Synchronized intermittent mandatory ventilation with and without pressure support ventilation in weaning patients with COPD from mechanical ventilation

“The role of a negative pressure ventilator coupled with oxygen helmet against COVID-19: a review”

Background

The coronavirus (SARS-COV-2) pandemic has provoked the global healthcare industry by potentially affecting more than 20 14 million people across the globe, causing lasting damage to the lungs, notably pneumonia, ARDS (acute respiratory distress 15 syndrome), and sepsis with the rapid spread of infection. To aid the functioning of the lungs and to maintain the blood oxygen 16 saturation (SpO₂) in coronavirus patients, ventilator assistance is required.

Materials and methods

The main purpose of this article is to outline the need 17 for the introduction of a non-invasive negative pressure ventilator (NINPV) as a promising alternative to positive pressure 18 ventilator (PPV) by elucidating the cons of non-invasive ventilators in clinical conditions like ARDS. Another motive is to 19 profoundly diminish the rate of infection spread by the employment of oxygen helmets, instead of endotracheal intubation in 20 invasive positive pressure ventilator (IPPV) or non-invasive positive pressure ventilator (NIPPV) like face masks and high-flow 21 nasal cannula (HFNC).

Result and conclusion

The integration of oxygen helmet with NPV would result in a number of notable facets including the 22 degree of comfort delivered to patients who are exposed to various ventilator-induced lung injuries (VILI) in the forms of 23 atelectasis, barotrauma, etc. Likewise, preventing the aerosol-generating procedures (AGP) diminishes the rate of nosocomial 24 infections and providing a better environment to both the patients and the healthcare professionals.

Controversies in Weaning from Mechanical Ventilation

Invasive mechanical ventilation can successfully support the patient with acute respiratory failure, but it is associated with considerable risks. Numerous complications of invasive mechanical ventilation have been identified, and these may contribute to increased mortality. Therefore after clinical improvement has occurred, considerable emphasis is placed on expeditiously freeing the patient from the ventilator. This process of getting a patient off mechanical ventilation has been variably termed weaning, liberation, or discontinuation (terms which may be used interchangeably), and can be further divided into “readiness testing” and “progressive withdrawal.” Over the last decade, new developments in our understanding of the process of weaning have provided investigators with the tools to address a number of key questions: How should readiness for weaning (and trials of spontaneous breathing) be determined? What is the role of weaning parameters in deciding when to initiate the weaning process? What is the best mode for conducting a spontaneous breathing trial and how should the patient be monitored? What are the mechanisms for weaning (and spontaneous breathing trial) failure? What is the best technique to facilitate progressive withdrawal? What other factors can facilitate liberation from mechanical ventilation? What are the risks of extubation failure and how can extubation outcome best be predicted? What is the role for protocols in facilitating weaning from mechanical ventilation?.

Automated weaning and SBT systems versus non-automated weaning strategies for weaning time in invasively ventilated critically ill adults

BACKGROUND

Automated systems use closed-loop control to enable ventilators to perform basic and advanced functions while supporting respiration. SmartCare™ is a unique automated weaning system that measures selected respiratory variables, adapts ventilator output to individual patient needs by operationalizing predetermined algorithms and automatically conducts spontaneous breathing trials (SBTs) when predetermined thresholds are met.

