“Weaning” refers to the transition from full mechanical ventilatory support to spontaneous ventilation with minimal support. “Liberation” refers to discontinuation of mechanical ventilation. This chapter focuses on the clinical assessment of readiness to wean, the technique for conducting a spontaneous breathing trial (SBT), and the assessment of readiness of extubation. In addition, we will review the evidence supporting various ventilator strategies in the difficult-to-wean patient.
Mechanically ventilated intensive care patients may be classified as simple to wean, difficult to wean, or prolonged weaning. Simple-to-wean patients are extubated on the first attempt, make up the vast majority of the patients in the intensive care unit (ICU; ∼69%), and have a low mortality rate (∼5%). The remaining cohort of difficult-to-wean (requiring up to three attempts or up to 7 days from the onset of weaning) or prolonged-wean (over three attempts or greater than 7 days from the onset of weaning) patients require greater effort to successfully liberate from mechanical ventilation. These difficult-to-wean and prolonged-wean patients have an associated higher mortality rate (∼25%).
Prolonged mechanical ventilation is associated with increased mortality and costs (mechanical ventilation costs > U.S. $2000/day). It has been estimated that the 6% of patients who require prolonged mechanical ventilation consume 37% of ICU resources, and 40% to 50% of the time spent undergoing mechanical ventilation occurs after this weaning process has started. In part, the reason is that more severely ill patients usually require longer periods of mechanical ventilation. Prolonged weaning may result, though, from an excessive use of sedatives, the absence of weaning-liberation protocols, and myriad of organizational and cultural factors that fail to optimize weaning conditions. In general, the duration of mechanical ventilation should be minimized, and liberation from mechanical ventilation should be considered as soon as possible.
Expert consensus has proposed that the weaning process be considered in the following six steps:
- 1.
Treatment of acute respiratory failure
- 2.
Clinical judgment that weaning may be possible
- 3.
Assessment of the readiness to wean
- 4.
An SBT
- 5.
Extubation
- 6.
Possibly re-intubation
Depending on the mechanism of acute respiratory failure—whether it is a problem of oxygenation, ventilation, or airway (or a combination)—most critically ill patients require a period in which they will require full ventilatory support after intubation. Consideration of the weaning process should begin very soon after intubation. Weaning involves several discrete logical and sequential steps. If patients fail to make sufficient progress, then a contingency plan is required. Failure to wean/liberate involves either (1) the failure of an SBT or (2) the need for re-intubation/ventilation or death within 48 hours of extubation.
Clinical Suspicion that Weaning May Be Possible
Because of the significant morbidity and mortality associated with prolonged mechanical ventilation, it is generally accepted that all ventilated ICU patients should be assessed for their readiness to wean at least on a daily basis. The importance of this “readiness” assessment has been highlighted by several trials that have demonstrated that weaning can be achieved in most patients after the first formal assessment of readiness and the finding that nearly 50% of unexpected self-extubations during the weaning process did not require re-intubation. The benefit of early weaning should be balanced against the significant morbidity and mortality associated with failed extubation. Two large prospective observational studies found a fivefold to tenfold increased mortality in patients requiring re-intubation. It is unclear, though, how much of this effect is confounded by population and disease severity differences.
Assessment of Readiness to Wean
The clinical assessment of readiness to wean is a two-step process that is based on (1) the assessment of predictors of weaning and (2) the successful completion of an SBT. Both of these steps require a reliable and reproducible institutional sedation strategy that maximizes the patient’s capability of being assessed and undergoing SBTs. Ventilator liberation protocols should be developed, locally, in concert with analgesia protocols. The concept of nocturnal rest, in conjunction with daytime respiratory muscle training, is an important one for those patients whose weaning is more difficult and prolonged.
