Subjective parameters
Resolution of the disease for which the patient needed ventilatory support
Good effective cough/ability to clear secretions
Absent or minimal tracheobronchial secretions
Clear sensorium/ no delirium
Absent or minimal neuromuscular weakness
Objective parameters
Respiratory rate ≤35/min
Heart rate ≤140/min
Resolution of hypotension or minimal need of vasopressors
SpO2 >89 % on ≤ FiO2 0.4 (or 
)
)PEEP ≤8 cmH2O
MIP ≤ −20 to −25 cmH2O
V T >5 ml/kg
RSBI <105 breaths/min/l
56.3 Pathophysiology of Weaning Failure
Patients who face weaning difficulties usually develop rapid shallow breathing on liberation from positive pressure ventilation. This results in dynamic hyperinflation and development of intrinsic positive end-expiratory pressure (iPEEP) [6, 20, 21]. The iPEEP, along with high ventilatory demand, leads to an increase in the respiratory work of breathing and causes respiratory muscle fatigue, thus causing weaning failure. The increase in respiratory work of breathing is further compounded by cardiovascular responses associated with removal of positive pressure ventilation, leading to an increase in the venous return along with escalation in resistance to left ventricular outflow [22]. An inappropriate cardiovascular response, especially in those with compromised left ventricular function, further increases the load on already-burdened respiratory muscles [23].
56.4 Assessment Tools for Weaning
A weaning assessment tool should ideally be able to correctly identify all the individuals who can be safely liberated from the ventilator. Patients who meet the weaning criteria should be screened with one of the weaning assessment tools. The various weaning assessment tools include respiratory frequency-to-tidal volume ratio, maximum inspiratory pressure, integrative weaning index, diaphragm ultrasound, and others. The currently available methods for weaning assessment are far from perfect, thus creating a need for an ideal assessment tool.
56.4.1 Respiratory Frequency-to-Tidal Volume Ratio (f/V T)
Also known as rapid shallow breathing index (RSBI), the f/V T is measured during spontaneous breathing for 1 min. During spontaneous breathing, the ventilator is set at a pressure support of 0 cm of H2O or continuous positive airway pressure (CPAP) of 0 cm H2O. A RSBI of 100 discriminates between successful weaning and failure, with a value of <100 suggesting a successful weaning trial with a sensitivity of 0.97 and a specificity of 0.65 [24].
56.4.2 Diaphragm Ultrasonography
Mechanical ventilation can lead to rapidly progressive diaphragmatic weakness and may hinder weaning from ventilation [25]. Bedside ultrasound is a useful modality to assess the diaphragm function. In a single-center study involving mechanically ventilated patients, diaphragm dysfunction (excursion of <10 mm on M-mode) could be identified in 29 % patients by using bedside ultrasound. Presence of diaphragmatic dysfunction on ultrasound assessment was associated with prolonged weaning time (17 vs 4 days, p < 0.01) and a longer time spent on mechanical ventilation (24 vs 9 days, p < 0.01) [26].
56.4.3 Integrative Weaning Index
A prospective study assessed a combination of several factors (respiratory system compliance x arterial oxygen saturation/f/V T ratio) to predict the weaning from mechanical ventilation. The Integrative Weaning Index (IWI) performed better than several other parameters, including RSBI, tidal volume (V t), tracheal airway occlusion pressure in the first 0.1 s (P 0.1), the product of P 0.1 and f/V t (P 0.1 × f/V t), respiratory rate (f), static compliance of the respiratory system (Cst), and ratio of arterial oxygen tension to fraction of inspired oxygen (PaO2/FiO2) with an area under the receiver-operating characteristic (ROC) curve of 0.96 [27].
56.5 Weaning Trials
Once the patient is assessed for weaning and is considered suitable for weaning, a weaning trial is instituted before extubation. An ideal weaning trial should be able to identify all the individuals who will successfully tolerate extubation. The weaning techniques that are used include SBT, automated tube compensation, pressure support ventilation (PSV), and synchronized intermittent mechanical ventilation (SIMV).