OBJECTIVES

The primary objective of this review was to compare weaning time (time from randomization to extubation as defined by study authors) between invasively ventilated critically ill adults weaned by automated weaning and SBT systems versus non-automated weaning strategies.As secondary objectives, we ascertained differences between effects of alternative weaning strategies on clinical outcomes (time to successful extubation, time to first SBT and first successful SBT, mortality, ventilator-associated pneumonia, total duration of ventilation, lengths of intensive care unit (ICU) and hospital stay, use of non-invasive ventilation (NIV), adverse events and clinician acceptance).The third objective of our review was to use subgroup analyses to explore variations in weaning time, length of ICU stay, mortality, ventilator-associated pneumonia, use of NIV and reintubation according to (1) the type of clinician primarily involved in implementing the automated weaning and SBT strategy, (2) the ICU (as a reflection of the population involved) and (3) the non-automated (control) weaning strategy utilized.We conducted a sensitivity analysis to evaluate variations in weaning time based on (4) the methodological quality (low or unclear versus high risk of bias) of the included studies.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) 2013, Issue 5; MEDLINE (1966 to 31 May 2013); EMBASE (1988 to 31 May 2013); the Cumulative Index to Nursing and Allied Health Literature (CINAHL) (1982 to 31 May 2013), Evidence-Based Medicine Reviews and Ovid HealthSTAR (1999 to 31 May 2013), as well as conference proceedings and trial registration websites; we also contacted study authors and content experts to identify potentially eligible trials.

SELECTION CRITERIA

Randomized and quasi-randomized trials comparing automated weaning and SBT systems versus non-automated weaning strategies in intubated adults.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and abstracted data according to prespecified criteria. Sensitivity and subgroup analyses were planned to assess the impact on selected outcomes of the following: (1) the type of clinician primarily involved in implementing automated weaning and SBT systems, (2) the ICU (as a reflection of the population involved) and (3) the non-automated (control) weaning strategy utilized.

MAIN RESULTS

We pooled summary estimates from 10 trials evaluating SmartCare™ involving 654 participants. Overall, eight trials were judged to be at low or unclear risk of bias, and two trials were judged to be at high risk of bias. Compared with non-automated strategies, SmartCare™ decreased weaning time (mean difference (MD) -2.68 days, 95% confidence interval (CI) -3.99 to -1.37; P value < 0.0001, seven trials, 495 participants, moderate-quality evidence), time to successful extubation (MD -0.99 days, 95% CI -1.89 to -0.09; P value 0.03, seven trials, 516 participants, low-quality evidence), length of ICU stay (MD -5.70 days, 95% CI -10.54 to -0.85; P value 0.02, six trials, 499 participants, moderate-quality evidence) and proportions of participants receiving ventilation for longer than seven and 21 days (risk ratio (RR) 0.44, 95% CI 0.23 to 0.85; P value 0.01 and RR 0.39, 95% CI 0.18 to 0.86; P value 0.02). SmartCare™ reduced the total duration of ventilation (MD -1.68 days, 95% CI -3.33 to -0.03; P value 0.05, seven trials, 521 participants, low-quality evidence) and the number of participants receiving ventilation for longer than 14 days (RR 0.61, 95% CI 0.37 to 1.00; P value 0.05); however the estimated effects were imprecise. SmartCare™ had no effect on time to first successful SBT, mortality or adverse events, specifically reintubation. Subgroup analysis suggested that trials with protocolized (versus non-protocolized) control weaning strategies reported significantly shorter ICU stays. Sensitivity analysis excluded two trials with high risk of bias and supported a trend toward significant reductions in weaning time favouring SmartCare™.

AUTHORS' CONCLUSIONS

Compared with non-automated weaning strategies, weaning with SmartCare™ significantly decreased weaning time, time to successful extubation, ICU stay and proportions of patients receiving ventilation for longer than seven days and 21 days. It also showed a favourable trend toward fewer patients receiving ventilation for longer than 14 days; however the estimated effect was imprecise. Summary estimates from our review suggest that these benefits may be achieved without increasing the risk of adverse events, especially reintubation; however, the quality of the evidence ranged from low to moderate, and evidence was derived from 10 small randomized controlled trials.

Noninvasive positive-pressure ventilation as a weaning strategy for intubated adults with respiratory failure

BACKGROUND

Noninvasive positive-pressure ventilation (NPPV) provides ventilatory support without the need for an invasive airway. Interest has emerged in using NPPV to facilitate earlier removal of an endotracheal tube and to decrease complications associated with prolonged intubation.

