Weaning from mechanical ventilation: Readiness testing

INTRODUCTION — Liberation from mechanical ventilation is a three-step process that involves readiness testing, weaning, and extubation. Criteria for readiness testing are reviewed here. Weaning and extubation are discussed separately. (See "Initial weaning strategy in mechanically ventilated adults" and "Extubation management in the adult intensive care unit".)

DEFINITIONS

Readiness testing — Readiness testing uses objective clinical criteria (and occasionally physiological tests) to determine whether a patient is ready to begin weaning from mechanical ventilation. (See 'Clinical criteria' below and 'Weaning predictors' below.)

Weaning — Weaning is the process of decreasing the degree of ventilator support and allowing the patient to assume a greater proportion of their own ventilation (eg, spontaneous breathing trials or a gradual reduction in ventilator support). (See "Extubation management in the adult intensive care unit" and "Initial weaning strategy in mechanically ventilated adults".)

Extubation — Extubation is the removal of the endotracheal tube and is the final step in liberation from mechanical ventilation support. Extubation is performed when the patient is successful at weaning and both airway patency and airway protection measures are in place. (See "Extubation management in the adult intensive care unit".)

GOALS — Readiness testing identifies patients who are ready to wean as well as those who are not ready to wean from mechanical ventilation. Identifying those who are ready to wean avoids unnecessary mechanical ventilation and thereby, also avoids the risk of death and complications related to mechanical ventilation (eg, pulmonary barotrauma, ventilator-associated lung injury) [1,2]. Similarly, identifying patients who are not ready to wean protects patients from the risks of premature weaning (eg, cardiovascular dysfunction, respiratory muscle fatigue, psychological distress). (See "Clinical and physiologic complications of mechanical ventilation: Overview" and "Ventilator-induced lung injury" and "Diagnosis, management, and prevention of pulmonary barotrauma during invasive mechanical ventilation in adults".)

DAILY SCREENING LIBERATION ASSESSMENT — We and others agree that patients who are mechanically ventilated for more than 24 hours should undergo daily ventilator liberation protocol assessment [3,4].

Liberation protocols contain guidance regarding readiness for weaning as well as the performance of subsequent weaning trials and extubation. Support for daily protocolized liberation strategies comes from the observation that clinicians consistently underestimate the capacity of patients to breathe independently from the ventilator [5-15] and data from randomized trials which have demonstrated benefit from protocol use when compared with usual care [4,16-19]. Consequently, some intensive care units (ICUs) have incorporated liberation protocols into routine care for mechanically ventilated patients.

However, considerable global variation exists in the use of protocols to guide weaning [20,21]. Best illustrating this variation was a multicenter, international prospective observational study of 1868 patients who were mechanically ventilated for at least 24 hours and admitted to one of 142 intensive care units (ICUs) in six geographic regions (Canada, US, UK, Europe, India, Australia/New Zealand) [21]. Written directives to screen for readiness ranged from 5 to 83 percent and were present in more than half of participating ICUs in Canada (56 percent), India (70 percent), and the United States (83 percent). However, written directives for SBT conduct were present in less than half of ICUs outside of the US, where directives were used in 78 percent of US ICUs. Similarly, the frequency of screening (eg, never, once daily, twice daily) also varied widely; once daily screening occurred in more than half of ICUs in Canada (67 percent), India (74 percent), and the US (83 percent), while lower rates were seen in the other regions.

Liberation protocols may not be beneficial in all settings and should be tailored to the patient population in which they will be applied. For example, liberation protocols have not been shown to be superior to usual care in highly staffed, closed ICUs in an academic hospital [22]. As another example, a liberation protocol did not improve outcome in neurologic patients [23], but adding a neurologic assessment led to a reduction in the need for reintubation among this population [24].

Data that support the value of liberation protocols that prompt spontaneous breathing trials (SBTs) are discussed separately. (See "Initial weaning strategy in mechanically ventilated adults", section on 'Daily spontaneous breathing trials (SBTs)'.)