56.5.1 Spontaneous Breathing Trial
SBT is the oldest and most commonly employed method used for weaning [16]. It involves removing the patient from the ventilator and providing supplemental oxygen by using a T-piece or T-tube device or a pressure support of 5–8 cm of H2O in adults [19, 28–30]. A pooled analysis of nine randomized trials comparing PSV SBT versus T-piece trial SBT did not find any difference between the methods or the pressure used in weaning success, intensive care unit (ICU) mortality, reintubation rate, length of stay in ICU or long-term weaning unit, and pneumonia [30]. However, PSV was more effective in predicting successful SBT in patients with simple weaning compared with T-piece trial [19, 30]. A few studies have also used CPAP of 5 cm H2O for SBT [30]. It is reasoned that provision of CPAP maintains functional residual capacity at a level similar to that following extubation. Further, CPAP also helps maintain the patency of small airways, especially in patients with COPD [31]. However, in patients with poor left ventricular function, provision of CPAP may falsely predict successful extubation [23].
56.5.2 Pressure Support Ventilation
PSV is another commonly used method for weaning. With PSV, all the breaths are patient triggered, flow cycled, and provide ventilatory support that is gradually reduced over time until patient is successfully liberated from mechanical ventilation. Both inspiratory positive airway pressure (IPAP) or pressure support and expiratory positive airway pressure (EPAP) or CPAP are reduced gradually by 1–2 cm of H2O until an acceptable IPAP of 5–7 cm of H2O and EPAP of 0–5 cm of H2O is reached [19].
56.5.3 Synchronized Intermittent Mandatory Ventilation
With SIMV, ventilation is assisted intermittently either as mandatory or as spontaneous breaths. During the spontaneous mode there is no support to the respiratory muscle, which have an additional burden to overcome dead space due to ventilatory circuit [5, 32]. This increased burden leads to fatigue of respiratory muscles and hence weaning failure and increased need for invasive mechanical ventilation. Therefore, the use of SIMV as a mode for weaning is discouraged. SIMV can also be combined with PSV, where the spontaneous breaths are assisted by pressure support. However, even this mode is inferior as the respiratory center and respiratory muscles have to alter their output in anticipation of the next breath, which may either be mandatory or spontaneous. Moreover, it does not does not allow partitioning of the work of breathing performed by either the ventilator or the patient [5, 32–35].
56.6 Role of NIV in Weaning
Noninvasive ventilation has been used in three different scenarios for weaning: (i) advancing extubation in patients with difficult or prolonged weaning (weaning strategy); (ii) avoidance of reintubation after extubation in patients with post-extubation respiratory failure (management strategy); and (iii) to prevent development of post extubation respiratory failure (prophylactic strategy). Herein, we discuss only the weaning strategy.
56.6.1 Rationale of NIV in Weaning Failure
By reducing the work of breathing and preventing the development of the rapid shallow breathing pattern, NIV may be useful where weaning has failed. Further, EPAP akin to positive end-expiratory pressure acts as an external splint and helps in avoiding dynamic hyperinflation seen in patients with COPD. NIV by its favorable cardiovascular effects may also facilitate weaning [3, 4, 6, 15, 36, 37].
56.7 Role of NIV in Difficult/Prolonged Weaning (Weaning Strategy)
NIV has been tried in the management of acute respiratory failure as a strategy to shorten the weaning process and facilitate liberation from invasive mechanical ventilation, especially in patients with COPD [38–40]. The trial design in most studies involved patients who failed SBT; they were subsequently randomized to continued invasive ventilation or extubated and initiated on NIV. The results of nine randomized controlled trials (RCTs) evaluating the role of NIV in augmenting extubation are summarized in Tables 56.2 and 56.3. Of the nine studies identified, three studies included patients with acute exacerbation of COPD (AECOPD) [38–40], while three studies encompassed patients with acute respiratory failure due to heterogeneous etiology (COPD, heart failure, pneumonia, thoracic trauma, and chest wall deformity) [41–43]. One study comprised patients with acute hypoxemic respiratory failure [44], and two studies involved patients with acute-on-chronic respiratory failure (COPD, persistent asthma, bronchiectasis, obesity hypoventilation syndrome, restrictive lung diseases, and others) [45, 46]. The most common weaning assessment tool applied in all the studies was SBT with duration ranging between 5 min and 2 h. Weaning success, as defined by the lack of need of reintubation within 48–72 h of extubation or hospital survival, was reported in eight studies. Other parameters reported included duration of invasive mechanical ventilation, length of ICU/hospital stay, hospital mortality, and complications associated with invasive mechanical ventilation and weaning.