OBJECTIVES

We evaluated studies in which invasively ventilated adults with respiratory failure of any cause (chronic obstructive pulmonary disease (COPD), non-COPD, postoperative, nonoperative) were weaned by means of early extubation followed by immediate application of NPPV or continued IPPV weaning. The primary objective was to determine whether the noninvasive positive-pressure ventilation (NPPV) strategy reduced all-cause mortality compared with invasive positive-pressure ventilation (IPPV) weaning. Secondary objectives were to ascertain differences between strategies in proportions of weaning failure and ventilator-associated pneumonia (VAP), intensive care unit (ICU) and hospital length of stay (LOS), total duration of mechanical ventilation, duration of mechanical support related to weaning, duration of endotracheal mechanical ventilation (ETMV), frequency of adverse events (related to weaning) and overall quality of life. We planned sensitivity and subgroup analyses to assess (1) the influence on mortality and VAP of excluding quasi-randomized trials, and (2) effects on mortality and weaning failure associated with different causes of respiratory failure (COPD vs. mixed populations).

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 5, 2013), MEDLINE (January 1966 to May 2013), EMBASE (January 1980 to May 2013), proceedings from four conferences, trial registration websites and personal files; we contacted authors to identify trials comparing NPPV versus conventional IPPV weaning.

SELECTION CRITERIA

Randomized and quasi-randomized trials comparing early extubation with immediate application of NPPV versus IPPV weaning in intubated adults with respiratory failure.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and abstracted data according to prespecified criteria. Sensitivity and subgroup analyses assessed (1) the impact of excluding quasi-randomized trials, and (2) the effects on selected outcomes noted with different causes of respiratory failure.

MAIN RESULTS

We identified 16 trials, predominantly of moderate to good quality, involving 994 participants, most with chronic obstructive pulmonary disease (COPD). Compared with IPPV weaning, NPPV weaning significantly decreased mortality. The benefits for mortality were significantly greater in trials enrolling exclusively participants with COPD (risk ratio (RR) 0.36, 95% confidence interval (CI) 0.24 to 0.56) versus mixed populations (RR 0.81, 95% CI 0.47 to 1.40). NPPV significantly reduced weaning failure (RR 0.63, 95% CI 0.42 to 0.96) and ventilator-associated pneumonia (RR 0.25, 95% CI 0.15 to 0.43); shortened length of stay in an intensive care unit (mean difference (MD) -5.59 days, 95% CI -7.90 to -3.28) and in hospital (MD -6.04 days, 95% CI -9.22 to -2.87); and decreased the total duration of ventilation (MD -5.64 days, 95% CI -9.50 to -1.77) and the duration of endotracheal mechanical ventilation (MD - 7.44 days, 95% CI -10.34 to -4.55) amidst significant heterogeneity. Noninvasive weaning also significantly reduced tracheostomy (RR 0.19, 95% CI 0.08 to 0.47) and reintubation (RR 0.65, 95% CI 0.44 to 0.97) rates. Noninvasive weaning had no effect on the duration of ventilation related to weaning. Exclusion of a single quasi-randomized trial did not alter these results. Subgroup analyses suggest that the benefits for mortality were significantly greater in trials enrolling exclusively participants with COPD versus mixed populations.

AUTHORS' CONCLUSIONS

Summary estimates from 16 trials of moderate to good quality that included predominantly participants with COPD suggest that a weaning strategy that includes NPPV may reduce rates of mortality and ventilator-associated pneumonia without increasing the risk of weaning failure or reintubation.

Noninvasive positive pressure ventilation as a weaning strategy for intubated adults with respiratory failure

BACKGROUND

Noninvasive positive pressure ventilation (NPPV) provides ventilatory support without the need for an invasive airway approach. Interest has emerged in using NPPV to facilitate earlier removal of an endotracheal tube and decrease complications associated with prolonged intubation.

