Description of the condition
Acute respiratory distress syndrome (ARDS) is a common, devastating clinical syndrome of acute lung injury that affects both medical and surgical patients. It is clinically characterized by severe hypoxaemia, radiographic evidence of bilateral pulmonary infiltration and absence of left heart failure. Acute lung injury (ALI) is a subset of ARDS with less severe impairment in oxygenation. A recent study has estimated the incidence of this disease to be between 15 and 34 cases per 100,000 inhabitants per year (Frutos-Vivar 2004). According to a prospective cohort study, the prevalence of ARDS and ALI was about 9% amongst intensive care patients, and 39.6% amongst ventilated patients (Roupie 1999). The mortality rate from ARDS and ALI is approximately 35% to 65% (Esteban 2008). There is also short- and long-term morbidity associated with these syndromes. Short-term morbidity leads to a prolonged stay in the intensive care unit and prolonged ventilator dependence (Davidson 1999). Those who survive the illness have a reduced health-related quality of life as well as cognitive impairment and high rates of disability (Dowdy 2006).
Recently, a new definition of ARDS, based on the degree of hypoxaemia, was proposed (Ranieri 2012).
Description of the intervention
Mechanical ventilation (MV) represents the main therapeutic support to maintain acceptable pulmonary gas exchange whilst treating the underlying disease. ARDS is associated with a reduction in static compliance (Marini 1990). As a result of this low compliance, high pressures are needed to obtain a sufficient tidal volume. The larger the tidal volume the higher the pressure required, which may lead to barotrauma, that is, alveolar rupture and radiological evidence of extra-alveolar air. In such patients, mechanical ventilation could lead to injury due to over-distension. This results from the distribution of the increased tidal volume to the high-compliance regions causing stretching and sheer forces on the alveolar wall (volutrauma) (Dreyfuss 1998). Plateau pressure, defined as airway pressure during the end-expiratory pause, roughly reflects the level of alveolar over-distension. Finally, parts of the lung are consolidated and are thus not recruitable. This leads to fewer alveoli truly being ventilated; hence a 'normal' tidal volume may not be appropriate for those remaining alveoli. Damage caused by MV to the lungs has been termed ventilator-induced lung injury (VILI).
How the intervention might work
Better understanding of the pathophysiology of ARDS and of VILI have led to the proposal that airway pressure and tidal volume should be limited in managing the ventilation of ARDS patients (Artigas 1998; Hickling 1990; Hickling 1994). This entails accepting a rise in the arterial partial pressure of carbon dioxide. In addition, cyclic inflation-deflation of injured lung units or alveoli can exacerbate lung injury (Dreyfuss 1998), and medium to high levels of positive end-expiration pressure (PEEP) should be used to keep alveoli open throughout the ventilatory cycle. Overall, this type of approach has been termed a lung protective ventilation strategy (Artigas 1998). Ventilation with lower tidal volumes was also associated with lower levels of systemic inflammatory mediators (Ranieri 1999).
Why it is important to do this review
Lowering the tidal volume is possibly not without hazards (Roupie 1999). Severe hypercapnia and acidosis can have adverse effects, including increased intracranial pressure, depressed myocardial contractility, pulmonary hypertension and depressed renal blood flow (Feihl 1994). The view that these risks are preferable to the higher plateau pressure required to achieve normocapnia has represented a substantial shift in ventilatory management.
The objective of this systematic review was to compare ventilation with a lower tidal volume, with lower or higher PEEP, and ventilation with conventional tidal volume, with lower or higher PEEP, to determine whether such a lung protective ventilation strategy (LPVS) reduced morbidity and mortality in critically ill adults affected by ALI or ARDS.
Criteria for considering studies for this review
Types of studies
We accepted only randomized controlled trials (RCTs) to guarantee control of selection bias. We excluded studies that, on closer scrutiny, were determined to be quasi-randomized or cross-over studies.
Types of participants
We included critically ill patients aged 16 years or older, intubated and ventilated and affected by ARDS or ALI from any cause as defined by the North-American-European Consensus Conference on ARDS (NAECC) (Bernard 1994) or by the Lung Injury Severity Score (LISS) (Murray 1988).
