
Source of Support: None, Conflict of Interest: None
Background Weaning failure is an important issue among critically ill patients. Currently, lung ultrasound (US) is a useful tool to early predict this problem. This study was conducted to assess the efficacy of lung ultrasound to predict early weaning failure in critically ill patients.
Patients and methods A total of 50 patients requiring mechanical ventilation (MV) were included in this prospective observational study. Patients were divided into two groups after 48 h: group NS included patients who were unsuccessfully weaned, either failed 30 min of spontaneous breathing trial (SBT) or reintubated within 24 h after being extubated, and group S included patients who were successfully weaned (extubated). Lung US was completed for all patients, 1 h before SBT while patients were mechanically ventilated, 30 min after SBT, and 6 h after extubation. Patients were prospectively followed up for the need for reintubation and mechanical ventilation or the need for non-invasive positive pressure ventilation (NIPPV) after successful extubation (primary endpoints). The length of Intensive care Unit (ICU) stay, duration of mechanical ventilation (MV) and ICU mortality (secondary end points).
Results Lung US done for all patients in the anterior, lateral, and posterior regions of the lung showed significantly high lung US score in group NS than group S (P<0.001). The number of patients who failed SBT, needed reintubation and MV or noninvasive positive pressure ventilation, after being successfully extubated were significantly high in group NS than group S (P<0.001). During the study period, there were a significant decrease in ICU stay, ventilator days, and patient mortality in Group S than Group NS (P<0.001).
Conclusions Lung US can accurately detect lung aeration changes and predict weaning outcome through estimating the lung US scoring system for lung aeration changes where high scores are associated with weaning failure.
Keywords: duration of mechanical ventilation, failed weaning, ICU mortality, length of ICU stay, lung ultrasound score, lung ultrasound, weaning
|How to cite this article:|
Hosny MR, Shoukry RA. Application of lung ultrasound for predicting outcome of weaning from mechanical ventilation. Res Opin Anesth Intensive Care 2020;7:211-9
|How to cite this URL:|
Hosny MR, Shoukry RA. Application of lung ultrasound for predicting outcome of weaning from mechanical ventilation. Res Opin Anesth Intensive Care [serial online] 2020 [cited 2020 Jul 7];7:211-9. Available from: http://www.roaic.eg.net/text.asp?2020/7/2/211/287987
| Introduction|| |
Patients require mechanical ventilation (MV) when their respiratory drive is inadequate to maintain ventilation because of a disease or medications. Overall, 80% of the patients requiring temporary MV do not require a gradual withdrawal process and can be disconnected within few hours of initial support. However, for some patients owing to comorbid problems, the process can be lengthy for more than 48 h and complex. Accordingly, applying spontaneous breathing trial (SBT) can be an important determinant for successful weaning from MV challenging the patients’ ability to breathe on their own for 30–120 min ,.
Postextubation distress is defined as reintubation or need for rescue noninvasive positive pressure ventilation (NIPPV) within 48 h following extubation ,. Most proposed predictors of postextubation distress are too complex for bedside use or have a limited predictive value ,. Therefore, no simple clinical indices are known to be powerful predictors of postextubation distress .
Ultrasound (US) examination is increasingly being used as a valuable bedside method in the diagnosis of various thoracic conditions in ICUs . US in a normal lung shows that the wide acoustic impedance between the pleura and underlying aerated parenchyma creates typical horizontal artifact defined as A lines . In the presence of variations in the relationship between the aerated and tissue fluid parts of the lung, vertical artifacts called B lines correlate with extravascular lung water (EVLW) increase .
The aim of this prospective study was to assess the efficacy of lung US as a tool to predict early weaning failure in critically ill patients.
| Patients and methods|| |
Approval of the research ethics committee was done, and written informed consents were obtained from patient’s next of kin, patients were enrolled to this prospective observational trial in Ain-Shams University Intensive Care Units over a period of one year. ClinicalTrials.gov registration number NCT03880864.
Patients included in the study were adults more than 18 years of age who needed MV for 48 h or more, and are eligible for SBT in Ain Shams University Intensive Care Units. Discontinuation from mechanical ventilation was attempted at clinical judgment of the medical team in charge, according to the consensus criteria, which included: reversal of the underlying cause for respiratory failure, adequate oxygenation (PaO2/FiO2 ratio ≥150), pressure support ≤7 cmH2O, FiO2 ≤ 0.35, negative inspiratory force (NIF) ≥ 20–30 cm H2O, respiratory rate (RR) ≤34, the ratio between respiratory frequency (f) and tidal volume (Vt) in liters f/Vt 60–105/L, pH >7.25. Included patients should be hemodynamically stable and had Glasgow coma scale (GCS) ≥ 12.