Predictors of Successful Weaning
The initial screening evaluation of readiness to wean is composed of a clinical examination and an assessment of several objective criteria (respiratory, cardiovascular, and neurologic) that aim to predict the likelihood of successfully weaning ( Table 9-1 ). Individually, these predictors are neither highly sensitive nor specific, but together with the clinical examination they allow the clinician to identify patients who will clearly not be suitable for weaning and who may have detrimental effects from aggressive reduction in ventilatory support. All other patients should undergo a SBT. This is an important point because (1) many patients who meet some but not all of the criteria for weaning will still successfully wean and (2) clinicians frequently underestimate the ability of patients to wean.
| Clinical assessment | Resolution of acute process requiring Intubation/ventilation Patient awake and cooperative Chest wall pain controlled Adequate cough Absence of excessive tracheobronchial secretions Absence of Nasal flaring Suprasternal and intercostal recession Paradoxical movement of the rib cage or abdomen |
| Objective measures | Respiratory stability: Oxygenation Sa o 2 >90% on F io 2 ≤0.4 Pa o 2 ≥ 50-60 mm Hg on F io 2 ≤0.5 Alveolar-arterial P o 2 gradient <350 mm Hg (F io 2 1.0) Pa o 2 /F io 2 ≥ 150 Respiratory stability: Function Respiratory rate ≤35 breath/min –1 Maximal inspiratory pressure ≤ −20 to −25 cm H 2 O Tidal volume > 5 mL/kg –1 Minute ventilation <10 L/min –1 No significant respiratory acidosis Respiratory rate/tidal volume <105 breath/min –1 /L –1 ∗ CROP index >13 mL/breath/min –1 † Integrative index of Jabour <4 per minute –1 ‡ IWI of ≥25 mL/cm H 2 O breaths/min –1 /L –1 § Cardiovascular stability Heart rate <140 beats/min –1 Systolic blood pressure >90 and <160 mm Hg Minimal inotropic/vasopressor support Neurologic function Including normal mentation on sedation |
∗ The respiratory rate/tidal volume ratio is also known as the RSBI.
† CROP index = [compliance (dynamic) × maximum inspiratory pressure × (arterial partial pressure of oxygen/alveolar partial pressure of oxygen)]/respiratory rate.
‡ Integrative index of Jabour = pressure time product × (minute ventilation to bring the Pa co 2 to 40 mm Hg/tidal volume during spontaneous breathing).
§ IWI = (compliance (static) × arterial oxygen saturation/(respiratory rate/tidal volume during spontaneous breathing).
Individual Limitations of the Readiness-to-Wean Predictors
It is important to be aware of the individual limitations of these prediction criteria because many have been examined only retrospectively, and of those who have been studied prospectively, many have demonstrated high false-positive and false-negative rates.
A minute ventilation less than 10 L/min is only associated with a positive predictive value of 50% and a negative predictive value of 40%. The maximal inspiratory pressure , a measure of respiratory muscle strength, was initially suggested to be a good indicator of weaning success. These findings were not replicated in subsequent trials.
Static compliance (i.e., tidal volume/plateau pressure–positive end expiratory pressure) has a low positive predictive value (60%) and negative predictive value (53%). Occlusion pressure (P0.1) is the airway pressure 0.1 second after the initiation of a spontaneous breath in a measure of respiratory drive. The results from studies determining the utility of this index have been conflicting to date.
A reduction in central venous saturation of more than 4.5% at the thirtieth minute of an SBT in patients who had failed their first T-tube SBT was an independent predictor of re-intubation, with a sensitivity of 88% and a specificity of 95%. A previous study showed that on discontinuation of the ventilator, mixed venous oxygen saturation fell progressively in the failure group ( P = .01) whereas it did not change in the success group.
A low left ventricular ejection fraction (LVEF) (36% [27 to 55] vs. 51% [43 to 55], P = .04), shortened deceleration time of E wave (DTE), and increased Doppler E velocity to tissue Doppler E’ velocity ratio (E/E′) assessed by transoesophageal echocardiography with an experienced operator were predictive of extubation failure in a prospective observational study. Given the expense and limited availability of expert transthoracic echocardiogram (TTE), though, we recommend that evidence of benefit of TTE-guided interventions should be available before its introduction into routine clinical practice.