CLINICAL CRITERIA — Clinical criteria that are listed in the table (table 1) should be used to identify patients who are ready to begin weaning [25]. While these criteria are widely used, it should be recognized that up to 30 percent of patients who never satisfy such criteria may be successfully weaned [9].

To be considered ready for weaning, patients should have all of the following:

●Improvement in the underlying cause of respiratory failure

●Adequate oxygenation

●An arterial pH >7.25

●Hemodynamic stability

●Ability to take spontaneous respirations

Other criteria that are ideally in place but not critical for readiness assessment include a hemoglobin level ≥7 g/dL, core temperature ≤38.5ºC, and an awake or easily arousable mental status (eg, sedation may need to be titrated to a target goal such as Richmond agitation-sedation scale [RASS] -2 to +1). (See "Sedative-analgesia in ventilated adults: Management strategies, agent selection, monitoring, and withdrawal".)

These criteria are derived from studies that predicted successful weaning with their use performed within the context of a liberation protocol:

●A randomized trial of 304 mechanically ventilated patients compared those who underwent readiness testing using objective clinical criteria alone with those who underwent readiness testing using objective clinical criteria plus a weaning predictor, the rapid shallow breathing index [26] (see 'Rapid shallow breathing index' below). The group that used objective clinical criteria alone took one day less to discontinue mechanical ventilation. There was no difference in length of stay or reintubation rate. This trial is discussed in greater detail separately. (See 'Clinical outcomes' below.)

●The Awakening and Breathing Controlled (ABC) trial screened 336 mechanically ventilated patients daily for adequate oxygenation (peripheral oxygen saturation [SpO₂] >88 percent while receiving a fraction of inspired oxygen [FiO₂] <50 percent and a positive end-expiratory pressure [PEEP] ≤8 cm H₂O), for hemodynamic stability and any spontaneous inspiratory effort during a five-minute period, as well as the absence of agitation, myocardial ischemia, and increased intracranial pressure [27]. Weaning predictors were not measured. Those who passed the screen underwent a spontaneous breathing trial (SBT). More than 50 percent of patients who underwent an SBT tolerated it, suggesting that these criteria are reasonable indicators of successful weaning.

The complication rate of using readiness criteria for subsequent SBTs appears to be low. In a study of more than 1000 patients who underwent an SBT following the use of clinical readiness weaning criteria, only one complication was identified (<0.1 percent) [17]. Another study of 19 patients who underwent an SBT following readiness criteria found that low frequency fatigue, which can hinder future weaning attempts, did not occur [28].

Required criteria — Discussed in this section are the required clinical criteria, that are necessary before a patient can be considered ready to wean (table 1).

The underlying indication for mechanical ventilation has improved — In most cases, the underlying condition that precipitated intubation and mechanical ventilation should be improving as evidenced by clinical, radiological, and/or laboratory parameters (eg, fluid overload, pneumonia, acute respiratory distress syndrome).

Adequate oxygenation — Thresholds that determine adequate oxygenation are empiric. We consider oxygenation adequate when either the ratio of arterial oxygen tension to fraction of inspired oxygen (PaO₂/FiO₂) is ≥150 mmHg or when the SpO₂ is ≥90 percent while receiving an FiO₂ ≤40 percent and a PEEP ≤5 cm H₂O. For patients who have chronic hypoxemia, a PaO₂/FiO₂ ratio ≥120 mmHg may be used instead.

A higher level of PEEP may be acceptable in some patients to avoid atelectasis (eg, 8 cm H₂O in patients with obesity or abdominal distension for another reason such as ascites, or patients with a narrow endotracheal tube [ETT; eg, <7cm]). Similarly, a slightly higher FiO₂ (eg, 0.5) may be acceptable if significant underlying lung disease is present.