OBJECTIVES

To summarize the evidence comparing NPPV and invasive positive pressure ventilation (IPPV) weaning on clinical outcomes in intubated adults with respiratory failure.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 2, 2008), MEDLINE (January 1966 to April 2008), EMBASE (January 1980 to April 2008), proceedings from four conferences, and personal files; and contacted authors to identify randomized controlled trials comparing NPPV and IPPV weaning.

SELECTION CRITERIA

Randomized and quasi-randomized studies comparing early extubation with immediate application of NPPV to IPPV weaning in intubated adults with respiratory failure.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trial quality and abstracted data according to prespecified criteria. Sensitivity and subgroup analyses were planned to assess the impact of (i) excluding quasi-randomized trials, and (ii) the etiology of respiratory failure on selected outcomes.

MAIN RESULTS

We identified 12 trials of moderate to good quality that involved 530 participants with predominantly chronic obstructive pulmonary disease (COPD). Compared to the IPPV strategy, NPPV significantly decreased mortality (relative risk (RR) 0.55, 95% confidence Interval (CI) 0.38 to 0.79), ventilator associated pneumonia (RR 0.29, 95% CI 0.19 to 0.45), length of stay in an intensive care unit (weighted mean difference (WMD) -6.27 days, 95% CI -8.77 to -3.78) and hospital (WMD -7.19 days, 95% CI -10.80 to -3.58), total duration of ventilation (WVD) -5.64 days (95% CI -9.50 to -1.77) and duration of endotracheal mechanical ventilation (WMD - 7.81 days, 95% CI -11.31 to -4.31). Noninvasive weaning had no effect on weaning failures or the duration of ventilation related to weaning. Excluding a single quasi-randomized trial maintained the significant reduction in mortality and ventilator associated pneumonia. Subgroup analyses suggested that the benefits on mortality and weaning failures were nonsignificantly greater in trials enrolling exclusively COPD patients versus mixed populations.

AUTHORS' CONCLUSIONS

Summary estimates from 12 small studies of moderate to good quality that included predominantly COPD patients demonstrated a consistent, positive effect on mortality and ventilator associated pneumonia. The net clinical benefits associated with noninvasive weaning remain to be fully elucidated.

Outcomes of patients ventilated with synchronized intermittent mandatory ventilation with pressure support: a comparative propensity score study

BACKGROUND

Few data are available regarding the benefits of one mode over another for ventilatory support. We set out to compare clinical outcomes of patients receiving synchronized intermittent mandatory ventilation with pressure support (SIMV-PS) compared with assist-control (A/C) ventilation as their primary mode of ventilatory support.

METHODS

This was a secondary analysis of an observational study conducted in 349 ICUs from 23 countries. A propensity score stratified analysis was used to compare 350 patients ventilated with SIMV-PS with 1,228 patients ventilated with A/C ventilation. The primary outcome was in-hospital mortality.

RESULTS

In a logistic regression model, patients were more likely to receive SIMV-PS if they were from North America, had lower severity of illness, or were ventilated postoperatively or for trauma. SIMV-PS was less likely to be selected if patients were ventilated because of asthma or coma, or if they developed complications such as sepsis or cardiovascular failure during mechanical ventilation. In the stratified analysis according to propensity score, we did not find significant differences in the in-hospital mortality. After adjustment for propensity score, overall effect of SIMV-PS on in-hospital mortality was not significant (odds ratio, 1.04; 95% CI, 0.77-1.42; P = .78).

CONCLUSIONS

In our cohort of ventilated patients, ventilation with SIMV-PS compared with A/C did not offer any advantage in terms of clinical outcomes, despite treatment-allocation bias that would have favored SIMV-PS.

Weaning and extubation readiness in pediatric patients

OBJECTIVE

A systematic review of weaning and extubation for pediatric patients on mechanical ventilation.

DATA SELECTION

Pediatric and adult literature, English language.

STUDY SELECTION

Invited review.