Types of interventions
A protective ventilation strategy that used lower tidal volume or low airway driving pressure (plateau 30 cm H
Types of outcome measures
Overall mortality, evaluated at hospital discharge (if this information was unavailable, mortality was evaluated at the end of the follow-up period scheduled for each trial)
There is evidence that the cause of death in ARDS is the development of organ failure (Monchi 1998), and that mechanical ventilation might be a factor leading to multiple organ failure. The duration of mechanical ventilation could also lead to the development of associated infectious diseases that could affect the patients' overall prognosis, such as ventilator-associated pneumonia (VAP). If mortality is equal, other outcomes become important to the patients such as long-term quality of life and cognitive impairment.
1. Development of multi-organ failure (MOF)
2. Duration of mechanical ventilation and total duration of mechanical support
3. Total duration of stay in intensive care unit and hospital
4. Long-term mortality
5. Long-term health-related quality of life
6. Long-term cognitive outcome
Search methods for identification of studies
In our original review (Petrucci 2004a) we searched the databases from inception until 2006.
In this updated review we searched the Cochrane Central Register of Controlled Trials (CENTRAL) in The Cochrane Library 2012, Issue 3 (Appendix 1); MEDLINE (2006 to September 2012) (Appendix 2); EMBASE (2006 to September 2012) (Appendix 3); CINAHL (2006 to September 2012) (Appendix 4) and the Web of Science (2006 to September 2012) (Appendix 5) using a combination of MeSH and text words.
We applied no language restrictions.
In addition, we handsearched references lists and abstracts and proceedings of scientific meetings held on the subject.
In our original review (Petrucci 2004a), we searched proceedings of the Annual Congress of the European Society of Intensive Care Medicine (ESICM) and of the American Thoracic Society (ATS) from 1993 to 2006. In this version we updated that search to September 2012.
Searching other resources
We searched the following web resources:
We contacted the original author(s) for clarification about content, study design and missing data, if needed.
Data collection and analysis
Selection of studies
Nicola Petrucci (NP) and Carlo De Feo (CDF) independently screened the results of the search strategy for potentially relevant trials and independently assessed them for inclusion based on the inclusion criteria. We resolved disagreements through discussion until a consensus was reached.
Data extraction and management
We employed the standard methods of the Cochrane Anaesthesia Review Group. We (NP, CDF) independently performed assessment of methodology and extraction of data, with comparison and resolution of any differences found at each stage.
Assessment of risk of bias in included studies
We judged the quality of each trial by whether or not the study design had minimized bias within the scope of the clinical context. The risk of bias in included studies was assessed by addressing six specific domains: random sequence generation, allocation concealment, blinding, incomplete outcome data, selective reporting, and other issues.
We defined high-quality trials as: controlled, appropriately randomized, having adequate concealment of allocation, and completeness of follow up according to an intention-to-treat analysis. We judged the concealment of allocation as adequate if the trials took adequate measures to conceal allocation through central randomization, such as serially numbered opaque envelopes or a table of random numbers. We judged the generation of allocation sequence as adequate where trials were deemed to have a satisfactory sequence generation (random numbers generated by computers, drawing of lots of envelopes).
For the individual trial results, we calculated relative risk (RR) with 95% confidence interval (CI) and number needed to treat (NNT) for categorical outcomes. We reported mean differences (MDs) (and 95% CIs) for continuous variables, if appropriate.
Measures of treatment effect
For the meta-analysis, we reported relative risk (RR), risk difference (RD) and 95% CI for dichotomous outcomes; and weighted mean differences (WMD) for continuous variables.
Unit of analysis issues
We did not have any unit of analysis issues.
Dealing with missing data
For all included trials, we registered the levels of attrition. We tried to contact main authors for missing data.
Assessment of heterogeneity
We performed a formal exploration of heterogeneity. Higgins developed a measure of the impact of heterogeneity in a meta-analysis that is independent of the number of studies (I
Assessment of reporting biases
We explored publication bias using a plot of study sample size against RR. In the absence of publication bias, a plot of study sample size (or study weight) versus outcome (that is log RR) should have a bell or inverted funnel shape with the apex near the summary effect estimate (funnel plot).