Arterial pH >7.25 — Severe acute acid-base disturbances, in particular, an acute metabolic, respiratory, or mixed acidosis, should be resolving or estimated to be close to baseline prior to weaning. In most cases, the pH is within normal limits. However, in patients with chronic metabolic or respiratory acid-base disturbances, we try to obtain or estimate baseline values and ventilate patients at a target value that is close to their baseline (eg, patients with chronic hypercapnia, renal tubular acidosis). Although arbitrary, the value of an arterial pH >7.25 is reasonable based upon the observation that for most spontaneously breathing patients, it may be tolerated without undue load on the respiratory system. Exceptions exist including patients with worsening acid-base disturbance, those who cannot generate sufficient minute ventilation to compensate for acute acidosis (eg, those with neuromuscular weakness).

Hemodynamic stability — We generally consider that patients are ready to wean if they are hemodynamically stable and without myocardial ischemia. The blood pressure thresholds below or above which it is unsafe for a patient to wean have not been established. However, it is reasonable to require that the mean arterial pressure be consistently >60 mmHg, and/or the systolic blood pressure be >90 mmHg and <180 mmHg, or estimated to be at the patients baseline. The use of vasopressors to maintain hemodynamic stability is acceptable, but only low and stable doses should be necessary (eg, dopamine <5 mcg/kg/minute, low-dose dobutamine in patients with heart failure).

Ability to initiate inspiration — For weaning trials to begin, patients should be able to initiate inspiratory effort. We assess this by determining that the patient is breathing above the set respiratory rate on the ventilator. For patients who are breathing at or below the set rate, we temporarily reduce the set rate to a lower value for a brief period and ensure that the patient is able to initiate spontaneous breaths above the newly set value (eg, set ventilator rate at six breaths per minute for one minute).

Additional or optional requirement — When assessing patients readiness to wean, we take into consideration several additional criteria that are also listed in the table (table 1) [3,4]. While it is ideal that these criteria are in place, they are less critical to the success of weaning than the required criteria listed above (see 'Required criteria' above). These include the following:

●Hemoglobin level ≥7 g/dL – Anemia reduces oxygen carrying capacity and can affect successful weaning. While in the past, any degree of anemia was considered a contraindication to weaning, studies performed since then suggest that hemoglobin levels of approximately 7 to 8 g/dL or greater are appropriate targets for safe weaning. For example, a secondary analysis of a randomized trial reported no difference in the rate of successful weaning when a restrictive blood transfusion strategy (ie, maintain hemoglobin level of 7 to 9 g/dL) was compared to a liberal strategy (ie, maintain a hemoglobin level of 10 to 12 g/dL; 78 versus 82 percent) [29]. Similarly, another single center retrospective study of 138 difficult to wean patients found a higher probability of weaning success in patients with a hemoglobin level of ≥8 g/dL compared with patients who had a hemoglobin <8 g/dL [30]. No difference in weaning success was found in patients who have hemoglobin levels of 8 to 10 g/dL compared with those who have hemoglobin levels >10 g/dL.

●Core temperature ≤38.5ºC – The rationale for this criterion is that the presence of fever makes successful weaning less likely because it increases the minute ventilation and, thus, increases work of breathing [31]. In addition, fever may also result in diminished respiratory muscle function (eg, patients with sepsis) [32]. However, a temperature threshold above which weaning is unsafe has not been identified. It should be noted that this applies to patients with actual fever at the time of assessment and does not apply to individuals who are afebrile for periods in between intermittent spiking of fever.

●A mental status that is either awake and alert or easily arousable – Although an awake or easily arousable patient is ideal for weaning and more importantly, extubation, it is not always feasible (eg, due to sedatives or delirium). It has been shown that an abnormal mental status (eg, Glasgow Coma Scale score between 8 and 15 (table 2) or Richmond agitation-sedation scale -2 to +1) does not appear to be associated with a higher rate of extubation failure [33,34]. Thus, as long as a patient can protect their airway, an abnormal mental status does not preclude weaning. Evaluation of the ability to protect the airway for extubation is described separately. (See "Extubation management in the adult intensive care unit", section on 'Assess airway protection'.)