DATA SOURCES

Literature review using National Library of Medicine PubMed from January 1972 until April 2008, earlier cross-referenced article citations, the Cochrane Database of Systematic Reviews, and the Internet.

CONCLUSIONS

Despite the importance of minimizing time on mechanical ventilation, only limited guidance on weaning and extubation is available from the pediatric literature. A significant proportion of patients being evaluated for weaning are actually ready for extubation, suggesting that weaning is often not considered early enough in the course of ventilation. Indications for extubation are even less clear, although a trial of spontaneous breathing would seem a prerequisite. Several indices have been developed in an attempt to predict weaning and extubation success but the available literature would suggest they offer no improvement over clinical judgment. Extubation failure rates range from 2% to 20% and bear little relationship to the duration of mechanical ventilation. Upper airway obstruction is the single most common cause of extubation failure. A reliable method of assessing readiness for weaning and predicting extubation success is not evident from the pediatric literature.

Oxygen consumption and PEEPe in ventilated COPD patients

The intrinsic positive-end-expiratory pressure (PEEPi) increases the inspiratory load, the cost of breathing and thus oxygen consumption (V(O2)). It has been shown that applying an extrinsic positive-end-expiratory pressure (PEEPe) reduces the inspiratory threshold load but the optimal PEEPe level is still in debate. We hypothesize that the best level of PEEPe could induce a decrease in V(O2) by reducing the V(O2) demands from PEEPi. Nine mechanically ventilated COPD patients were included. The level of PEEPe was determined in accordance with the static PEEPi. V(O2) was measured using an automatic gas analyser during synchronized intermittent mandatory ventilation (SIMV): without PEEPe, with a PEEPe equal to 50% of static PEEPi and with a PEEPe equal to 100% of static PEEPi. Static PEEPi appeared to be significantly correlated with the degree of airflow obstruction (FEV1) (P<0.05). Applying a PEEPe equal to static PEEPi resulted in a significant decrease in V(O2) (P<0.05) whereas the change in V(O2) proved to be unpredictable for a PEEPe level of 50% of static PEEPi. In conclusion, V(O2) decreases progressively when increasing PEEPe up to a level equal to 100% of static PEEPi. Thus, in mechanically ventilated COPD patients with a FEV1 < or = 1000 ml, applying a PEEPe of 5 cmH2O should be recommended.

Noninvasive positive pressure ventilation as a weaning strategy for intubated adults with respiratory failure

BACKGROUND

Noninvasive positive pressure ventilation (NPPV) provides ventilatory support without the need for an invasive airway. Interest has emerged in using NPPV to facilitate earlier removal of the endotracheal tube and decrease complications associated with prolonged intubation.

OBJECTIVES

To summarize the evidence comparing NPPV and invasive positive pressure ventilation (IPPV) weaning on clinical outcomes in intubated adults with respiratory failure.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (The Cochrane Library, issue 2, 2003), MEDLINE (January 1966 to July 2003) and EMBASE (January 1980 to July 2003) for randomized controlled trials comparing NPPV and IPPV weaning. Additional data sources included personal files, conference proceedings and author contact.

SELECTION CRITERIA

Randomized and quasi-randomized studies comparing early extubation with immediate application of NPPV to IPPV weaning in intubated adults with respiratory failure.

DATA COLLECTION AND ANALYSIS

Two reviewers independently assessed trial quality and abstracted data according to prespecified criteria. Sensitivity and subgroup analyses were planned to assess the impact of (i) excluding quasi-randomized trials and (ii) the etiology of respiratory failure on outcomes.

