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The simple weaning group includes patients who successfully pass the initial SBT and are successfully extubated on the first attempt. This group, named group 1, represents ,69% of weaning patients. Prognosis in this group is good, with an ICU mortality of ,5% and an in-hospital mortality of ,12%. The remaining patients (,31%) represent groups 2 and 3. In this population, ICU mortality is ,25%. Group 2, difficult weaning, includes patients who require up to three SBT or as long as 7 days from the first SBT to achieve successful weaning. Group 3, prolonged weaning, includes patients who require more than three SBT or .7 days of weaning after the first SBT. Indeed, in the study by ESTEBAN et al., the 75th percentile for duration of weaning for all patients, for those with COPD and those with acute respiratory distress syndrome (ARDS) was 4, 5 and 6 days, respectively. Moreover, estimating from the survival curves from the studies by ESTEBAN et al. and BROCHARD et al. , about half of the patients who failed initial SBT (group 2) still required mechanical ventilation at day 7. Therefore, the current authors estimate that ,15% of patients would be in the prolonged weaning group (group 3). Recommendations Patients can be readily categorised into three groups based on the difficulty and duration of the weaning process. Clear definitions of the timing, stages and outcomes of the weaning process will support further research in weaning. Clinicians should evaluate readiness for weaning early in the hospital course to allow prompt initiation of the weaning process. An efficient weaning process reduces time on the ventilator and improves patient outcomes. Further research Further study is needed of the effectiveness of NIV in the process and outcomes of weaning. The proposed definitions of the weaning process and the groups of patients who undergo weaning need to receive careful testing and scrutiny. Studies of the patients’ outcomes in the difficult weaning and prolonged weaning groups are needed. QUESTION 2: WHAT IS THE PATHOPHYSIOLOGY OF WEANING FAILURE? A thorough and systematic search for potentially reversible pathologies should be conducted in all patients who did not fulfil simple weaning as previously defined. Using the definitions put forth in this statement, this places the patient in group 2, and reversible aetiologies for weaning failure may be categorised as follows: respiratory load, cardiac load, neuromuscular competence (central and peripheral), critical illness neuromuscular abnormalities (CINMA), neuropsychological factors, and metabolic and endocrine disorders . These same factors should be sought in group 3 patients, who have required mechanical ventilation for 7 days. The pathophysiology of weaning failure in group 3 may be complex and multifactorial. Irreversible lesions may become apparent at this time point or clearly delineated issues (e.g. cardiac dysfunction or CINMA) may be difficult to optimise or slow to resolve. In this group of chronically critically ill patients it is imperative that there is a disciplined approach to ongoing surveillance for any reversible or remediable factors. Respiratory load The decision to attempt discontinuation of mechanical ventilation has largely been based on the clinician’s assessment that the patient is haemodynamically stable, awake, the disease process has been treated adequately and that indices of minimal ventilator dependency are present (e.g. inspiratory oxygen fraction (FI,O2) f0.5, positive end-expiratory pressure (PEEP) f8 cmH2O, arterial oxygen tension (Pa,O2)/FI,O2 .20.0 kPa (150 mmHg), arterial oxygen saturation o90%).
The success of weaning will be dependent on the ability of the respiratory muscle pump to tolerate the load placed upon it. This respiratory load is a function of the resistance and compliance of the ventilator pump. Normal respiratory system resistance is ,5 cmH2O?L-1?s -1. Normal respiratory system static compliance is 0.06–0.1 L?cmH2O-1. Improvement in static lung compliance may alert the clinician to the potential for weaning from mechanical ventilation and has been incorporated into a computer-driven model where a plateau pressure of f30 cmH2O for a tidal volume (VT) o8 mL?kg-1 is included in the algorithm prior to decreasing levels of pressure support ventilation (PSV). Maximum inspiratory pressure has been used to assess inspiratory muscle strength, measured either after negative pressure is maintained for 1 s after a forceful expiration to residual volume or using a unidirectional expiratory valve. A maximum inspiratory pressure value of f-20– -25 cmH2O has been referenced as a predictive index for successful weaning. An excess work of breathing (WOB) may be imposed by inappropriate ventilator settings (e.g. inadequate inspiratory flow rate or flow trigger setting) resulting in ventilator dysynchrony . Components of such dysynchrony include the lead time between onset of inspiratory effort and the onset of flow delivery, respiratory effort after onset of flow, ineffective triggering, expiratory effort prior to switchover from mechanical inflation to exhalation, and the effect of high intrinsic PEEP (representing a threshold load). Reduced pulmonary compliance may be secondary to pneumonia, cardiogenic or noncardiogenic pulmonary oedema, pulmonary fibrosis, pulmonary haemorrhage or other diseases causing diffuse pulmonary infiltrates. Pneumonia is common in critically ill patients, either as the primary admission diagnosis or as a subsequent ventilator-acquired pneumonia. Ventilator-acquired pneumonia has a variable incidence of 9– 67% and an attributable mortality of up to 50%. Shortening the weaning process is an important goal in reducing the incidence of ventilator-acquired pneumonia. However, adequate treatment of the underlying disease process is a prerequisite to extubation. The need for reintubation is associated with a 4.5-fold increased risk of nosocomial pneumonia. Therapeutic guidelines and recommendations for reducing the incidence of ventilator-acquired pneumonia have been recently published. Reduced pulmonary compliance attributable to cardiogenic or noncardiogenic pulmonary oedema should be considered, with optimisation of cardiac function and clearance of lung water instituted as clinically indicated. Reduced pulmonary compliance secondary to pulmonary fibrosis (acute or pre-morbid) or reduced chest wall compliance secondary to kyphoscoliosis may be less amenable to treatment (more likely to represent group 3 patient profile). The splinting effect of obesity, abdominal distension or ascites may be further considerations. Additional load may be related to reversible airway bronchoconstriction, which should be addressed prior to SBT.