Role of diaphragmatic ultrasonography — Data suggest that diaphragmatic ultrasonography may predict weaning success. However, more robust data are needed before ultrasonography can be used routinely to predict successful weaning. As an example, a meta-analysis of 19 studies demonstrated that diaphragmatic excursion (sensitivity 80 percent, specificity 80 percent) and diaphragmatic thickening fraction (sensitivity 85 percent, specificity 75 percent) can predict successful weaning [35].

Diaphragmatic excursion is dependent upon diaphragmatic contraction and inspired volume. The latter is influenced by the degree of ventilatory support. Thus, measurements of diaphragmatic excursion are best taken during spontaneous unsupported breathing. One prospective study of 50 patients studied during spontaneous breathing (T-tube) reported that the diaphragmatic excursion cut-off value predictive of weaning was 10 mm with a specificity of 78 percent and sensitivity of 82 percent [36]. Diaphragmatic thickness cut-off value predictive of weaning was 21.5 percent with a specificity of 71 percent and sensitivity of 76 percent.

FOLLOW-UP

Patients who are ready to wean — For patients who are deemed ready to wean, we perform a weaning trial, which predicts the patient's potential for spontaneous breathing following extubation. Details regarding methods of weaning are provided separately. (See "Initial weaning strategy in mechanically ventilated adults".)

Readiness testing is imperfect, and some patients deemed ready to wean fail a subsequent spontaneous breathing trial (SBT). Failed weaning has not been shown to be harmful if it is well monitored and the patient is returned to full ventilatory support at the first sign of intolerance (ie, ventilatory fatigue should be avoided). This justifies repeated daily readiness assessments and SBTs in those deemed ready to wean, provided the reason for weaning failure is investigated and treated (eg, excess sedation, development of myocardial ischemia, neuromuscular weakness, electrolyte disturbances).

Patients not ready to wean — For patients who do not meet readiness criteria, we continue to treat the underlying disorder or complications of mechanical ventilation until readiness criteria can be eventually met. In many cases, if the patient improves, they can undergo a weaning trial when ready. If they do not improve, then we assess the patients for long term mechanical ventilation with a tracheostomy. Management of the difficult to wean patient is discussed separately. (See "Management of the difficult-to-wean adult patient in the intensive care unit".)

Patients with uncertainty — For patients in whom uncertainty exists as to whether the readiness criteria will predict a successful weaning trial, we sometimes use a weaning predictor to identify potential candidates suitable for weaning or to confirm lack of readiness to wean (eg, patients with borderline readiness criteria or suspected respiratory muscle weakness). Use of weaning predictors is most pertinent among patients in whom the risk associated with a failed spontaneous trial is significantly elevated (eg, patients with prolonged mechanical ventilation, patients with critical care neuromyopathy).

Among the predictors, the rapid shallow breathing index (RSBI) is our preferred weaning predictor because it is well studied, easy to measure, and no alternative predictor has been shown to be superior. (See 'Rapid shallow breathing index' below.)

●For patients who have an RSBI <105 breaths/minute/L (measured without ventilatory support), we initiate a weaning trial.

●For patients who have an RSBI ≥105 breaths/minute/L (measured without ventilatory support), we maintain full ventilatory support.

For patients in whom neuromuscular weakness is suspected, we also sometimes measure the maximal inspiratory pressure and/or diaphragmatic ultrasound at the bedside to confirm our suspicion. (See 'Predictors not routinely used' below.)

Weaning predictors — Some clinicians use physiological tests (weaning predictors) in addition to clinical criteria to predict whether a patient is likely to tolerate weaning, while others, including us, only use them when doubt exists over a patient's readiness to wean. Among the predictors, the RSBI is the most widely used and extensively studied (table 3). All other tests generally have poor predictive capacity or require complex testing.

Rapid shallow breathing index — The RSBI is the ratio of respiratory frequency to tidal volume (f/VT).

Measurement — We measure the f and VT using a hand-held spirometer attached to the endotracheal tube while a patient is breathing room air for one minute without any ventilator assistance [37]. The spirometer measures the total volume inspired and expired in one minute (ie, minute ventilation) while the operator counts the actual respiratory rate. The VT can be calculated by dividing the minute ventilation by the f. The f and VT can then be used to calculate the RSBI.