MAIN RESULTS

We identified eleven trials, of which five were included, involving 171 participants with predominantly chronic obstructive pulmonary disease. Overall, the included studies were of moderate to good quality. Compared to the IPPV strategy, the NPPV strategy decreased mortality (RR 0.41, 95% CI 0.22 to 0.76), the incidence of ventilator associated pneumonia (RR 0.28, 95% CI 0.09 to 0.85), intensive care unit length of stay (WMD -6.88 days, 95% CI -12.60 to -1.15), hospital length of stay (WMD -7.33 days, 95%CI -14.05 to -0.61), total duration of mechanical support (WMD -7.33 days, 95% CI -11.45 to -3.22) and the duration of endotracheal mechanical ventilation (WMD -6.79 days, 95% CI -11.70 to -1.87). There was no effect of NPPV on weaning failures or the duration of mechanical support related to weaning and insufficient data to pool adverse events or quality of life. Excluding a single quasi-randomized trial maintained the significant reduction in mortality and ventilator associated pneumonia. Subgroup analyses suggested that the mortality benefit of the NPPV approach is greater in patients with chronic obstructive pulmonary disease.

REVIEWER'S CONCLUSIONS

Summary estimates from five studies of moderate to good quality demonstrated a consistent positive effect on overall mortality. At present, use of NPPV to facilitate weaning in mechanically ventilated patients, with predominantly chronic obstructive lung disease, is associated with promising, although insufficient, evidence of net clinical benefit.

Trials comparing alternative weaning modes and discontinuation assessments

We identified 16 randomized controlled trials (RCTs) of methods for weaning patients from mechanical ventilation, 8 of which were trials of discontinuation assessment strategies, 5 of which were trials of stepwise reduction in mechanical ventilatory support, and 3 of which were trials comparing alternative ventilation modes for weaning periods lasting < 48 h. We found that different thresholds for deciding when a patient is ready for a trial of spontaneous breathing, different criteria for a successful trial, and different thresholds for extubation may overwhelm the impact of alternative ventilation strategies. Nevertheless, the results of these studies suggest the possibility that multiple daily T-piece weaning or pressure support may be superior to synchronized intermittent mandatory ventilation. Other RCTs suggest that early extubation with the back-up institution of noninvasive positive-pressure ventilation as needed may be a useful strategy in selected patients.

Update on patient-triggered ventilation

Physiologic studies have demonstrated short-term benefits of triggered ventilation over conventional ventilation. The results of the randomized trials are disappointing. Meta-analysis has highlighted that the only significant difference in outcomes on PTV compared with conventional ventilation is a shorter duration of weaning. A few of the trials included infants with meconium aspiration syndrome and congenital pneumonia, but most infants randomized had RDS. In addition, a high proportion of the infants included in the meta-analysis were from two trials in which the SLE 2000 and airway pressure triggering system were mainly used. We cannot confidently conclude that in a population of infants with another respiratory disorder or even in those with RDS supported by an alternative triggering system, a different result might have been achieved. In addition, the benefits of PTV demonstrated in physiologic studies are largely related to achieving synchronized ventilation. In none of the randomized trials was any attempt made to determine if the infants were breathing synchronously with their ventilators. Before dismissing PTV for use in the management of infants with acute respiratory distress, an appropriately designed trial needs to take place. Essential, before any such trial, is identification of optimum method of PTV delivery, which may be disease specific.

Assessment of factors predicting outcome of acute respiratory distress syndrome in North India

OBJECTIVES

Information on the spectrum and outcome of acute respiratory distress syndrome (ARDS) in tropical countries is scanty. This study was designed to assess the factors predicting the outcome of ARDS in North India.

METHODOLOGY

Consecutive patients requiring mechanical ventilation for ARDS over a 2 year period at the Respiratory Intensive Care Unit (RICU) of a tertiary referral hospital were studied. Hospital survival was correlated with age, aetiology, disease severity scores (APACHE III, SAPS II, lung injury score) and organ failure using univariate analysis. Factors significantly influencing mortality were examined by multivariate analysis to identify factors independently affecting outcome.

RESULTS

Sepsis (28.6%), followed by malaria (21.4%), were the commonest risk factors. Seven out of eight patients (87.5%) with sepsis died. The presence of sepsis, more than three organ failure prior to admission, APACHE III score > 57 and SAPS II score > 39 were significantly associated with mortality. Only APACHE III score > 57 or SAPS II score > 39 were, however, independently predictive of a poor outcome following multivariate analysis.

