Evidence Report/Technology Assessment: Number 23
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Under its Evidence-based Practice Program, the Agency for Healthcare Research and Quality (AHRQ) is developing scientific information for other agencies and organizations on which to base clinical guidelines, performance measures, and other quality improvement tools. Contractor institutions review all relevant scientific literature on assigned clinical care topics and produce evidence reports and technology assessments, conduct research on methodologies and the effectiveness of their implementation, and participate in technical assistance activities.
Overview / Reporting the Evidence / Methodology / Findings / Future Research / Availability of Full Report
Mechanical ventilation refers to the use of life-support technology to perform the work of breathing for patients who are unable to do so on their own, and the majority of critically ill patients in most modern intensive care units (ICUs) require a period of this treatment. The use of prolonged mechanical ventilation is associated with nosocomial pneumonia, cardiac-associated morbidity, and death.
Discontinuing mechanical ventilation prematurely, however, may result in reintubation, which is associated with similar complications to prolonged ventilation. Thus, optimal weaning—minimizing the duration of mechanical ventilation without incurring substantial risk of reintubation, and thus preventing important complications—plays a crucial role in the management of critically ill patients.
Due to the costs and morbidity and mortality rates associated with the use of life-support technology, the Agency for Healthcare Research and Quality (in 1997) charged the McMaster University Evidence-based Practice Center (EPC) with producing an evidence report on issues related to weaning patients from mechanical ventilation.
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Reporting the Evidence
The following key questions defined the parameters of the investigation:
- When should weaning be initiated?
- What criteria should be used to initiate the weaning process?
- What are the most effective methods of weaning from mechanical ventilation?
- What are the optimal roles of non-physician health care professionals in facilitating safe and expeditious weaning?
- What is the value of clinical practice algorithms and computers in expediting weaning?
To address these questions, the EPC team of investigators retrieved all relevant randomized controlled trials (RCTs) and clinical observational studies on weaning from mechanical ventilation. They were interested in:
- Patients who had received mechanical ventilation.
- Any strategies designed to facilitate weaning and extubation.
- Predictors of weaning and extubation in all critically ill patients.
- Predictors of the duration of weaning in patients with Chronic Obstructive Pulmonary Disease (COPD) or patients following cardiac surgery.
In addition, the investigators collected and analyzed study data from the following areas:
Populations. The investigators included all studies of adult and pediatric patients who were mechanically ventilated and had either an endotracheal tube or tracheostomy tube. They excluded studies of highly specific populations and studies in neonates.
Settings. They included studies conducted in ICUs, intermediate care units, step-down units, and post-anesthetic recovery rooms. The team excluded studies of home ventilation for children or adults, and chronic ventilation settings.
Interventions. The investigators included any ventilation or weaning strategy (e.g., mode, method, procedure, protocol, timing, operator, computer, tracheostomy, noninvasive ventilation modes, adjunctive holistic aids, and other miscellaneous approaches) geared to facilitate weaning and/or extubation. They excluded mechanical ventilation methods and interventions whose influence on the duration of ventilation had already been summarized in a recent systematic review (e.g., sedation in the ICU, and optimal timing of tracheotomy).
Predictors. The team included predictors of weaning and/or extubation success, and predictors of duration of mechanical ventilation in cardiac surgery and COPD patients. They excluded predictors of self-extubation.
Outcomes. They included all clinical outcomes and excluded studies that reported exclusively physiologic outcomes. The team included studies reporting the endpoint of patients' experiences and nurses' assessments of patients' experiences to address question No. 4 (regarding the important role of nurses in optimizing the weaning process).
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To identify relevant studies, the investigators searched MEDLINE®, EMBASE, HEALTHStar, CINAHL, the Cochrane Controlled Trials Registry, and the Cochrane Data Base of Systematic Reviews from 1971 to 1998. They also examined reference lists and personal files, and hand searched Respiratory Care. However, they did not explicitly search for unpublished literature. In addition, they retrieved all articles that either of two reviewers of the titles and abstracts considered possibly eligible. The same two reviewers examined the full text and made final decisions regarding eligibility based on the above inclusion and exclusion criteria.
Five respiratory therapists and five intensivists participated in data abstraction and rating the methodologic quality of all eligible RCTs and non-randomized controlled cohort studies. All of the studies addressed treatment issues and provided quantitative data concerning predictors of weaning and extubation. Two reviewers abstracted the data and assessed the methodologic quality of each study.
Methodologic features of the randomized trials that were abstracted by the reviewers included:
- The method of randomization and whether randomization was concealed.
- The extent to which groups were similar with respect to important prognostic factors.
- Whether investigators conducted an intention-to-treat analysis.
- Whether patients, clinicians, and those assessing outcomes were blind to allocation.
- The extent to which the groups received similar co-interventions.
- Reporting of the reasons for study withdrawal.
For non-randomized controlled clinical trials, they considered the extent to which groups were similar with respect to important prognostic factors, whether the investigators adjusted for differences in prognostic factors, and the extent to which groups received similar co-interventions.
For studies addressing predictors of weaning success, the team considered:
- Whether investigators enrolled a representative sample of patients.
- Whether those making weaning decisions or assessing outcomes were blind to predictor variables.
For qualitative studies, the team considered whether participants were relevant to the research question. If the selection of participants was well reasoned, the team then examined:
- Whether the data collection methods were appropriate for the research objectives.
- Whether the data collection was comprehensive enough to support rich and robust descriptions of the observed events.
- Whether the data were appropriately analyzed and the findings adequately corroborated.
To synthesize the data from RCTs comparing weaning interventions, the team abstracted or when necessary calculated effect sizes in terms of relative risks and associated 95 percent confidence intervals for binary outcomes and mean differences, and 95 percent confidence intervals for continuous variables.
