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(Chest. 2003;124:337-343.)
© 2003 American College of Chest Physicians

A Meta-analysis of Nocturnal Noninvasive Positive Pressure Ventilation in Patients With Stable COPD*

Peter J. Wijkstra, MD, PhD; Yves Lacasse, MD; Gordon H. Guyatt, MD, FCCP; Ciro Casanova, MD, PhD; Peter C. Gay, MD, FCCP; Jeffry Meecham Jones, MD and Roger S. Goldstein, MD, FCCP

* From the Division of Respiratory Medicine (Dr. Goldstein), University of Toronto, West Park Hospital, Toronto, ON, Canada; Department of Home Mechanical Ventilation (Dr. Wijkstra), Department of Pulmonary Diseases, University Hospital Groningen, the Netherlands; Centre de Pneumologie (Dr. Lacasse), Hopital Laval, Ste-Foy, QC, Canada; Department of Clinical Epidemiology and Biostatistics (Dr. Guyatt), McMaster University, Hamilton, ON, Canada; Department of Pulmonary and Internal Medicine (Dr. Casanova), Hospital Universitario de Canarias, Tenerife, Spain; Division of Pulmonary and Critical Care Medicine and Internal Medicine (Dr. Gay), Mayo Clinic, Rochester, MN; and Department of Thoracic Medicine (Dr. Meecham Jones), Gloucestershire Royal Hospital, Gloucester, UK.

Correspondence to: Peter J. Wijkstra, MD, PhD, University Hospital, Department of Pulmonary Diseases/Home Mechanical Ventilation, Postbox 30001, 9700 RB, Groningen, the Netherlands; e-mail:p.j.wijkstra{at}int.azg.nl


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: The potential benefits of noninvasive positive pressure ventilation (NIPPV) for patients with COPD remains inconclusive, as most studies have included only a small number of patients. We therefore undertook a meta-analysis of randomized controlled trials (RCTs) that compared nocturnal NIPPV with conventional management in patients with COPD and stable respiratory failure.

Design: RCTs were identified from several sources, such as MEDLINE, EMBASE, and CINAHL. In addition, records were identified through hand searching of abstracts from meetings of the American Thoracic Society, the American College of Chest Physicians, and the European Respiratory Society.

Patients: Patients with COPD according to the definition of the American Thoracic Society.

Interventions: NIPPV applied via a nasal or facemask for at least 5 h/d for at least 3 weeks. Patients in the actively treated group continued to receive the usual management for COPD. The control group received the same management as the study group but did not receive NIPPV.

Measurements and results: PaCO2, PaO2, 6-min walking distance (6MWD), respiratory muscle function, FEV1, vital capacity, and sleep efficiency (time asleep as a percentage of total time in bed) were used as outcome measures. The publications were reduced to 10 potentially eligible articles from 164 publications retrieved from computer searches and 8 further abstracts. Four trials were finally included in the meta-analysis. The only outcome for which the confidence intervals excluded zero was maximal inspiratory pressure (PImax). The confidence intervals for the other outcomes included zero. The mean treatment effects for FEV1 and PImax were small, whereas it was moderate for the 6MWD. Small negative effects were found for the outcomes of vital capacity, PaCO2, and sleep efficiency.

Conclusions: This meta-analysis of 3 months of NIPPV in patients with stable COPD showed that ventilatory support did not improve lung function, gas exchange, or sleep efficiency. The high upper limit of the confidence interval for the 6MWD suggested that some people do improve their walking distance. The small overall sample size precluded a clear clinical direction regarding the effects of NIPPV in patients with COPD.

Key Words: COPD • meta-analysis • nocturnal noninvasive positive pressure ventilation • respiratory insufficiency


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
COPD is an important cause of morbidity and mortality worldwide. Although there are a wide range of therapeutic approaches to assist patients with this condition, only the provision of long-term oxygen therapy (LTOT) for patients with resting awake hypoxemia has been shown to prolong life.1 2 Other treatments are largely symptomatic. Clinicians frequently prescribe bronchodilators to improve airflow and relieve dyspnea. Even with the optimal use of pharmacotherapy, patients with COPD often have dyspnea that limits their exercise tolerance and health-related quality of life (HRQOL). Randomized trials have demonstrated that respiratory rehabilitation improved dyspnea, exercise tolerance, and HRQOL.3 4 5 6 As a result, rehabilitation has become part of the standard of care for the more severely affected patients. Newer approaches to the management of COPD, such as lung transplantation and lung volume reduction surgery,7 are likely to benefit only a small number of highly selected individuals.