Exclusion criteria were patients who were aged less than 18 years, BMI more than or equal to 35 kg/m2, or who were tracheostomized. Patients who were paraplegic with spinal cord injury above C8, those who had any kind of cardiac arrhythmias or had severe ICU-acquired neuropathy, and patients with planned prophylactic noninvasive ventilation.
Patients were divided into two groups after 48 h: group NS included patients who were unsuccessfully weaned, either failed 30 min of SBT, had been reintubated or needed NIPPV within 48 h after being extubated, and group S included patients who were successfully weaned (extubated).
All patients who were included in the study were observed for 48 h after passing the successful SBT and were monitored for the occurrence of reintubation after being extubated and the need of NIPPV. The patients were demographically classified according to baseline clinical characteristics, which included mean age, sex, BMI, simplified acute physiologic score II, complete medical history, and physical examination, which included vital signs and monitoring of SPO2 1 h before SBT, 30 min after SBT, and 6 h after extubation using arterial blood gases. Transthoracic echocardiography was completed for all patients who were included in the study to diagnose any systolic or diastolic dysfunction.
Lung US (Kontron Medical, Cedex, France) was completed for all patients before weaning by trained investigators using a 2- to 4-MHz convex probe, 1 h before SBT while patients were mechanically ventilated, 30 min after SBT, and 6 h after extubation in a 10-min period, which is the time required for assessing the whole lung. It is to be noted that the same investigator performed the different lung US at each point of the study.
Lung US findings of the upper and lower parts of the anterior, lateral, and posterior regions of the right and left chest wall were carefully examined making 12 lung regions ([Figure 1]) .
|Figure 1 Lung ultrasound score . Lung US of the upper and lower parts of the anterior, lateral and posterior regions of the right and left chest wall were carefully examined making 12 lung regions . US, ultrasound.|
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Four US aeration patterns were defined: normal aeration is represented by the presence of lung sliding and horizontal A lines, or less than three vertical B lines; a score of 0 is assigned to a lung region if all of the intercostal spaces show normal aeration. A moderate loss of aeration is characterized by multiple regularly or irregularly spaced B lines that originate from pleural line or from small juxta-pleural consolidations; a score of 1 is assigned to a lung region if all of the intercostal spaces show a moderate loss of aeration. Severe loss of aeration is characterized by the presence of coalescent B lines in several intercostal spaces, occupying the whole intercostal space; a score of 2 is assigned to the examined region. Complete loss of lung aeration, as observed in lung consolidation, is characterized by tissue echogenicity with static or dynamic air bronchograms; a score of 3 is assigned to the examined region. The scores of the 12 examined regions range between 0 and 36 .
Patients who were enrolled in the study were prospectively followed up primarily (primary endpoint) for the need for reintubation and MV and the need for NIPPV after being successfully extubated, and secondarily (secondary outcomes) for the length of ICU stay and duration of MV, and ICU mortality.
IBM SPSS statistics (V. 24.0, 2016; Armonk, NY: IBM Corp., USA) was used for data analysis. Data were expressed as mean±SD for quantitative parametric measures in addition to both number and percentage for categorized data. Comparison between two independent mean groups for parametric data was done using Student t test. χ2 test was used to study the association between each two variables or comparison between two independent groups regarding the categorized data. Comparison between two proportions as regards univariant categorized data. The probability of error at 0.05 was considered significant, whereas 0.01 and 0.001 are highly significant.
Sample size calculation
Using Med Calc program, setting alpha error at 5% and power at 80%. Result from previous study (Banerjee and Mehrotra)  showed that LUS has an area under curve of 0.78 for prediction of successful weaning which was present in 75% of cases. Based on this, the needed sample is 50 cases with taking in consideration 10% dropout rate.
| Results|| |
All 50 patients included in this study fulfilled the inclusion and exclusion criteria mentioned earlier, and they were divided in two parallel groups: group NS (27 patients) and group S (23 patients). The study group included 28 (56.0%) men and 22 (44.0%) women, with a mean (SD) age of 51.89 (14.58) years in group NS and 47.52 (14.6) years in group S (P˃0.05), mean (SD) BMI of 35 (4.3) kg/m2 in group NS and 33.2 (5.2) kg/m2 in group S (P>0.05). On admission to ICU, median value of simplified acute physiologic score II was 36 for group NS and 34 for group S (P>0.05). The cause of ICU admission of the studied group included 15 (30%) cases of pneumonia, five (10%) chest trauma, 10 (20%) chronic obstructive pulmonary disease (COPD), nine (18%) decompensated heart failure, one (2%) near drowning, six (12%) ischemic heart disease, and four (8%) after operation (P>0.05).