However, if a spirometer is not available or the patient cannot breathe room air, the RSBI may be calculated using the ventilator [38-41]. We set the ventilator to a pressure support level of 0 cm H₂O and a positive end-expiratory pressure (PEEP) of 0 cm H₂O, without flow or pressure trigger for one minute [40,41]. The tidal volume can then be determined by the ventilator. However, the respiratory rate should be manually counted since the ventilator may underestimate the respiratory rate if the patient makes inspiratory efforts that are not sensed by the ventilator. Such unmeasured inspiratory efforts falsely lower the RSBI, particularly in patients who have chronic obstructive lung disease with dynamic hyperinflation [42].

Interpretation — An RSBI ≥105 breaths/minute/L (ie, a negative RSBI) indicates that a patient is likely to fail weaning while a positive test RSBI <105 breaths/minute/L is more likely to undergo successful weaning. However, many experts adopt an individual approach to interpreting the threshold value and allow for factors that may falsely alter it. For example, several factors have been shown to increase the RSBI, including a narrow endotracheal tube (eg, ≤7 cm), female gender, sepsis, fever, supine position, anxiety, suctioning, and chronic restrictive lung disease [43,44]. Thus, interpretation may need to be adjusted individually under these circumstances.

Evidence suggests that a negative RSBI (RSBI ≥105 breaths/minute/L) is better at identifying patients who will fail weaning than a positive RSBI (RSBI <105 breaths/minute/L) is at identifying patients who can be successfully weaned [37,45,46]. RSBI was originally described in a prospective cohort study that evaluated 64 mechanically ventilated patients [37]. An RSBI ≥105 breaths/minute/L was associated with weaning failure, while an RSBI <105 breaths/minute/L predicted weaning success with a sensitivity, specificity, positive predictive value, and negative predictive value of 97, 64, 78, and 95 percent, respectively [37]. The pretest probability of weaning success in the study population was approximately 60 percent. When these data were used to calculate likelihood ratios, the LR+ was 2.7 and the LR- was 0.05 (table 4). This indicates that there is only a small increase in the probability of weaning success among patients with a positive RSBI (<105 breaths/minute/L). In contrast, there was a large increase in the probability of weaning failure among patients with a negative RSBI (≥105 breaths/minute/L). These findings were supported by a systematic review of 20 RSBI studies [45] and a Bayesian analysis of the same studies [46].

Clinical outcomes — Routine use of the RSBI has not been shown to decrease duration of weaning or mechanical ventilation. As examples:

●In one trial, 304 mechanically ventilated patients had their RSBI measured daily and all of the patients underwent daily screening [26]. Patients randomized to RSBI-dependent weaning underwent an SBT if they passed all components of the screening and had an RSBI <105 breaths/minute/L. In contrast, patients randomized to RSBI-independent weaning underwent an SBT if they passed all components of the screening, regardless of their RSBI. The group that underwent RSBI-dependent weaning took one day longer to discontinue mechanical ventilation. There was no difference in the total duration of mechanical ventilation, length of stay, or reintubation rate.

●In a meta-analysis that pooled data from 48 studies, sensitivity for RSBI in predicting successful extubation was 83 percent, while specificity was low (58 percent) [47]. Results were consistent among several subgroups. The corresponding likelihood ratios (LR+ of 2.0, LR- of 0.3) translate to only a small increase in the probability of weaning success with a positive RSBI and a moderate probability of weaning failure with a negative RSBI.

Predictors not routinely used — Several other weaning predictors have been described but have poor predictive capacity, are investigational, have poorly defined thresholds, or require complex maneuvers for calculation. These predictors are listed in the table (table 3).

