However, the increase in capillary pulmonary pressure occurring during the transition from mechanical to spontaneous ventilation depends on the type of weaning trial. The standard test for extubation readiness is the spontaneous breathing trial (SBT) performed using the T-tube by simply disconnecting the patient from the ventilator and providing additional oxygen. The weaning trial can also be performed without disconnecting the patient from the ventilator using a low level of pressure support, although respiratory rate and tidal volume are continuously monitored on the ventilator screen. Cabello et al. compared three modalities of trial before extubation in a selected population of patients with difficult weaning. A SBT on a T-tube was compared with a low pressure support level (7 cmH2O) with or without PEEP. They showed that the patient effort was higher during a T-tube trial than during a pressure support trial. During the pressure support trial, addition of PEEP further decreased both the effort and capillary pulmonary pressure, suggesting that weaning trials must be done without PEEP to unmask latent cardiac dysfunction . In a large, multicentre, randomized controlled trial, although the proportion of patients who failed the first trial was higher using T-tube than using a pressure support trial , the rate of patients who were extubated after 48 h was similar when the weaning trial was performed using T-tube or pressure support trial. Other studies suggested that some patients who failed a T-tube trial could immediately succeed a pressure support trial [21,34] and could be extubated without an increased risk of extubation failure . Overall, this suggests that either the T-tube trial slightly delays weaning readiness due to higher respiratory muscle effort or, on the contrary that the pressure support trial may expose to a higher risk of reintubation. The use of pressure support was justified by reducing the imposed work of the ventilator circuit and the endotracheal tube . However, the postextubation period is characterized by a relatively high upper airway resistance and an overall work of breathing similar before and after removal of endotracheal tube . Therefore, addition of even low levels of pressure support may lead to underestimate the risk of extubation failure in some patients .
The two goals of a weaning trial are the early detection of patients who are able to breathe without a ventilator, in order to avoid complications of prolonged mechanical ventilation and the identification of patients who are not able to breathe spontaneously to avoid extubation failure and its potential complications. In the vast majority of patients, the main objective is the early detection of weaning trial success and a short trial is fully effective . However, a more challenging SBT using prolonged T-tube trial might be especially interesting in a population in which the risk of reintubation is particularly high [2▪▪,5].
Regardless of the weaning strategy used in ICUs, early identification of patients who are able to breathe spontaneously results in better outcomes, and it has been clearly demonstrated that the use of a weaning protocol, including daily screening followed by weaning trial and systematic extubation if successful, shortened intubation time without an increased risk of reintubation [13,38]. This strategy is usually caregiver-driven but can also be computer-driven by an automatic system, keeping the patient in a ‘respiratory comfort zone’ on the basis of respiratory rate, tidal volume and end-tidal CO2. This automated-weaning system allows to facilitate the weaning by gradually decreasing the level of pressure support and to perform a pressure support weaning trial when the lower level of assistance has been reached, suggesting to the clinician that the patient can be separated from the ventilator when this test is successful . A randomized controlled trial has revealed that automated-weaning system shortened overall ventilation time as compared with usual care , and a recent study [41▪▪] found similar results in postsurgical patients, but only in those after cardiac surgery. Although automated weaning could have no beneficial results compared to a nurse-driven protocol in units with a high nurse ratio and staffing , most studies that have compared automated-weaning system versus standard weaning found either reduction or similar duration of intubation time.
The SBT is meant to accurately predict the tolerance of unassisted breathing after extubation. However, it does not predict well the consequences of the tube removal in terms of upper airway patency and lower airway protection, removal of secretions and, ultimately, the ability to sustain spontaneous breathing. Interestingly, it has been suggested that extubation success may be well correlated to the patient's subjective perception of his ability to breathe without the ventilator .
Extubation failure may be caused by hidden factors such as delirium, prolonged sedation or ICU-acquired weakness. Delirium is frequent in the ICU and is a predictor of higher mortality . Acute brain dysfunction may favour extubation failure through disturbance of consciousness, agitation or sedation induced by medications, aspiration or refusal of treatment. ICU-acquired paresis occurs in about 25% of patients after prolonged mechanical ventilation  and can affect both peripheral and respiratory muscles  leading to prolonged weaning [51–53] and a higher risk of extubation failure . A recent study [54▪▪] showed that diaphragm dysfunction assessed by ultrasonography was associated with weaning failure. Diaphragm dysfunction at the time of extubation may correlate clinically with alveolar hypoventilation and cough inefficacy, subsequently increasing the risk of failure.