Table 56.2
Summary of studies describing use of noninvasive pressure ventilation (NIV) in difficult weaning
Author/year of study | Type of study | No. of patients | Comparator strategy (n) | Cause of respiratory failure | Weaning trial given | Weaning success (p value) |
|---|---|---|---|---|---|---|
Nava et al. (1998) [38] | RCT | 50 | NIV vs PSV (25 vs 25) | AECOPD | T-piece trial | 22/25 vs 17/25 (0.002) |
Girault et al. (1999) [46] | RCT | 33 | NIV vs PSV (17 vs 16) | AECOPD, restrictive lung disease, mixed lung disease | 2 h T-piece trial | 13/17 vs 12/16 (>0.05) |
Ferrer et al. (2003) [41] | RCT | 43 | NIV vs conventional weaning strategy (21 vs 22) | AECOPD, heart failure, pneumonia, thoracic trauma, post-operative | T-piece trial | 18/21 vs 16/22 (>0.05) |
Trevisan at al. (2008) [43] | RCT | 65 | NIV vs IMV (28 vs 37) | AECOPD, heart failure, pneumonia, thoracic trauma, post-operative | 30 min T-piece trial | 15/28 vs 15/37 (NA) |
Prasad et al. (2009) [40] | RCT | 30 | NIV vs PSV (15 vs 15) | AECOPD | 2 h T-piece trial | NA |
Girault et al. (2011) [45] | RCT | 208 | NIV vs PSV vs oxygen therapy (69 vs 69 vs 70) | Chronic hypercapnic respiratory failure due to COPD, persistent asthma, bronchiectasis, obesity-hypoventilation syndrome, chest wall deformity, sequelae of pulmonary tuberculosis | 5 mins-2 h T-piece trial | 46 vs 32 vs 20 (<0.001) |
Vaschetto et al. (2012) [44] | RCT | 20 | NIV vs PSV (10 vs 10) | Acute hypoxemic respiratory failure | 30 min SBT | 9/10 vs 5/10 |
Tawfeek et al. (2012) [42] | RCT | 42 | NIV vs SIMV (21 vs 21) | AECOPD, heart failure, pneumonia, thoracic trauma, post-operative, neuromuscular disease | 2 h SBT | 18/21 vs 11/21 (<0.05) |
El-Shimy et al. (2013) [39] | RCT | 40 | NIV vs SIMV (20 vs 20) | AECOPD | 0.5–2 h SBT | 17/20 vs 15/20 (0.049) |
Table 56.3
Outcome parameters in studies describing noninvasive ventilation as a weaning strategy
Author/year | Total duration of IMV, in days | Total duration of ventilatory support (both NIV and IMV), in days | Length of ICU stay, in days | Length of hospital stay, in days | In hospital deaths (n) | Complication related to IMV and weaning (n) |
|---|---|---|---|---|---|---|
Nava et al. (1998) [38] | 10.2 ± 6.8 vs 16.6 ± 11.8 | NA | 15.1 ± 5.4 vs 24 ± 13.7 | NA | 2 vs 7 | NA |
Girault et al. (1999) [46] | 4.56 ± 1.85 vs 7.69 ± 3.79 | 11.54 ± 5.24 vs 3.46 ± 1.42 | 12.35 ± 6.82 vs 14.06 ± 7.54 | 27.12 ± 14.33 vs 27.69 ± 13.09 | 0 vs 2 | 6 vs 9 |
Ferrer et al. (2003) [41] | 9.5 ± 8.3 vs 20.1 ± 13.1 | 11.4 ± 8 vs 20.1 ± 13.1 | 14.1 ± 9.2 vs 25 ± 12.5
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