When patients with COPD acquire acute respiratory failure, noninvasive positive pressure ventilation (NIPPV) provides a safe, effective way of stabilizing the arterial blood gas (ABG) levels while avoiding the risks and inconvenience of intubation.8 9 Short-term NIPPV has therefore become an accepted management approach for patients with acute hypercapnia; however, it remains unclear as to whether NIPPV can also play a useful role in improving either ABG levels or functional ability among stable patients with chronic respiratory failure. Theoretical reasons why it might include resetting of the respiratory center to improve daytime gas levels; resting dysfunctional respiratory muscles, thereby increasing their daytime strength and endurance; improving peripheral muscle function from a better milieu (pH, PaO2, PaCO2); and preventing repeated nocturnal arousals, thereby improving the quality of sleep.

Many of the trials of NIPPV in patients with stable COPD have been uncontrolled10 11 12 or included only small numbers of patients.13 14 Although the types of patients most likely to benefit from NIPPV have been suggested,15 the equivocal evidence prevented a consensus as to the indications for NIPPV among those with stable COPD.16 We therefore undertook a meta-analysis of individual data from randomized controlled trials (RCTs) that compared NIPPV with conventional management of patients with COPD and stable respiratory failure. Although the number of trials was small, this meta-analysis provides an up-to-date summary of the results of this new management approach. Primary outcome measures included gas exchange, pulmonary function, functional exercise capacity, and HRQOL.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Search Strategy
RCTs were identified from several sources. Through the Cochrane Airways group registry, we searched for RCTs in MEDLINE, EMBASE, and CINAHL in all languages. The following terms were used to identify trials related to NIPPV: nasal ventilation or positive pressure or NIPPV. The bibliographies of all relevant articles were hand searched for additional articles that may contain RCTs. In addition, records were identified through hand searching of abstracts from meetings of the American Thoracic Society (1980 to 2000), the American College of Chest Physicians (1980 to 2000), and the European Respiratory Society (1987 to 2000).

Eligibility Criteria
Study Population: The study population included patients with stable COPD according to the definition of the American Thoracic Society17 : a disease state characterized by the presence of airflow obstruction due to chronic bronchitis or emphysema; and the airflow obstruction is generally progressive, may be accompanied by airway hyperactivity, and may be partial reversible.

Intervention: The intervention in the treated group was nocturnal NIPPV applied via nasal or facemask for at least 5 h/d for at least 3 weeks. Patients in the actively treated group continued to receive the usual management for COPD. The control group received the same management as the study group but did not receive NIPPV.

Study Selection
Two primary reviewers (P.J.W., R.S.G.) assessed all abstracts that were identified either after the search by the Cochrane airways group or from the hand search. Both reviewers independently selected trials for inclusion according to prior agreement regarding the study population and the intervention. If one of the reviewers concluded an abstract might be eligible, the complete article was retrieved and reviewed in detail by both reviewers. If the reviewers disagreed, a third reviewer arbitrated. We requested the individual data from the authors of the eligible primary studies.

Types of Outcome Measures: The primary outcome measures were PaCO2, PaO2, 6-min walking distance (6MWD), dyspnea during daily activities, HRQOL, and respiratory muscle function (muscle endurance or muscle strength). Secondary outcomes included lung function (FEV1 and vital capacity) and sleep efficiency (time asleep as a percentage of total time in bed).

Validity Assessment: We considered two important potential sources of bias that have proved to be major determinants of the magnitude of the effect size in clinical trials: unconcealed randomization and unblinded study personnel. The former has been associated with an overestimation of the treatment effect by up to 40%,18 and the latter may result in differential interpretations or encouragement during test performance.19 If the original publication did not specify details pertaining to randomization, blinding, dropouts, and withdrawals,20 the principal investigator obtained clarification from the primary author. No attempt was made to attribute a global score of scientific quality to each trial; instead, the validity assessment for each aspect was considered separately.