We pooled the results from the original RCTs using a fixed-effect model, or a random-effects model for I
Subgroup analysis and investigation of heterogeneity
We planned subgroup analysis to determine whether the results differed by one of the following.
- Severity of disease: ARDS (more severe impairment) or ALI (less severe impairment)
- Aetiology of ARDS and ALI (i.e. pneumonia, trauma, sepsis, etc)
Delivery of interventions
- To determine whether the comparison between low tidal volume and normal tidal volume was different if a plateau pressure of greater than 30 to 35 cm H
20 was used
We performed a subgroup analysis based on the plateau pressure in the control groups.
We also performed sensitivity analyses based on whether the outcome assessors were blinded and by imputing values for dropouts.
Description of studies
Results of the search
In our previous updated review (Petrucci 2007) we found 10 studies that were of potential relevance, of which we eventually excluded four studies (Brower 2004; Esteban 2000; Ranieri 1999; Rappaport 1994). In this updated review we searched the literature from 2007 to 2012 (Figure 1).
|Figure 1. Study flow diagram.|
We did not find any studies, in addition to our previous six trials (Amato 1998; ARDS Network 2000; Brochard 1998; Brower 1999; Stewart 1998; Villar 2006), that were assessed for methodological quality (see Characteristics of included studies table). All of the included studies were multi-centre trials.
The total number of patients randomized in each study varied from 52 (Brower 1999) to 861 (ARDS Network 2000). All studies included ARDS patients but some investigations tended to use the LISS definition (Amato 1998; Brochard 1998) and some the NAECC definition of ARDS (ARDS Network 2000; Brower 1999; Villar 2006). The average age at randomization with standard deviation (SD) varied from 33 ± 13 (Amato 1998) to 59 ± 17 (Stewart 1998). The European study (Brochard 1998) was unique in the inclusion of patients with single organ failure (lung injury) only. The time that had elapsed from eligibility to randomization ranged from one hour to 36 hours. In one study (Villar 2006), only patients who demonstrated persistent ARDS 24 hours after initially meeting the ARDS criteria were enrolled.
All the trials assessed the baseline risk by combining several prognostic variables into a severity score (APACHE II or APACHE III score), which provides initial risk stratification for severely ill, hospitalized patients and a risk estimate for hospital mortality for individual intensive care unit (ICU) patients (Knaus 1985; Knaus 1991). The higher the score the higher is the relative risk of hospital death. The APACHE II score at baseline was in the range of 17 ± 8 SD (Brochard 1998) to 28 ± 7 SD (Amato 1998). Two trials used the APACHE III score (ARDS Network 2000; Brower 1999). The APACHE III score ranged from 81 ± 28 SD (ARDS Network 2000) to 90 ± 26 SD (Brower 1999). The severity of impairment of lung function was reported according to the partial pressure of arterial oxygen to the fraction of inspired oxygen ratio (PaO
Delivery of the interventions for each trial varied. Five trials (ARDS Network 2000; Brochard 1998; Brower 1999; Stewart 1998; Villar 2006) set the tidal volume based on body weight, without the use of a pressure-volume (PV) curve. However, in the ARDS Network, Brower, Stewart and Villar trials the tidal volume was set according to predicted (or ideal) body weight (IBW), which was calculated according to the gender and height of the patient. IBW is, on average, 20% less than measured body weight. When transformed to ml/kg of measured body weight the mean tidal volume in the ARDS Network trial ranged from 9.4 to 9.9 ml/kg in the control group, which was quite similar to values used in the other trials, and 5.2 ml/kg in the low tidal volume group, which was lower than in other trials. Two trials (Amato 1998; Villar 2006) compared the effect of a combined strategy composed of a low tidal volume and relatively high PEEP titrated according to the PV curve. Amato added an intermittent recruitment manoeuvre, and used in the control group a ventilatory strategy aimed at normalizing the partial pressure of carbon dioxide, without any limitation in peak inspiratory pressure. Based on aggregate data, all patients in the lung protective ventilation groups received tidal volumes significantly lower than those in the conventional ventilation groups. Limits of airway plateau pressure ranged from 22 to 30 cm H