Furthermore, they reviewed the interventions and outcomes and decided when it was or was not legitimate to pool across studies. When pooling was not appropriate, the team divided studies into categories according to similarity of interventions. All of the pooled analyses were based on a random effects model that included differences between studies in calculating the variance estimate. This provided the strategy for final estimates of all treatment effects.
For nonrandomized controlled studies that compared alternative weaning interventions, the investigators used a similar methodology for calculating point estimates and confidence intervals for individual studies, but made no attempt to pool data across studies.
For observational studies addressing prediction of successful weaning and duration of ventilation, the team categorized studies according to the outcome of interest. For each predictor in every study in which the data were available, they constructed a 2x2 table examining the presence or absence of the predictor in relation to the success or failure of the weaning process. This allowed calculation of the sensitivity and specificity of the tests and their associated 95 percent confidence intervals and odds ratios, as well as their 95 percent confidence intervals and the associated likelihood ratios. They then organized the observational studies according to predictors of interest.
The team defined predictors as relevant if they showed potential for differentiating success from failure. They retained all predictors for which results were presented in 2x2 table if there was an associated likelihood ratio (LR) of >2 or <0.5. When results were presented as means and standard deviations of the success and failure groups, they included predictors if the difference in means between the two groups was greater than one half of the smaller of the standard deviations of the two groups. Where appropriate, the team pooled the observational data to narrow the 95 percent confidence intervals.
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After conducting a thorough investigation of all relevant studies, clinical trials, and scientific literature, the team was able to report the following findings:
- The issue concerning the optimal start of weaning is confounded by alternative definitions of weaning: one reasonable conceptualization is weaning beginning with the onset of mechanical ventilation. Research to date suggests that the best answer to "when to start weaning" is to develop a protocol implemented by nurses and respiratory therapists that begins testing for the opportunity to reduce support very soon after intubation and reduces support at every opportunity.
- Differences in clinicians' intuitive threshold for reduction or discontinuation of ventilatory support have a greater impact on failure of spontaneous breathing trials or on reintubation than do modes of weaning. When clinicians set a high threshold, many patients who could tolerate weaning remain on mechanical support longer than necessary.
- For step-wise reductions in mechanical support, pressure support mode or multiple daily T-piece trials may be superior to intermittent mandatory ventilation.
- For trials of unassisted breathing, low levels of pressure support may be beneficial.
- There may be substantial benefits to early extubation and institution of noninvasive positive pressure ventilation for patients who are alert, cooperative, and ready to breathe without an artificial airway.
- Following cardiac surgery, early extubation is unequivocally achieved with a variety of anaesthetic interventions and ICU protocols; however, the corresponding reduction in ICU stay is generally small and the impact on complications, though rare, remains unclear.
- While steroids can reduce postextubation stridor in children, their impact on reintubation in children and adults remains uncertain.
- Most theoretically plausible predictors of weaning and extubation success have no predictive power. Those with some predictive power include the rapid shallow breathing index which has been intensely studied, P0.1/MIP, and the CROP index. However, these are relatively weak predictors of weaning success.
Tests are rarely useful in increasing the probability of weaning success; however, on occasion, they can lead to moderate reductions in the probability of success. In general, weaning predictors were probably found to perform poorly because physicians had already considered the results when they selected their patients for study.
- The role of computerized weaning protocols has not been established.
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For future research on mechanical ventilation, the team recommends the following:
- The examination of alternative weaning strategies should enroll homogeneous patient groups: those whose likely period of additional ventilation is a few hours, for example, and those whose likely period is a few days. Patients after cardiac surgery constitute another population that should be considered separately.
- In the setting of a high threshold for extubation associated with low failure rates, investigators would require trials of thousands of patients to demonstrate differences between techniques; and tens of thousands to demonstrate differences in complications of failed extubation. Investigators should establish plausible event rates before embarking on clinical trials.
- Investigators should attempt to elucidate the trade-off between decreasing duration of time on a ventilator and the increase in reintubation rates associated with a low-weaning threshold (e.g., what reduction in duration of time on a ventilator would warrant an increase in reintubation rates from 5 percent to 10 percent?). This work should involve attention to the important consequences of prolonged ventilation or reintubation, including nosocomial pneumonia, cardiac morbidity, and death.
- Investigators should launch trials examining the use of noninvasive positive pressure ventilation (NPPV) in reducing the duration of intubation and total mechanical support. Future research should also explore the optimal timing and management of NPPV for weaning purposes, its effect on morbidity (e.g., pneumonia), length of ICU stay, and mortality.
- Investigators should launch additional randomized trials of weaning protocols implemented by respiratory therapists and nurses. These trials should evaluate the differential impact of protocols in different types of patients and in ICUs with different organizational structures (e.g., open versus closed units, and teaching versus community hospitals). The influence of different protocols and their impact on ICU and hospital length of stay and costs are important future considerations.
- A more fruitful line of investigation, rather than further research seeking powerful predictors of successful weaning or extubation, might be randomized trials of weaning protocols that decrease the duration of mechanical ventilation without substantially increasing rates of failed extubation.
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Availability of Full Report
The full evidence report from which this summary was derived was prepared for the Agency for Healthcare Research and Quality by the McMaster University Evidence-based Practice Center under contract No. 290-97-0017. Printed copies may be obtained free of charge from the AHRQ Publications Clearinghouse by calling 800-358-9295. Requestors should ask for Evidence Report/Technology Assessment No. 23, Criteria for Weaning From Mechanical Ventilation (AHRQ Publication No. 00-E029).
The Evidence Report is also online on the National Library of Medicine Bookshelf.
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AHRQ Publication No. 00-E028
Current as of June 2000