Statistics: The principal investigators of the trials included in the meta-analysis provided the individual data for each of the subjects who completed their study ("per-protocol" analysis). In the case of crossover trials, we considered only the first study period (prior to the crossover). Study outcomes were expressed in the same units across the trials. For each individual and for each outcome, we calculated the absolute change in score. Within the trials, the treatment effect was then determined from the difference between the mean changes in the treatment and control groups. The treatment effects were weighted by the inverse of the population variance and combined according to a random-effects model.21 We report only the pooled treatment effects and associated 95% confidence intervals. Homogeneity was tested by the method described by Fleiss.22 Statistical significance (p < 0.1 from the test of homogeneity) suggested that the observed differences in the treatment effects was significantly greater than expected by chance if all studies shared a common treatment effect.21 Subgroup analysis would be feasible if sufficient studies and a large enough sample size were to be included in the analysis and if significant heterogeneity was found among the outcomes of the trials. Two respirologists (P.J.W., R.S.G.) identified, a priori, potential sources of heterogeneity among the primary and secondary outcomes. We postulated the following sources of heterogeneity: (1) patients with greater hypercapnia might benefit more from NIPPV; (2) the benefits of NIPPV might be greater in patients who received adequate training with NIPPV; (3) the benefits of NIPPV might be greater among those who used it for longer periods; (4) the greater the span between inspiratory and expiratory positive airway pressure, the greater the benefit of NIPPV; and (5) the more supervision in the home setting, the greater the benefits of NIPPV.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The publications were reduced to 10 potentially eligible articles from 164 publications retrieved from computer searches and 8 further abstracts identified. Four trials were excluded for the following reasons: the study was not randomized23 and the duration of bilevel pressure ventilation was too short (< 5 h per night),24 25 and the training period of bilevel pressure ventilation was too short (< 3 weeks).26 Two publications (abstracts) pertained to long-term European multicenter studies of NIPPV in patients with COPD. They have not yet been submitted for publication; therefore, no additional data could be obtained from the authors of these studies.27 28 Both reviewers (P.J.W., R.S.G.) agreed to include four articles in the meta-analysis (weighted {kappa} = 0.91). The four trials included in the meta-analysis13 14 29 30 are summarized in Table 1 . The authors of these articles provided individual data for each of the subjects in these trials, and their individual treatment effects are summarized in Table 2 . Two studies13 30 both concealed randomization and blinded outcome assessments. Outcomes of the meta-analysis are presented in Table 3 . The only outcome for which the confidence intervals excluded zero was maximal inspiratory pressure (PImax). The confidence intervals for the other outcomes included zero. The mean treatment effects for FEV1 and PImax were small, whereas it was moderate for the 6MWD. Small negative effects were found for the outcomes of FVC, PaCO2, and sleep efficiency. The results were homogeneous across all the outcomes (p > 0.1), with the exception of maximal expiratory pressure (PEmax). This might have been due to a lack of power. Given the small number of studies and sample size, a meaningful subgroup analysis was not feasible.


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  		Table 1.. RCTs of Nocturnal NIPPV*

 

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  		Table 2.. Treatment Effect for Each Outcome in the Trials Included in the Meta-analysis*

 

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  		Table 3.. Primary Results of the Meta-analysis

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
This meta-analysis showed that nocturnal NIPPV for 3 months in hypercapnic patients with COPD did not have clinically or statistically significant effects on lung function, gas exchange, or sleep efficiency. The small sample size precludes a definitive statement regarding the clinical implications of NIPPV, other than stating that at present there is insufficient evidence from short-term trials to support its widespread use. The small positive treatment effects for PImax and PEmax are unlikely to be clinically relevant. The improvement of 27.5 m in the 6MWD could be more encouraging, especially given the high upper limits of the confidence interval for the 6MWD. Guyatt et al31 showed that even when the mean difference between an intervention group and a control group is appreciably less than the minimal clinically important difference, which is 55 m for the 6MWD,32 treatment might still have an important impact on many patients. The upper limit of the confidence interval of 82 m for the 6MWD in this meta-analysis suggests that it remains possible that NIPPV might have substantial beneficial effects on walking in at least some patients. Finally, we did assess treatment effects for dyspnea. The reason is that in three studies four different scales were used: Casanova et al30 used both the Medical Research Council dyspnea scale and Borg scale, while Strumpf et al14 used the Mahler scales of dyspnea index. Finally, Gay et al13 used a subjective commentary.

The design of this meta-analysis included only studies in which nocturnal NIPPV was applied for at least for 5 h per night. This excluded two studies that reported beneficial effects from NIPPV administered for 2 h during the day,24 25 which could be criticized on the basis of insufficient acclimatization. In keeping with the application of mechanical ventilatory support for patients with thoracic restriction or neuromuscular conditions, both of which have been found to be beneficial for patients in respiratory failure,10 33 we considered nighttime ventilation to be the most appropriate clinical approach and reasoned that several hours would be required to achieve therapeutic goals. Furthermore, a minimum duration of 3 weeks was chosen, as from our own clinical experience we were aware that it might take up to 2 weeks just for mask fitting, adjustment, and patient familiarization with noninvasive ventilation. Therefore, a study in which NIPPV was assessed for only 2 weeks26 was excluded from the analysis.

The use of NIPPV in patients with stable COPD has been controversial, with some reports suggesting effectiveness and others suggesting no beneficial effects. An interesting debate as to whether NIPPV was effective in patients with COPD was published16 ; in this report, it was suggested that studies with positive findings probably included patients who had higher levels of daytime hypercapnia than those with no observed effects of NIPPV. Uncontrolled studies that included patients with a higher level of hypercapnia then we had in our analysis (60 mm Hg vs 52 mm Hg) did show decreases in PaCO2,12 34 a decrease in the duration of hospital admissions,11 and an improvement in quality of life.34

Several issues remain outstanding and might influence the outcome of clinical trials of NIPPV. The most appropriate bilevel pressure ventilation settings need to be established. It is unclear whether pressures of 10 to 14 cm H2O13 26 are the optimal pressures for improving ventilation in patients with COPD. In the only RCT that demonstrated positive results, Meecham Jones and Paul29 used mean inspiratory positive airway pressures of 18 cm H2O (range, 16 to 22 cm H2O), suggesting that perhaps higher ventilating pressures might be more effective.