The studies did not report all the considered outcomes. The definition of overall mortality differed between studies. Mortality was measured at a cut-off point of day 60 in one study (Brochard 1998), day 180 in one study (ARDS Network 2000) and at hospital discharge in two studies (Brower 1999; Stewart 1998). In one study (Amato 1998) both mortality at day 28 and at hospital discharge were reported. ICU and hospital mortality were reported in one study (Villar 2006). Importantly, in ARDS Network 2000 patients were followed until discharged home or for 180 days, whichever occurred first. This may be considered equivalent to hospital mortality. Among the secondary outcome measures considered for this review, only three trials reported the duration of mechanical ventilation (Brochard 1998; Brower 1999; Stewart 1998). In the ARDS Network study, this outcome was reported as the median number of days, without further description. However, the absence of a strict protocol for weaning in all the trials, apart from the ARDS Network study (ARDS Network 2000), makes this outcome difficult to evaluate objectively, although important for clinical and economic implications. Similarly, organ failures were reported using different measures. Two trials did not report this outcome at all (Amato 1998; Brower 1999). Brochard 1998 reported that 24 patients in each group suffered from organ failures without further details, whereas Stewart 1998 reported a mean value of two organ failures per patient in each group. ARDS Network 2000 reported the number of days without non-pulmonary organ failure at day 28. Other secondary outcomes considered relevant for this review were not reported in the studies.
We found two additional studies that were of potential relevance from our updated search (2007 to September 2012) (Meade 2008; Mercat 2008). These studies used low tidal ventilation in both arms. The groups differed by the level of PEEP (Mercat 2008) or the level of PEEP and recruitment manoeuvres (Meade 2008). Therefore, these trials were eventually excluded. Data from one trial (ARDS Network 2000) were used for subsequent multiple publications (see 'Additional Table 1'). We excluded these papers.
Risk of bias in included studies
All included studies were randomized controlled trials. All the studies found similarity of the two study groups at the time of randomization with respect to important determinants of outcome.
All the studies reached a high level of quality (Figure 2).
|Figure 2. Risk of bias summary: review authors' judgements about each risk of bias item for each included study.|
In five studies (Amato 1998; ARDS Network 2000; Brochard 1998; Brower 1999; Villar 2006) sample size was calculated before the beginning of the study. With the exception of the Amato trial, which reached the target sample (53 patients), the other four studies were stopped early. Brochard 1998 based the calculation on a 30% to 40% reduction of hospital mortality. They estimated a sample of 240 patients and the trial was stopped at 116 because protective ventilation was clearly detrimental (RR = 1.23). Brower 1999 used an estimation of treatment effect based on reversal of respiratory failure as the target outcome for the calculation of sample size. The study was designed to detect a 30% reduction in respiratory failure. They estimated a sample of 130 patients, but the trial was stopped at 52 because the second interim analysis showed that it was unlikely that beneficial treatment effects on reversal of respiratory failure could be demonstrated if the trial was to continue. In ARDS Network 2000, the calculation of sample size was based on a 10% reduction in cumulative mortality at day 180. The estimated sample consisted of 1000 patients and the trial was stopped after enrolling 861 patients because the interim analysis demonstrated that lower tidal volume ventilation was beneficial. The ARDS Network trial, in its design, tried to detect the smallest difference in mortality between groups. Villar 2006 calculated sample size based on a 20% reduction in absolute mortality. The trial was stopped after enrolling 103 patients due to the benefits of low tidal volume ventilation. None of the trials reported a long-term follow up.
Five included studies had low risk of selection bias in the random sequence generation domain. One study had an unclear risk (concealment of allocation was unclear) (Brower 1999).
In one trial (Amato 1998) it was clearly stated that the study was not blind. No mention of blinding was found in all the other studies. Protocols for management of mechanical ventilation were found in all the studies, thus minimizing performance bias. A protocol for weaning was found only in the ARDS Network study (ARDS Network 2000). For outcomes assessed in this review, it was unclear to whom the blinding referred and so there was an unclear risk of detection bias in all the included studies.