B – type natriuretic peptide (BNP) and N – terminal pro – BNP levels either at baseline or the relative change during an SBT have been associated with extubation failure due to heart failure. There was significant heterogeneity, though, between results, which may be explained by the different populations studied, fluid balance, the use of cardioactive drugs, and underlying cardiovascular or renal dysfunction.
The rapid shallow breathing index (RSBI) (respiratory rate/tidal volume) measured over 1 minute in the spontaneously breathing patient has demonstrated a high sensitivity (97%) and a moderate specificity (65%) for predicting patients who will subsequently successfully pass an SBT compared with the other predictors. The measurement of RSBI value may be affected by the airway pressure protocol. In prospective studies, RSBI values were significantly lower in patients while they were on a continuous positive airway pressure (CPAP) of 5 cm H 2 O compared with T-piece (median 71 vs. 90 breaths/L/min) or a spontaneously breathing room air trial without ventilator support (median 36 vs. 71 breaths/L/min).
The trend rather than an individual value of RSBI may be a better predictor of weaning success. RSBI remained unchanged or decreased in successful extubation; in contrast, RSBI tended to increase in those who failed extubation in three prospective observational studies. Although many clinicians use RSBI in their clinical practice, there is some controversy to its utility: one small randomized controlled trial (RCT) reported that the use of this measure prolonged weaning time and did not reduce the incidence of extubation failure or tracheostomy. This trial was small, though, and there was a high likelihood of selection bias and crossover in the non-RSBI utilization arm. Results from another RCT suggested that the predictive value of RSBI may be increased using automatic tube compensation (ATC).
Overall, individual “predictor” criteria should not be considered as reliable indicators to predict successful weaning. When combined with the clinical examination, though, they may assist the clinician to identify patients who will clearly not be suitable for weaning and who may have detrimental effects from an unnecessary SBT.
The failure of these individual indices to predict successful weaning prompted the authors to combine several individual indices in an attempt to increase specificity and sensitivity. However, these predictors ( Table 9-1 ) are more complex and are more commonly used in clinical trials than in routine clinical practice.
A Compliance, Respiratory rate, arterial Oxygenation and maximal inspiratory Pressure (CROP) index (see Table 9-1 ) more than 13 mL/breath/min has prospectively determined a positive predictive value of 71% and a negative predictive value of 70% to predict weaning success. A Jabour pressure time product (see Table 9-1 ) less than 4 per minute has been shown in a retrospective study to have a positive predictive value of 96% and a negative predictive value of 95%.
An integrative weaning index (IWI) (see Table 9-1 ) of 25 mL/cm H 2 O breaths/min/L or more has been shown in a prospective study to have a positive predictive value of 0.99 and a negative predictive value of 0.86 to predict weaning success ( n = 216 in the prospective-validation group). Future research is required to identify simple predictors that are sufficiently sensitive and specific to predict successful weaning. In the absence of such measures, the clinician should have a low threshold for conducting a daily SBT.
Spontaneous Breathing Trial
The initiation of the weaning process is defined as the commencement of the first SBT. There are several techniques that can be used to conduct an SBT. These include techniques such as (1) T-tube/T-piece, (2) pressure support ventilation (PSV), or (3) ATC, all of which may be used with or without CPAP. Failure of an SBT is defined as the development of respiratory (function or oxygenation), cardiovascular, or neurological instability and is determined by clinical assessment and objective testing during the trial ( Table 9-2 ). ∗
∗ References .
There appears to be little predictive advantage by increasing the duration of the SBT assessment to greater than 20 to 30 minutes. Prospective studies have demonstrated that most patients successfully pass their first SBTs and more than 60% of patients successfully wean †† References .