CONCLUSIONS

Sepsis, associated with a very poor outcome, and malaria were important risk factors for the development of ARDS. APACHE III score > 57 or SAPS II score > 39 were associated with increased risk of mortality.

Weaning from prolonged mechanical ventilation

The development of weaning failure and need for PMV is multifactorial in origin, involving disorders of pulmonary mechanics and complications associated with critical illness. The underlying disease process is clearly important when discussing mechanisms of ventilator dependence; interventions therefore must be tailored to individual patients. Unfortunately, the main conclusion that can be drawn from the sum of the studies investigating patients on PMV to date is that an evidence-based approach to weaning is not possible and more research needs to be done. New studies need to incorporate severity-of-illness scores and an assessment of principal and comorbid conditions to allow for comparison of the findings from different centers. The best approach to a patient requiring PMV after exclusion of easily treatable conditions is not known. The literature regarding both acute and chronic cases suggests that a systematic approach to weaning involving the participation of multiple caregivers, including nurses, physicians, and respiratory, physical, and speech therapists facilitates liberation from MV. Although a gradual decrement in ventilator support would seem prudent, Scheinhorn et al have begun to identify a subpopulation of patients who can tolerate an acceleration of the weaning process. Given the known complications associated with MV, it is crucial that further research be performed to identify patients as soon as they are capable of breathing spontaneously. The literature demonstrates through multiple studies that satisfactory patient outcomes are attainable and can be achieved at LTAC facilities in a more cost-effective manner than in an ICU setting. The trend toward the concentration of patients into specialized regional weaning centers should facilitate the research process and continue to improve outcomes in this population.

Is there a preferred technique for weaning the difficult-to-wean patient? A systematic review of the literature

OBJECTIVE

To answer the following question: In difficult-to-wean patients, which of the three commonly used techniques of weaning (T-piece, synchronized intermittent mandatory ventilation, or pressure support ventilation) leads to the highest proportion of successfully weaned patients and the shortest weaning time?

DATA SOURCES

Computerized literature searches in MEDLINE (1975-1996), Cinahl (1982-1996), and Healthplan (1985-1996), exploding all Mesh headings pertaining to Mechanical Ventilation and Weaning. Searches were restricted to the English language, adults, and humans. Personal files were hand searched, and references of selected articles were reviewed.

STUDY SELECTION

a)

POPULATION

Patients requiring a gradual weaning process from the ventilator (either requiring prolonged initial ventilation of >72 hrs or a failed trial of spontaneous breathing after >24 hrs of ventilation); b)

INTERVENTIONS

At least two of the following three modes of weaning from mechanical ventilation must have been compared: T-piece, synchronized intermittent mandatory ventilation, or pressure support ventilation; c)

OUTCOMES

At least one of the following: weaning time (time from initiation of weaning to extubation) or successful weaning rate (successfully off the ventilator for >48 hrs); and d)

STUDY DESIGN

Controlled trial.

DATA EXTRACTION

Two reviewers independently reviewed the articles and graded them according to their methodologic rigor. Data on the success of weaning and the time to wean were summarized for each study.

DATA SYNTHESIS

The search strategy identified 667 potentially relevant studies; of these, 228 had weaning as their primary focus, and of these, 48 addressed modes of ventilation during weaning. Only 16 of these 48 studies had one of the specified outcomes, and only ten of these were controlled trials. Of the ten trials, only four fulfilled all our selection criteria. The results of the trials were conflicting, and there was heterogeneity among studies that precluded meaningful pooling of the results.