Incomplete outcome data
Most studies included complete follow up and intention-to-treat analysis. In the ARDS Network trial 31 patients (3.6%) were lost to follow up (ARDS Network 2000). Twenty-two of them were patients who were still hospitalized when the trial was stopped, and for nine patients the outcome was unknown (legend, Figure 1 in the original trial). See the 'Characteristics of included studies' for a more detailed description of individual trial quality.
Risk of reporting bias was rated as low in all trials.
Other potential sources of bias
No other potential sources of bias were found.
Effects of interventions
The ARDS Network trial coordinator and the corresponding author of the Brochard study were requested to supply further data on mortality at day 28. Unfortunately this information has not been made available. Therefore, we extracted the numerical data from figure 1 and figure 5 as presented in the original publications (ARDS Network 2000; Brochard 1998). The total number of randomized patients was 1297. Globally, when considering mortality at the end of the follow-up period, as reported by the trialists, the results from the trials clustered between a RR of 0.6 and 1.23, with overlapping CIs (see Additional Table 2). Overall, the test for heterogeneity yielded a borderline result (P = 0.10; 5 degrees of freedom). The I
Overall mortality at the end of the follow-up period for each trial showed a trend toward a reduction but it did not reach statistical significance. Using a random-effects model, the RR was 0.86 (95% CI 0.69 to 1.06), absolute risk difference of -6% (95% CI -16% to 3%) with moderate heterogeneity (I
We stratified the trials based on comparable outcomes. Using hospital mortality (Amato 1998; ARDS Network 2000; Brower 1999; Stewart 1998; Villar 2006) in 1181 patients, the point estimate was in favour of the low tidal volume ventilation with both the fixed-effect and random-effects models: RR 0.80 (95% CI 0.69 to 0.92), absolute RD -8% (95% CI -13% to -2%); and RR 0.81 (95% CI 0.66 to 0.98), absolute risk reduction -9% (95% CI -18% to 0.0%), respectively. The I
Mortality at day 28 (Amato 1998; ARDS Network 2000; Brochard 1998) in 1030 patients was significantly lower in the patients with the lung protective ventilation strategy, fixed-effect model RR of 0.74 (95% CI 0.61 to 0.88); the absolute risk difference was -10% (95% CI -15% to -4%). When applying a random-effects model the RR of 0.73 was largely unchanged (95% CI 0.61 to 0.87) but the upper limit of the 95% CI of the RD approached unity (absolute RD -12%, 95% CI -23% to 0%). The test for heterogeneity gave a P value of 0.22 (df = 3), I
Amongst the secondary outcomes, there were only sufficient data to assess the association between the lung protective ventilation strategy and the duration of mechanical ventilation (see Additional Table 3). Three trials looked at this endpoint (Brochard 1998; Brower 1999; Stewart 1998) and enrolled a total of 288 patients. No additional data were requested from the authors. There was a trend toward a lower duration of mechanical ventilation in patients with the protective ventilation strategy, but this reduction was not statistically significant (fixed-effect model WMD -0.83, 95% CI -1.92 to 0.27, random-effects model WMD 0.38, 95% CI -3.06 to 3.82; test for heterogeneity P = 0.22, df = 3, I
We found insufficient data in the trials to perform subgroup analyses assessing the effects of age, severity of disease, and aetiology of ARDS and ALI. Similarly, the studies did not report enough data to perform a subgroup analysis based on concomitant treatments. Subgroup analysis based on the severity of the disease also was not feasible because all the studies included patients with more severe impairment of lung function (PaO
We performed subgroup analysis based on the delivery of the interventions and comparing the overall estimate of effect of treatment on all-cause mortality at the end of the follow-up period between the trials with a 'low pressure' control group (mean plateau pressure 31 cm H
Although publication bias is not the only cause of an asymmetrical plot, the symmetrical shape of the funnel plot of precision by effect size showed that the overall effect of meta-analysis in this review was not affected by publication bias or biased inclusion criteria. However, the small numbers of trials included should suggest a cautious interpretation.