Vital signs including heart rate, RR, core body temperature, and mean arterial blood pressure were monitored continuously during the study period, but the displayed data were recorded during 1 h before SBT, 30 min after SBT, and 6 h after extubation. There was a significant increase in RR during 1 h before SBT, 30 min after SBT, and 6 h after extubation in group NS than group S, but mean arterial blood pressure values were only significantly increased in group S than group NS during 30 min after SBT and 6 h after extubation, whereas heart rate values were only significantly increase in group NS than group S during 30 min after SBT and 6 h after extubation, and temperature was only significantly increase in group NS than group S 6 h after extubation ([Table 1]).
|Table 1 Comparison between the two studied groups according to mean arterial blood pressure, heart rate, respiratory rate, and temperature|
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Arterial blood gases were compared among both groups, which showed significant differences regarding pH, PaCO2, HCO3, and hypoxic index 6 h after extubation ([Table 2]).
|Table 2 Comparison between the two studied groups according to arterial blood gas|
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Transthoracic echocardiography was completed for all patients who were included in the study regarding systolic, diastolic, and combined systolic and diastolic dysfunction ([Table 3]). There were significantly more patients who had diastolic dysfunction and combined dysfunction in group NS than in group S (P=0.041 and P=0.005 respectively), but there was no significant difference in number of patients who had systolic dysfunction (P=1.000).
|Table 3 Comparison between the two studied groups according to echo studies|
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Fluid balance state was assessed in both groups every 4 h during the study period, but the displayed data recorded 24 h before SBT showed significantly high positive fluid balance in group NS (13 patients) than group S (four patients) ([Figure 2]).
|Figure 2 Bar chart presenting positive fluid balance state 24 h before SBT between group NS and group S. SBT, spontaneous breathing trial.|
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Lung US was completed for all patients in the anterior, lateral, and posterior regions of the lung, 1 h before SBT while patients were mechanically ventilated, 30 min after SBT, and 6 h after extubation. Lung US score was significantly high in group NS than group S ([Figure 3]).
|Figure 3 Bar chart presenting lung ultrasound score in the anterior, lateral and posterior lung region between group NS and group S.|
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Lung US was completed for all patients, 1 h before SBT while patients were mechanically ventilated, 30 min after SBT, and 6 h after extubation. Lung US score was significantly high in group NS than group S ([Table 4]).
|Table 4 Comparison between the two studied groups according to lung ultrasound score|
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The number of patients who failed SBT, or needed reintubation and mechanical ventilation or non-invasive positive pressure ventilation (NIPPV) after being successfully extubated are represented in ([Table 5]).
|Table 5 The number of patients who failed spontaneous breathing trial, needed reintubation, or noninvasive positive pressure ventilation|
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During the study period, there was a highly significant increase in ICU stay, ventilator days, and patient mortality in group NS than group S, with a mean (SD) of 17.93 (4.4) in group NS and 6.35 (2.57) in group S, 9.3 (1.79) in group NS and 4.26 (1.32) in group S, and nine (33.3%) patients in group NS and no mortality in group S, respectively (P<0.001).
During the study period, diagnostic accuracy of lung US score in weaning outcome was assessed ([Table 6]).
| Discussion|| |
In the intensive care, weaning failure is often due to multiple factors and not based on the result of a single test . The objective of the study is to evaluate the usefulness of lung US in predicting weaning failure from MV in critically ill patients. The assumption of the study is lung US can detect changes in lung aeration and EVLW when weaning from positive pressure ventilation.
Previous clinical trials reported that lung US and quantification of B lines is as a novel tool to detect overhydration. A study done by Enghard et al. , aimed to evaluate a four-region protocol of lung US to determine the pulmonary fluid status in ventilated patients in ICU and showed that there was a significant correlation between presence and the extent of B lines detected by lung US and pulmonary water status as assessed by EVLW index. Sensitivity and specificity were 92.1 and 91.7%, respectively, for the lung US score to detect EVLW above the value of 7 ml/kg.
A study by Anile et al. , done on 19 consecutive critically ill patients requiring MV showed a significant correlation between the number of lung quadrants positive for B lines and EVLW which is closely related to mortality in the emergency and the critical care.
In 2014, a study was conducted by Volpicelli et al.  to assess whether B lines in combination with left ventricular ejection fraction assessment can be used to differentiate between low/high pulmonary artery occlusion pressure (PAOP) and detection of EVLW and concluded that B lines allow good prediction of EVLW and of limited value in prediction of PAOP. Combining B lines with estimation of LVEF at transthoracic US may improve the prediction of PAOP.
In a study by Bouhemad et al. , the accuracy of lung US in the detection and efficacy of antimicrobials in treatment of ventilator-associated pneumonia (VAP) and the occurrence of lung aeration was examined. A total of 30 patients on MV proved to have VAP were enrolled in the study. Chest radiography, lung US, and computed tomography chest were done on days 0 and 7. The study illustrated that lung US is more appropriate than chest radiography for quantifying lung aeration in patients with VAP who are successfully treated with antibiotics.