●Measurements of oxygenation and gas exchange – Although adequate oxygenation is an essential clinical criterion to consider when deciding whether a patient is ready to wean (see 'Clinical criteria' above), it is a poor weaning predictor when used alone to predict weaning outcome. These include the following:

•The ratio of arterial oxygen tension to fraction of inspired oxygen (PaO₂/FiO₂)

•The ratio of arterial oxygen tension to alveolar oxygen tension (PaO₂/PAO₂)

•The alveolar-arterial (A-a) oxygen gradient

•Measures of dead space

●Measurements of load on the respiratory system and respiratory capacity (some require special equipment) ─ None of the following parameters have been shown to consistently predict successful weaning or failure to wean:

•Minute ventilation [45]

•Respiratory system compliance [37,48]

•Work of breathing [49-53]

•Oxygen cost of breathing [54-56]

•Occlusion pressure (P0.1) [57-59] and (P0.1/maximum inspiratory pressure ratio) [60,61]

•Maximal inspiratory pressure

•Gastric mucosal acidosis [62,63]

•Diaphragmatic ultrasound [64-68]

•Esophageal pressure [45]

•Diaphragmatic pressure during a spontaneous tidal breath [45]

•Tension-time index [45]

●Multicomponent integrative indices – RSBI is the best example of an integrated index. While several of the more complicated integrated indices looked promising, none have been confirmed as having the high level of accuracy that was initially reported. These include:

•Inspiratory effort quotient (IEQ) [69]

•The CROP index (Compliance, Rate, Oxygenation, Pressure) [37]

•The CORE index (Compliance, Oxygenation, Respiration, Effort) [70]

•Weaning Index (WI) [71]

•Integrative weaning index (IWI) [48]

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Weaning from mechanical ventilation".)

SUMMARY AND RECOMMENDATIONS

●Discontinuing mechanical ventilation is a three-step process that consists of readiness testing, weaning, and extubation. During readiness testing, objective clinical criteria are used to determine whether a patient is ready to begin weaning. Weaning is the process of decreasing the degree of ventilator support and allowing the patient to assume a greater proportion of their own ventilation. Extubation is the removal of the endotracheal tube (ETT). (See 'Introduction' above and 'Definitions' above.)

●The purpose of readiness testing is to identify patients who are ready to wean, since clinicians tend to underestimate the capacity of patients to breathe independently. Early identification of patients who may be extubated avoids risk of death and complications related to mechanical ventilation (eg, pulmonary barotrauma, ventilator-associated lung injury). Readiness testing is also intended to identify patients who are not ready for weaning, thereby protecting them against the potential risks of premature weaning (eg, cardiovascular dysfunction, respiratory muscle fatigue, psychological distress). (See 'Goals' above.)

●For patients who are mechanically ventilated for more than 24 hours, we perform daily ventilator liberation protocol assessment rather than non-protocolized assessment. However, wide variation exists in practice. (See 'Daily screening liberation assessment' above.)

●Reasonable objective clinical criteria used to predict readiness for weaning are listed in the table (table 1). (See 'Clinical criteria' above.)

●Based on the assessment, we suggest the following approach (see 'Follow-up' above):

•For patients who are deemed ready to wean, we suggest a weaning trial (Grade 2C). (See 'Patients who are ready to wean' above.)

•A weaning trial is not typically performed in patients who do not meet readiness criteria. Daily readiness assessment should be continued. (See 'Patients not ready to wean' above.)

•For patients in whom uncertainty exists, we sometimes use a weaning predictor to identify potential candidates suitable for weaning or to confirm lack of readiness to wean. Among the predictors (table 3), the rapid shallow breathing index (RSBI) is our preferred weaning indicator because it is well studied, easy to measure, and no alternative weaning predictor has been shown to be superior. However, there is no evidence that RSBI-dependent weaning improves clinical outcomes, such as duration of weaning, duration of mechanical ventilation, length of stay, or reintubation rate. (See 'Patients with uncertainty' above and 'Weaning predictors' above.)

-For patients who have an RSBI <105 breaths/minute/L (measured without ventilatory support), we initiate a weaning trial.

-For patients who have an RSBI ≥105 breaths/minute/L, we maintain full ventilatory support.

•Several other weaning predictors have been described but have poor predictive capacity, are investigational, have poorly defined thresholds, or require complex maneuvers for calculation (table 3).