We excluded one trial that was clearly unblinded and that used a different method to deliver the intervention in the treatment group (Amato 1998) in 53 patients. The test for heterogeneity changed slightly (P = 0.13, df = 3). Overall mortality at the end of the follow-up periods was also changed slightly (fixed-effect model RR 0.87, 95% CI 0.75 to 1.01, absolute RD -5%, 95% CI -11% to 0%; random-effects model RR 0.97, 95% CI 0.76 to 1.24, absolute RD -2%, 95% CI -11% to 8%). We evaluated the impact of the largest study (ARDS Network 2000) in 861 patients by excluding it from the analysis. The effect of lung protective and conventional treatment strategies on all-cause mortality disappeared (fixed-effect model RR 0.92, 95% CI 0.75 to 1.12; random-effects model RR 0.90, 95% CI 0.67 to 1.15).
The ARDS Network investigators reported that nine patients (hospital mortality unknown) and 22 additional patients (still hospitalized when the trial was stopped) were censored in the survival plot (Legend, Figure 1) (ARDS Network 2000). There was no later information about the eventual outcome of those patients. We performed a sensitivity analysis imputing values for the 31 dropouts according to 'best-worse' case analysis (see 'Comparisons and data' 11 and 12). For the best-case analysis, the 16 and 15 patients were added back and all added control patients were assumed to have died with no additional deaths in the treatment group. Thus the counts were: treatment (129 plus 0)/(416 plus 16) = 129/432; and control (165 plus 15)/(414 plus 15) = 180/429. The test for heterogeneity was P = 0.04, I
Other secondary outcomes
Although there is evidence that the cause of death in ARDS is the development of organ failure (Monchi 1998), only three studies reported that outcome, each of them using different criteria to define organ failure thus preventing us from combining the results in a summary estimate. In only two studies (ARDS Network 2000; Villar 2006) the ventilatory strategy affected the development of organ failure, reduced in the protective arm, whereas in the 'non-beneficial' studies (Brochard 1998; Stewart 1998) the occurrence of MOF was similar in both groups.
The long-term cognitive outcome and costs were not assessed by the trials.
Can we attribute all the differences between the two groups to differences in the tidal volumes used? This is a controversial issue. A study by the ARDS Network (Brower 1999) (the so-called ALVEOLI study) reported no difference in outcome when ARDS patients were ventilated with low tidal volume and high or low PEEP. That means that PEEP is not as important as tidal volume in changing outcomes. Villar 2006 confirms this finding as when a high-low PEEP and large tidal volume difference was used between groups the difference in outcomes appeared again. Therefore, the tidal volume difference clearly had an impact on mortality. Two recent trials (Meade 2008; Mercat 2008) compared ventilation with lower or higher PEEP in patients ventilated with the routine use of low tidal volumes, finding no difference in mortality. This finding confirms that tidal volume is the major determinant of outcome.
Meta-analysis performed by stratifying trials according to 'high plateau pressure' and 'low plateau pressure' in the control group confirmed that when delivery of conventional tidal volume was associated with a plateau pressure of 31 cm H
Plateau pressure reflects both pulmonary and chest compliance and, therefore, the same limit of pressure may reflect lower tidal volumes if the chest is stiff, and vice versa. Patients with potentially altered chest compliance were excluded in all the studies. Nevertheless, in some patients with ARDS flattening of the pressure-volume (PV) curve may be due to an increase in chest wall elastance related to abdominal distension (Ranieri 1997). Intra-abdominal pressure (a possible confounding factor) was not accounted for or controlled in the selected studies. Although abdominal pressure, a sort of baseline variable, should be distributed evenly between the groups, intra-abdominal pressure should be measured in each ARDS patient to establish whether plateau pressure actually reflects pulmonary pressure rather than a decrease in chest wall compliance.