( Table 9-3 ). Interestingly, to date, trials have not demonstrated that any one of these techniques is superior in its ability to predict weaning success ( Table 9-3 ). Clinicians still need to be aware of the relative advantages and disadvantages of each technique, though.| Clinical assessment | Agitation and anxiety Reduced level of consciousness Significant sweating Cyanosis Evidence of increased respiratory muscle effort Increased accessory muscle usage Facial signs of distress Dyspnea |
| Objective measures | Respiratory stability: Oxygenation Pa o 2 ≤50-60 mm Hg on F io 2 ≥0.5 or Sa o 2 <90% Respiratory stability: Function Pa co 2 >50 mm Hg or an increase in Pa co 2 >8 mm Hg pH < 7.32 or a decrease of pH ≥ 0.07 pH units Respiratory rate/tidal volume >105 breath/min –1 /L –1 ∗ Respiratory rate >35 breath/min –1 or increase ≥50% Cardiovascular stability Heart rate >140 beats/min –1 (or increase ≥20%) Systolic blood pressure >180 mm Hg (or increase ≥20%) Systolic blood pressure <90 mm Hg Significant cardiac arrhythmias Neurologic function Reduced level of consciousness |
∗ The respiratory rate/tidal volume is also known as the RSBI.
| Author | Year | Number | Passed Initial SBT | Extubated at 48 hr (From All Extubated) | Method |
|---|---|---|---|---|---|
| Trials Describing Success Rate of Initial SBT and Extubation | |||||
| Brochard | 1994 | 456 | 347 (76%) | 330(95%) | T-piece |
| Esteban | 1995 | 546 | 416 (76%) | 358 (86%) | T-piece |
| Vallverdu | 1998 | 217 | 148 (68%) | 125 (84%) | T-piece |
| Esteban | 1999 | 526 | 416 (79%) | 346(82%) | T-piece |
| Trials Describing Success Rate of Initial SBT and Extubation with Differing Techniques | |||||
| Esteban | 1997 | 484 | 397(82%) | 323(81%) | PSV/T-piece |
| Subgroup | 236 | 205(86%) | 167 (81%) | PSV 7 cm H 2 O | |
| Subgroup | 246 | 192(78%) | 156 (81%) | T-piece | |
| Farias | 2001 | 257 | 201 (78%) | 173(86%) | PSV/T-piece |
| Subgroup | 125 | 99 (79%) | 79 (80%) | PSV 10 cm H 2 O | |
| Subgroup | 132 | 102(77%) | 89 (87%) | T-piece | |
| Haberthur ∗ | 2002 | 90 | 78 (87%) | 62 (79%) | ATC/PSV/T-piece |
| Subgroup | 30 | 29(96%) | 25 (86%) | ATC | |
| Subgroup | 30 | 23(77%) | 18 (78%) | PSV 5 cm H 2 O | |
| Subgroup | 30 | 24(80%) | 19 (79%) | T-piece | |
| Matić | 2004 | 260 | 200 (77%) | Not specified | PSV/T-piece |
| Subgroup | 110 | 80 (73%) | Not specified | T-piece | |
| Subgroup | 150 | 120 (80%) | Not specified | PSV | |
| Cohen | 2006 | 99 | 90 (91%) | 73 (74%) | ATC/CPAP |
| Subgroup | 51 | 49 (96%) | 42 (82%) | ATC | |
| Subgroup | 48 | 41 (85%) | 31 (65%) | CPAP | |
| Cohen | 2009 | 190 | 161(85%) | 139(86%) | ATC/PSV |
| Subgroup | 87 | 81(93%) | 71(88%) | ATC | |
| Subgroup | 93 | 80(86%) | 68(85%) | PSV | |
| Figueroa-Casas | 2010 | 118 | 108 (92%) | 115 (97%) | ATC/CPAP |
| Subgroup | 58 | 56 (97%) | 57 (99%) | ATC | |
| Subgroup | 60 | 52 (87%) | 58 (97%) | CPAP | |
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