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12. Weaning from Ventilation
Mechanical ventilation (MV) is a life-saving strategy for critically ill patients, while unnecessarily prolonged ventilation is also associated with risks of ventilator-associated pneumonia (VAP), respiratory muscle weakness, prolonged duration of MV, and increase of hospital mortality . Therefore, weaning from the ventilator should be at the earliest possible time. As an important part of MV, weaning accounts for approximately 40% of the time patients spend on invasive ventilation . It usually refers to discontinuation of MV and extubating of any artificial airway (coma patients excluded), which has been a challenging problem faced by clinicians in the intensive care unit (ICU). Both premature and delay of weaning is associated with adverse outcome . To identify the appropriate time for weaning, some procedures have been proposed, including a screening test of the patient’s readiness to weaning and an unassisted breathing trial . Approximately two-thirds of patients could be extubated after their first successful spontaneous breathing trial, and those who failed may need other gradual approaches .
12.2 Readiness to Wean from Mechanical Ventilation
Recovery from the disease for which the patient was ventilated;
Hemodynamic stability with no need or minimal vasopressors;
No continuous sedation that may affect the patient’s ability to protect the airway;
Adequate oxygenation defined as PaO2/FiO2 ≥150 mmHg requiring positive end-expiratory pressure (PEEP) ≤8 cmH2O.
Daily screening is commonly recommended as standard care . While, a study conducted by Burns et al. found that more frequent screening was associated with a nonsignificantly shorter time to successful extubation, more successful extubations, and a trend toward shorter hospital stay .
Moreover, some parameters have been proposed to predict weaning outcomes, such as rapid shallow breathing test (respiratory frequency/tidal volume, f/VT), maximal inspiratory pressure (MIP), occlusion pressure at 100 ms (P 0.1), respiratory rate, tidal volume, minute volume, and so on. Rapid shallow breathing test is the most commonly used, and a high ratio (f/VT >100–105 breaths/min/L) is associated with a weaning failure . MIP is commonly used to assess the inspiratory muscle strength, and a value of ≤−20 ~ −25 cmH2O could predict successful weaning. However, the result may be affected by many factors, which may lead to underestimating patients’ strength [10, 11]. P 0.1 is related with inspiratory effort: P 0.1 value = 3.5 cmH2O corresponds to an inspiratory effort of approximately 0.75 J/L. Inspiratory effort values lower than 0.75 J/L could predict successful weaning . While, the accuracy of these predictors remains controversial, and they are not recommended as routine clinical practice [13–15]. Decision-making should be based on the evaluation of the patients.
12.3 Spontaneous Breathing Trials
Synchronized intermittent mechanical ventilation (SIMV);
Low-level pressure support ventilation (PSV ≤ 7 mmHg);
Continuous positive airway pressure ventilation (CPAP);
Unassisted breathing through a T-piece circuit.
The duration of SBT should be at least 30 min and can last up to 120 min to test the patient’s tolerance to spontaneous breathing [1, 2]. Theoretically, the endotracheal tube has a respiratory resistance which may increase the patient’s breathing work. The use of low-level PSV or CPAP could compensate for the resistance to breathing through the endotracheal tube . However, Straus et al. reported that a 120-min breathing trial through an endotracheal tube did not increase breathing work . The rate of successful extubation was similar between 30 and 120 min with either T-piece or PSV [3, 20]. It has been recently reported that the success rates of both SBT and extubation in patients with PSV mechanical ventilation mode are higher than that in patients with T-piece ventilation, without increasing the risk of reintubation. This proposes a shorter and less demanding strategy for SBT [18, 21]. For patients with chronic obstructive pulmonary disease (COPD) or heart failure, applying PEEP could decrease both the effort and capillary pulmonary pressure, suggesting that weaning trials without PEEP may expose underlying lung and cardiac dysfunction . So far, the optimal strategy of weaning patients from the ventilator is controversial, which may depend on the condition of patients.
Agitation or anxiety;
Hypoxemia (PaO2 ≤ 50–60 mmHg and FiO2 > 0.5, or SPO2 < 90%);
Hypercapnia (PaCO2 ≥ 50 mmHg or an increase in PaCO2 > 8 mmHg);
pH <7.32 or reduced by ≥0.07;
Tachypnea (respiratory rate >35 breaths/min or increased by ≥20% for more than 5 min);
Tachycardia (heart rate >140 beats/min; or increased by ≥20%);
Bradycardia (sustained heart rate decrease ≥20%);
Hypertension (systolic BP >180 mmHg or increased by ≥20%);
Hypotension (systolic BP <90 mmHg);
Severe cardiac arrhythmias.
12.4 Failure of Weaning from the Ventilator
Weaning failure is identified as the failure of SBT or the need for reintubation within 48 h after extubation, occurring in 10–20% of patients [5, 25]. According to the results of the first SBT and length of the weaning process, patients could be classified as following :
Simple Weaning: patient successfully passes the first SBT and is extubated on the first attempt (70% of weaning patients);
Difficult Weaning: patient fails the first SBT, and requires up to three SBT or not more than 7 days from the first SBT to achieve successful weaning (15% of weaning patients);
Prolonged Weaning: The patient requires more than three SBT or more than 7 days of weaning after the first SBT (15% of weaning patients).