Summary of main results
In our previous updated review we found evidence that a ventilation strategy using a tidal volume equal or less than 7 ml/kg of measured body weight and plateau pressure less than 31 mm H
In all the selected studies, although the primary purpose of the investigators was to compare two different tidal volumes other elements of the ventilatory strategy were associated with it. The experimental intervention included permissive hypercapnia, variable levels of PEEP and low plateau airway pressure. The traditional intervention consisted of higher tidal volume, normocapnia, lower levels of PEEP and potentially higher plateau pressures. Some of these co-interventions were unavoidable and consequent to the nature of the main intervention. Therefore, the studies performed a comparison of two approaches rather than two single interventions and caution is required in interpreting these results, especially when analyses have been inspired by looking at the available aggregate data, which makes it difficult to assess the respective importance of each factor.
A possible interpretation of the discordant results, as proposed by Gattinoni (Gattinoni 2002), could involve variations of trans-pulmonary pressure, which is the distending force of the lung, in the individual patient. The high volume might induce lung damage when the resulting trans-pulmonary pressure is high. Conversely, when trans-pulmonary and airway pressure are within the safe limits high or intermediate tidal ventilation (8 to 10 ml/kg) could be used thus avoiding potentially deleterious effects of low tidal volume, Thus, when chest wall compliance is low a higher plateau pressure may be necessary to reach the same trans-pulmonary pressure, without an increase in tidal volume.
Some questions still remain open. The treatment may be effective only in a subgroup of patients. The lower tidal volume ventilation may be clinically worthwhile only in the more severely ill patients.
Quality of the evidence
The issue of adverse effects of a lower tidal volume was not tackled in the studies. Specifically, the impact of acidosis and hypercapnia on the development of organ failure was not clear.
Ventilation with lower tidal volume can be very effective for short-term lung recovery but its impact on the development of organ failure and long-term recovery is still uncertain or unknown.
Potential biases in the review process
The statistic I
There is a 'hidden' source of heterogeneity. Although statistical tests of heterogeneity were not significant, we showed that studies were clinically different in some points. Furthermore, most of the trials did not report protocols of concomitant treatments and associated diseases (that is ventilator-associated pneumonia).
There is a subgroup of patients that can be ventilated with lower tidal volumes or volumes in the conventional range, as long as the plateau pressure is kept below 31 cm H
Agreements and disagreements with other studies or reviews
A previous meta-analysis (Eichacker 2002) investigated whether differences in treatment effect could be explained by differences in plateau pressure associated with either the control or low tidal volumes. The authors concluded that treatment in controls differed from current practice in terms of too-high tidal volumes and plateau pressure, and this difference may have influenced the outcomes in two trials (Amato 1998; ARDS Network 2000). Although the method applied in the work by Eichacker has been questioned (Petrucci 2003), his study suggests that as long as tidal volumes produce airway pressures considered safe (31 cm H
Implications for practice
Overall, the relative risk of death at day 28 is reduced by using ventilation with a lower tidal volume. Hospital mortality is also beneficially affected but there is insufficient evidence to draw any conclusions about morbidity and long-term outcomes. Ventilation with a higher tidal volume and higher plateau pressure is associated with increased risk of death, but the independent contribution of higher tidal volume (over-distension) or higher plateau pressure (barotrauma) cannot be identified. Lower tidal volume ventilation may be preferable when lung recovery is a priority.
Implications for research
Further data are required to assess the long-term health-related quality of life, long-term cognitive outcomes and cost. Large, adequately designed trials with subgroup analysis would be able to determine whether an 'intermediate' tidal volume of 8 to 10 ml/kg IBW would be beneficial, too.
We would like to thank Prof Marcus Müllner, Prof Nathan Pace, Dr Asima Bokhari, Dr Mark Davies, Janet Wale, Nete Villebro and Kathie Godfrey for commenting on the original review (Petrucci 2004a), and Dr Harald Herkner and Prof Nathan Pace for commenting on the updated review. We would like to thank Karen Hovhannisyan for help in searching the literature.