Postextubation laryngeal oedema is due to the pressure exerted by the endotracheal tube and is favoured by the conditions of intubation and the duration of mechanical ventilation . Laryngeal oedema occurs in about 5–15% of the patients [56–60], more often in women [56,60], with a low patient's height/tube diameter ratio . A good marker of severe laryngeal oedema is the absence of air leak when the sealing balloon cuff of the endotracheal tube is deflated. A low cuff-leak volume (<110–130 ml) measured by the difference between the insufflated volume and the expired volume in assist-control volume mode after deflating the balloon may be useful in identifying patients at risk for postextubation stridor [57,58]. However, although the absence of air leak is a good predictor of laryngeal oedema, the presence of detectable leak does not rule out the occurrence of upper airway obstruction . Upper airway obstruction was found to be the cause of extubation failure in 7–20% of the cases [3,10,61], but reached 38% in a large multicentric study focusing on postextubation stridor . In this latter study, administration of methylprednisolone prior to extubation reduced the incidence of stridor and the rate of reintubation due to laryngeal oedema . Recently, it has been found that a majority of patients ventilated more than 24 h exhibited laryngeal lesions, suggesting that this may favour postextubation respiratory distress by increasing work of breathing and/or favouring aspiration through glottis dysfunction. Interestingly, unlike other causes, when reintubation is purely linked to transient laryngeal oedema, it does not seem to be associated with a poor prognosis .
The use of NIV to treat postextubation respiratory distress or as a prophylactic measure to avoid respiratory distress needs to be distinguished. The literature suggests that prophylactic NIV after extubation may be useful to prevent acute respiratory failure in selected populations [62–64], whereas NIV employed for treating postextubation acute respiratory failure has no proven benefit  and can even increase mortality by delaying reintubation . However, NIV could reduce the risk of reintubation in postoperative patients after major elective abdominal surgery  or lung resection , and could even reduce mortality in this latter group. Indications and results of postextubation NIV on outcome are summarized in Table 3[11,62–67,68▪▪].
The results of large randomized controlled trials give an overall incidence of extubation failure relatively ‘low’ (10–20%) for the general ICU population. However, the individual risk of reintubation can become unacceptably high in some at-risk populations with an extremely high mortality in case of extubation failure.
Papers of particular interest, published within the annual period of review, have been highlighted as:
Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 72).
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A prospective observational study showing that failed extubation was followed by marked clinical deterioration, suggesting a direct effect on patient outcomes. Age and underlying chronic cardiac/respiratory failure were the only factors associated with extubation failure.
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A secondary analysis of a large prospective cohort trial analysing outcomes of mechanically ventilated patients according to the new weaning classification. In the 6% of patients who were classified as prolonged weaning (>7 days), there was a significantly longer length of stay and higher ICU mortality.
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In this large prospective cohort trial including 1152 mechanically ventilated patients who succeeded a weaning trial, 16% were reintubated within 48 h after extubation and reintubation was independently associated with mortality.
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25▪. Zapata L, Vera P, Roglan A, et al. B-type natriuretic peptides for prediction and diagnosis of weaning failure from cardiac origin. Intensive Care Med 2011; 37:477–485.
A prospective observational study showing that increases in natriuretic peptides during a SBT could predict weaning failure of cardiac origin. BNP performed better than NT-proBNP.
26▪. Ouanes-Besbes L, Dachraoui F, Ouanes I, et al. NT-proBNP levels at spontaneous breathing trial help in the prediction of postextubation respiratory distress. Intensive Care Med 2012; 38:788–795.
This study suggests that high levels of BNP before SBT may predict postextubation respiratory failure.
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28▪▪. Mekontso Dessap A, Roche-Campo F, Kouatchet A, et al.
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This study suggests that negative balance using diuretics may hasten extubation, especially in patients with left ventricular dysfunction.
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A prospective observational study in a noncardiac population of mechanically ventilated patients suggesting that left ventricular diastolic dysfunction measured with echocardiography before the SBT was associated with weaning outcome.
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This study suggests that the loss of lung aeration during a successful SBT may help to predict postextubation respiratory distress.
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This prospective controlled study compares an automated weaning protocol with a standardized written protocol and shows no differences in the overall ventilation times in surgical patients.
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This study shows that diaphragmatic dysfunction diagnosed by ultrasonography is seen in around 30% of patients without previous diaphragmatic diseases and is associated with weaning failure.
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