It has been proven that prolonged weaning was associated with increased mortality and morbidity in the ICU [26, 27]. Weaning failure is usually caused by the imbalance between the respiratory load and the patient’s ability to cope with it [28, 29].
Excess respiratory loads can be caused by an increase of respiratory resistance due to the endotracheal tube and heat and moisture exchange devices in patients undergoing a T-piece SBT, and which could be compensated by the use of low-level PSV or CPAP strategy of SBT .
Diaphragmatic dysfunction leading to reduced inspiratory muscle strength is one of the major causes of weaning failure [30, 31]. The diaphragm is the major inspiratory muscle, and its function can be assessed by diaphragm ultrasonography, including diaphragmatic excursion (DE) and diaphragm thickening fraction (DTF). DE is defined as the distance that diaphragm moves during the respiratory cycle [32, 33]. Values of DE ranged from 10 to 14 mm during normal spontaneous breathing and 25 mm for maximal inspiratory effort indicate diaphragmatic dysfunction . DTF reflects the magnitude of diaphragmatic effort and could predict successful weaning. It can be calculated as (thickness at the end-inspiration—thickness at the end-expiration)/thickness at the end of the expiration . A cutoff of DTF more than 30–36% was associated with successful weaning [36, 37].
Cardiac dysfunction can cause a decrease of aeration of the pulmonary parenchyma and pulmonary compliance, and lead to weaning failure. Discontinuation from a ventilator may cause a decrease of the intrathoracic pressure, an increase in the systemic venous pressure gradient, preload and afterload, and result in cardiac dysfunction . An increase of B-type natriuretic peptides (BNP) after discontinuing from the ventilator usually indicates cardiac dysfunction. The BNP-guided weaning strategy could shorten the weaning process compared with the usual medical strategy, although it did not change ICU mortality . Echocardiography could help assess the cardiac function, and pulmonary echography could help assess the status of aeration of the pulmonary parenchyma .
Considering the heterogeneity of patients, causes of weaning failure varied. The clinician should comprehensively evaluate the patient and find out reasons. For patients experiencing difficult or prolonged weaning, gradual reduction of ventilatory support could be attempted.
Noninvasive ventilation (NIV) is suggested as an alternative to invasive ventilation for those with COPD, which may reduce the duration of invasive ventilation and mortality [41, 42]. While among other patients the use of NIV remains controversial . Besides, NIV may cause aspiration, gastric distension, and facial skin breakdown, and efficiency is dependent on patients’ cooperation. Therefore, NIV is not generally recommended as prophylactic use after extubation .
The high-flow nasal cannula (HFNC) can deliver humidified and heated gas at a maximum flow of 50–60 L/min (inspired oxygen fractions up to 95–100%) thereby generate a low level of PEEP, which may reduce breathing work and improve oxygenation compared with conventional oxygen therapy . Patients with hypercapnic respiratory failure and heart dysfunction may benefit from HFNC therapy . However, the evidence supporting its use remains limited, and HFNC is not recommended as a routine therapy.
12.5 Protocolized Weaning
Considering the variation of clinical practice in different ICUs, protocolized weaning is proposed intending to facilitate and standardize the weaning process and reduce MV duration and ICU mortality. Protocolized weaning refers to a written list used to guide discontinuation from the ventilator, including the first two of the following criteria, with or without the third one .
A list of criteria for assessing the patient’s readiness to discontinue from the ventilator, which should be screened daily.
Structured guidelines for reducing ventilatory support such as SBT or gradual reduction in ventilator support to achieve weaning.
A list of criteria for assessing the patient’s readiness to be extubated.
Protocolized weaning could reduce the duration of mechanical ventilation and the length of ICU stay, compared with usual weaning practice . However, the efficiency of weaning protocols may be influenced by the differences in weaning protocols and different healthcare providers, including nurses, respiratory therapists (RTs), and physicians. Considering the wide variety of patient populations, it is necessary to make appropriate weaning protocols for specific types of patients. Furthermore, optimized sedation strategies can also reduce the duration of MV. Therefore, protocolized sedation, daily interruption of sedatives, and intermittent use of sedatives have been recommended .
Patients without brain injuries are usually extubated after a successful SBT. Extubation failure is defined as a need for reintubation or ventilatory support within 48 h after scheduled extubation . It may be associated with age, chronic comorbidities, altered consciousness, upper airway function, and lower airway protection [48, 49]. Approximately 10–25% of patients require reintubation or mechanical ventilation after a successful SBT and reintubation is associated with prolonged duration of mechanical ventilation and ICU stay, as well as higher mortality.
Upper airway obstruction after extubation, usually due to laryngeal edema, is one of the main causes of reintubation, occurring in approximately 5–15% of patients [50, 51]. A cuff-leak test (cuff-leak volume <130 mL or 12%) and the presence of risk factors could help identify high-risk patients . Although a positive cuff-leak test could screen for high-risk patients, a negative cuff-leak test cannot rule out the occurrence of upper airway obstruction.