Data and analyses
- Top of page
- Authors' conclusions
- Data and analyses
- What's new
- Contributions of authors
- Declarations of interest
- Sources of support
- Index terms
Appendix 1. CENTRAL and The Cochrane Library search strategy
#1 MeSH descriptor Respiratory Distress Syndrome, Adult explode all trees
#2 acute lung injury
#3 Adult Respiratory Distress Syndrome
#4 Acute Respiratory Distress Syndrome
#5 ARDS or ALI
#6 (#1 OR #2 OR #3 OR #4 OR #5)
#7 MeSH descriptor Tidal Volume explode all trees
#8 artificial near ventilation
#9 tidal volume
#10 protective near ventilation
#13 (#7 OR #8 OR #9 OR #10 OR #11 OR #12)
#14 (#6 AND #13)
Appendix 2. Search strategy for MEDLINE (OvidSP)
1. exp Respiratory Distress Syndrome, Adult/ or Adult Respiratory Distress Syndrome/ or Acute Lung Injury/ or Acute Respiratory Distress Syndrome/ or ARDS.mp. or ALI.mp.
2. exp tidal volume/ or exp respiration, artificial/ or tidal volume.mp. or (protective adj3 ventilat*).mp. or (pressure* adj3 limited*).mp. or LPVS.mp.
3. 1 and 2
4. ((randomized controlled trial or controlled clinical trial).pt. or randomized.ab. or placebo.ab. or clinical trials as topic.sh. or randomly.ab. or trial.ti.) not (animals not (humans and animals)).sh.
5. 3 and 4
Appendix 3. Search strategy for EMBASE (OvidSP)
1. respiratory distress syndrome/ or respiratory distress/ or acute lung injury/ or adult respiratory distress syndrome/ or acute respiratory failure/ or adult respiratory distress syndrome.mp. or acute lung injury.mp. or acute respiratory distress syndrome.mp. or (ards or ali).mp.
2. tidal volume/ or artificial ventilation/ or tidal volume.mp. or (protective adj3 ventilation).ti,ab. or pressure* limited*.mp. or LPVS.ti,ab.
3. 1 and 2
4. (placebo.sh. or controlled study.ab. or random*.ti,ab. or trial*.ti,ab. or ((singl* or doubl* or trebl* or tripl*) adj3 (blind* or mask*)).ti,ab.) not (animals not (humans and animals)).sh.
5. 3 and 4
Appendix 4. Search strategy for CINAHL (EBSCOhost)
S1 ( (MH "Respiratory Distress Syndrome, Acute") OR (MH "Respiratory Distress Syndrome") OR (MH "Acute Lung Injury") ) OR ( ARDS or ALI ) OR ( (Respiratory Distress Syndrome and (acute or adult)) )
S2 (MM "Tidal Volume") OR TI ( artificial and ventilation ) OR TX tidal volume OR AB ( protective and ventilation ) OR AB pressure limited
S3 S1 and S2
Appendix 5. Search strategy for ISI Web of Science
#1 TS=(adult respiratory distress syndrome or acute lung injury or acute respiratory distress syndrome or ards or ali)
#2 TS=(tidal volume) or TS=(protective SAME ventilation) or TS= LPVS
#3 TS=(random* or placebo* or multicenter* or prospective) or TS=(trail* SAME (clinical or controlled))
#4 #1 and #2 and #3
Last assessed as up-to-date: 3 September 2012.
Protocol first published: Issue 4, 2002
Review first published: Issue 3, 2003
Contributions of authors
Conceiving the review: Nicola Petrucci (NP)
Co-ordinating the review: NP
Undertaking manual searches: NP and CDF
Screening search results: NP
Organizing retrieval of papers: NP
Screening retrieved papers against inclusion criteria: NP
Appraising quality of papers: NP and CDF
Abstracting data from papers: NP and CDF
Writing to authors of papers for additional information: NP
Data management for the review: NP
Entering data into Review Manager (RevMan): NP
RevMan statistical data: NP
Double entry of data: (data entered by person one: NP ; data entered by person two: CDF)
Interpretation of data: NP and CDF
Statistical inferences: NP
Writing the review: NP
Guarantor for the review (one author): NP
Declarations of interest
Sources of support
- Azienda Ospedaliera Desenzano, Desenzano del Garda, Italy.
- No sources of support supplied
Medical Subject Headings (MeSH)
MeSH check words
* Indicates the major publication for the study