CONCLUSIONS

There are few trials designed to determine the most effective mode of ventilation for weaning, and more work is required in this area. From the trials reviewed, we could not identify a superior weaning technique among the three most popular modes, T-piece, pressure support ventilation, or synchronized intermittent mandatory ventilation. However, it appears that synchronized intermittent mandatory ventilation may lead to a longer duration of the weaning process than either T-piece or pressure support ventilation. Finally, the manner in which the mode of weaning is applied may have a greater effect on the likelihood of weaning than the mode itself.

Weaning from mechanical ventilation

For most mechanically ventilated patients, weaning can be accomplished quickly and easily. However, there is a smaller group of ventilated patients who fail to wean and remain ventilator-dependent. These patients account for a significant amount of health care costs and pose a great challenge for clinicians. Detailed knowledge of the etiology and pathophysiology of weaning failure is very important for the "treatment" of difficult to wean patients, and is thoroughly presented in this article. Based on this physiological background, strategies and techniques are proposed that are useful for the gradual transition to spontaneous ventilation.

Weaning from mechanical ventilation

For the ventilator-dependent patient, weaning should be accomplished by withdrawing support safely, efficaciously, and efficiently. Success depends largely on physiologic determinants of respiratory system function, avoidance of ventilator-associated complications, and attention to patient readiness. Recent clinical trials, predictors of weaning, current techniques of weaning, the concept of reloading the respiratory pump, and determinants of ventilator dependency are all discussed.

Weaning from ventilatory support

Resumption of spontaneous unassisted breathing after an episode of acute respiratory failure often is achieved without major difficulty. In a significant number of patients however, weaning from mechanical ventilation is a long and difficult process that markedly increases the duration of mechanical ventilation and consumes a significant fraction of critical care resources. Some criteria have been suggested to predict early and more accurately the moment the patient is ready to be separated from the ventilator. At the present time, the f/VT ratio (rapid shallow breathing index) appears to yield the best predictive power. None of these indices, however, is powerful enough to be relied on solely, and their use should be limited to that of aids to the critical care physician. The inability to sustain spontaneous ventilation usually is the consequence of an imbalance between respiratory demand and respiratory muscle capacity. Increased elastic workload, increased resistive workload, and increased VE are the main causes of excessive demand imposed on the respiratory system. Respiratory muscle pump failure usually relates to peripheral nerve dysfunction or muscular dysfunction. Left ventricular dysfunction also is an important cause of weaning failure. The usual methods of weaning from mechanical ventilation are T-piece trials with abrupt definitive discontinuation of mechanical ventilation if tolerated or with progressive intermittent trials, IMV, and PSV. All have their advantages and disadvantages, and the method of weaning per sé is not the only critical factor. Although their conclusions were different regarding the best method of weaning, however, two recent clinical trials suggest that ventilatory management has a major influence on the outcome of weaning from mechanical ventilation in difficult-to-wean patients. The global management of such patients requires a systematic approach with consideration of all factors involved in the process of separation from the ventilator. New computer-assisted systems already are tested and, in the future, may provide a significant advantage in the management of weaning from mechanical ventilation.

Synchronous intermittent mandatory ventilation modes compared with patient triggered ventilation during weaning

The efficacy of combining rate and pressure reduction during weaning by synchronous intermittent mandatory ventilation (SIMV) were compared with weaning by patient triggered ventilation (PTV) (pressure reduction alone) in two randomised trials. Regardless of ventilation mode, pressure was reduced to the same level according to the size of the infant. In the first trial, the SIMV rate was also reduced progressively to a minimum of 20 breaths/minute, and in the second to five breaths/minute. Forty premature infants aged 15 days of age or less were randomly allocated into each trial. No significant differences were found in the first trial between ventilation modes in either the duration of weaning or the number of infants in whom weaning failed. In the second trial, the duration of weaning was shorter by PTV than by SIMV (median 24 hours, range 7-432 v 50 hours, range 12-500; p < 0.05); weaning failed in two infants in the PTV group and in five in the SIMV group. It is concluded that weaning by a combination of pressure and rate reduction, such as can be achieved during SIMV, offers no significant advantage over pressure reduction alone.