Another study by Mallamaci et al.  performed lung US on 75 patients before and after hemodialysis. Patients before dialysis were classified into three groups according to lung congestion severity, depending on the number of bilateral B lines (mild, <14; moderate, 14–30; and severe >30); they found a significant reduction in the number of the B lines after dialysis.
In accordance with the previous studies, we evaluated the evidence of using lung US as a bedside tool to help intensivists in their weaning decision. Lung US was completed for all patients and showed that lung US score was significantly high in group NS than group S, 1 h before SBT while patients were mechanically ventilated, 30 min after SBT, and 6 h after extubation.
Lung US demonstrated significant changes in lung aeration during SBT in the unsuccessful group: a score above 19 shows 100% positive predictive value and 100% specificity for reintubation. A score more than 18 shows positive likelihood ratio of 6.81. As a score of less than 10 illustrates 100% negative predictive value, no patients with less than a score of 10 were reintubated. These results were matching with results published by Soummer et al. , which showed that lung US can predict the weaning outcome and a score of more than 17 and less than 12 demonstrated more than 90% sensitivity and specificity in predicting weaning failure and success, respectively
Non-invasive ventilation (NIV) is used in acute exacerbation of COPD and to prevent intubation and ventilation .It seems logical that NIV may avert reintubation after failed extubation  A study found that immediate application of NIV after extubation led to reduction in reintubation rates and ICU mortality  Another study reported that NIV is useful in preventing reintubation and mortality in patients with hypercapnia during SBT . In our study the decision to put patients whom had respiratory distress on NIV or to do reintubation, was left to the intensivist who was in charge.
Lung US score before SBT showed a significant difference between both groups. It was higher in the failed weaning group. A study by Antonio et al.  was conducted on mechanically ventilated patients when lung US was performed just before the SBT. Three or more B lines in single view were called B pattern. B predominance was defined as a B pattern on at least one of the four anterior chest zones. There was a significant correlation between B predominance and weaning failure after successful extubation.
Many studies reported that surgical ICU patients are easier to be weaned from MV than patients intubated owing to medical reasons ,. In our study, the number of patients ventilated owing to medical causes largely outweighed the number of patients intubated due to surgical causes or postoperatively, and this may explain why the number of patients in the weaning failure group is more than the number of patients in the successful weaning group.
Weaning from MV imposes additional work on the cardiovascular system (CVS) and can provoke or unmask left ventricular diastolic dysfunction, which is characterized by decrease myocardial compliance . The number of patients in the failed group in the current study diagnosed with diastolic dysfunction before SBT outweighed the number of their counterparts in the successful group.
Our observation is similar to a recent study  that obtained mitral standard inflow analysis and additional Doppler Echo cardiograph variables before and during SBT in 50 critically ill patients without previous cardiac disease for assessment of left ventricular diastolic dysfunction and filling pressures. Twenty three patients failed to wean, and weaning failure was significantly associated with left ventricular diastolic dysfunction.
We observed in the current study that patients with positive fluid balance before attempting to wean were more likely to fail weaning. Fluid balance state was assessed in both groups every 4 h during the study period, but the displayed data recorded 24 h before SBT showed significantly high positive fluid balance in group NS (13 patients) than group S (four patients). Antonio et al.  calculated positive fluid balance in the last 48 h before SBT in patients with COPD; they found that patients with positive fluid balance had high incidence of weaning failure.
Weaning failure is a well-known factor for causing increased incidence of morbidity and mortality in intensive care patients. When it comes to the length of stay in ICU, the mean value in group S was 6.35 (2.57) days compared with 17.39 (4.4) days in group S (P<0.001). The group S was on MV for 4.26 (1.32) days, whereas the group NS stayed for 9.3 (1.79) days (P<0.001). These results are matching with the results in the study by de Lassence et al. 2002 , where ICU length of stay was higher in the failed weaning group compared with successful weaning group (22 vs. 9 days, respectively; P<0.0001) and days on MV were higher in the failed weaning group compared with successful weaning group (17 vs. 6 days, respectively; P<0.0001).
The mortality rate in the current study was 33.3% in the group NS, which correlates with the higher mortality rate of 55% in the failed weaning patients in the study by Shin et al. . It is to be noted that to date there are no simple clinical indices known to be powerful predictors of postextubation failure, and deterioration of lung aeration is one of the most critical pathophysiology in weaning failure ,.
| Conclusion|| |
Lung US can accurately detect lung aeration changes through estimating the lung US scoring system for lung aeration changes which are associated with weaning failure, and until more randomized control studies with larger sized population could be conducted, lung US can be considered as a safe tool for predicting weaning from MV in critically ill patients.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]