Another important issue may be the extent of the nocturnal hypoventilation. In one study,29 patients with the greatest nocturnal hypoventilation benefited most from NIPPV. These were the patients in whom the PCO2 decreased the most at night and the ones with the greatest decrease in resting daytime PaCO2 (r = 0.69, p = 0.01). If this is the case, subjects with the higher drop in nocturnal PCO2 might be the most suitable for NIPPV.

Unlike patients with thoracic restriction or those with neuromuscular conditions, patients with COPD require a longer period of adjustment to mechanical ventilatory support. It is therefore possible that additional training with a longer time for phasing in assisted ventilation might result in better acceptance, greater usage, and improved outcomes from NIPPV. Alternatively, only patients who can be trained to use NIPPV and in whom the PCO2 falls during ventilatory support should be enrolled in an RCT.

Alternatively, it might be that those with the greatest respiratory muscle dysfunction are most likely to benefit from the muscle rest associated with NIPPV.35 This was the hypothesis behind a major trial of negative pressure ventilation (NPV) reported by Shapiro et al,36 who randomized 184 patients with severe COPD to active or sham ventilation with a poncho wrap negative pressure ventilator. There were no significant changes in 6MWD, ABG levels, or respiratory muscle strength. Although the investigators encouraged the patients to use the ventilator for at least 5 h/d, the average duration of use was closer to 3 h and the intensity of the treatment intervention was quite variable. Celli et al37 and Zibrak et al38 also failed to identify improvements in ABG levels or respiratory muscle strength with NPV. As with NIPPV, these studies with negative findings included patients with a mean PaCO2 of 45 mm Hg, whereas the studies with positive findings included patients with higher PaCO2 levels.39 40 41 Neither studies on NPV or NIPPV have clarified whether, in the few patients who did improve, the benefits related to improvements in respiratory muscle function or in gas exchange.35

The duration of ventilatory support may also be relevant. Studies in this meta-analysis were of relatively short duration (3 months); even if their results were positive, survival would not likely have been influenced. Two European studies reported in abstract form27 28 are still underway. Clini and Sturani27 compared 42 patients with severe COPD who received LTOT and NIPPV with 42 patients who received LTOT alone. Cumulative days spent in hospital due to respiratory exacerbations showed a trend in favor of those receiving NIPPV (12.6 ± 7.9 days vs 16.9 ± 10.3 days, respectively). Although it seems interesting to include this trail in the meta-analysis, it is questionable what it adds because only blood gas levels were assessed after 3 months. Muir et al28 compared 60 patients with severe COPD who received LTOT and NIPPV with 62 patients who received LTOT alone. After a median follow-up of 4.7 years, there were no significant differences in survival between the groups, with the exception of patients > 65 years old in whom survival was better in the NIPPV plus LTOT group.

It is possible that the combination of NIPPV and supervised rehabilitation might be more effective than NIPPV alone. Garrod et al42 randomized 45 patients with severe COPD (mean PaCO2 of 45 mm Hg) to receive 12 weeks of NIPPV plus exercise training or exercise training alone. Although the two groups did not differ in HRQOL, the shuttle walk distance improved significantly more (72 m, p < 0.01) in those receiving NIPPV and rehabilitation. Perhaps among those patients with hypercapnia NIPPV might be a useful adjunct to rehabilitation.

In summary, in this meta-analysis of NIPPV in patients with COPD, 3 months of ventilatory support did not improve lung function, gas exchange, or sleep efficiency. The high upper limit of the confidence interval for the 6MWD suggested that some people might improve their walking distance. The small overall sample size precluded a clear clinical direction regarding the effects of NIPPV in patients with COPD. Additional studies with larger sample sizes that address patient selection, ventilator settings, training, and length of ventilation, as well as adjunctive measures such as rehabilitation, are necessary to clarify the role of this treatment.


   Acknowledgements
 
We thank Carol C. Carlisle (Rhode Island Hospital, Providence, RI) for helping us with the data from their study,14 which was included in this meta-analysis.


   Footnotes
 
Abbreviations: ABG = arterial blood gas; HRQOL = health-related quality of life; LTOT = long-term oxygen therapy; NIPPV = noninvasive positive pressure ventilation; NPV = negative pressure ventilation; PEmax = maximal expiratory pressure; PImax = maximal inspiratory pressure; RCT = randomized controlled trial; 6MWD = 6-min walking distance

Received for publication April 15, 2002. Accepted for publication December 6, 2002.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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