To the Editors:
We read with interest the systematic review of Kolodziej et al. 1 about noninvasive positive-pressure ventilation (NPPV) in severe stable chronic obstructive pulmonary disease (COPD). First of all, we would like to compliment the authors on their excellent review. It is extremely important that good-quality reviews are published in the field of NPPV in severe stable COPD. The development of new therapeutic options in these patients is increasingly being recognised as urgently needed 2.
However, we would like to comment on the conclusions Kolodziej et al. 1 draw in their review. They conclude that bilevel NPPV used in a select proportion of patients with severe stable COPD can improve gas exchange, exercise tolerance, dyspnoea, work of breathing, frequency of hospitalisation, health-related quality of life and functional status. Following this, they suggest an adjunctive role for the use of bilevel NPPV in the management of chronic respiratory failure due to COPD.
The first remark we would like to make is that their conclusions were based mostly on nonrandomised controlled trails (RCTs). Combined analysis of the results of the RCTs did not show the effect on arterial blood gases, exercise tolerance, work of breathing or hospitalisations. Evidence of an improved health-related quality of life was derived from only two studies 3, 4. Furthermore, in the study by Garrod et al. 4, the NPPV group had very low baseline Chronic Respiratory Questionnaire scores, which may have influenced their positive outcome.
Secondly, Kolodziej et al. 1 pooled studies that differed in length, control intervention and type of ventilation (daytime and nocturnal). They assessed their data on heterogeneity in study quality, patients, interventions and measurement of outcomes, and they showed that heterogeneity was evident in many parameters. This prohibits strong conclusions that NPPV is as effective in severe stable COPD.
In our opinion, in the review by Kolodziej et al. 1, there was limited discussion about the importance of achieving effective ventilation. It is suggested that with higher hours of ventilatory use, greater reduction in hypercapnia can be achieved. While this might be true for nocturnal ventilation, with daytime ventilation, considerable effects might be achieved with a reduction in hours of NPPV use. Of the RCTs included, a significant reduction in hypercapnia during spontaneous breathing of room air was shown only in the study by Díaz et al. 5. This study, and the more recent study of the same group 6, showed that considerable effects can be achieved with 3 h of NPPV during the daytime. During the night, increased upper airway resistance, decreased respiratory drive and less supervision might lead to delivery of a reduced volume to the patient. Therefore, correct monitoring of whether or not effective ventilation is achieved is very important, especially during the night. Kolodziej et al. 1 address this issue of more dynamic monitoring of effectiveness of NPPV. However, they imply that dynamic monitoring by transcutaneous measurements is preferable to arterial blood gases alone. Transcutaneous measurement of carbon dioxide with current techniques tends to drift overnight 7. In our opinion, measuring multiple arterial blood gas samples during NPPV is the gold standard. Unfortunately, until now, no RCT has monitored the effectiveness of their intervention in this way.
Our second remark relates to the importance of using high inspiratory pressures. Even higher pressures than used in most RCTs might be necessary to achieve normocapnia 8, although no clear evidence exists on exactly how high pressures should be.
Finally, we would like to comment on the selection of appropriate patients. Patients with very severe COPD seem to benefit most. Kolodziej et al. 1 emphasise that patients with severe hyperinflation may benefit most. However, too little evidence currently exists to make a clear statement about whether patients should be selected on the basis of the severity of chronic respiratory failure, hyperinflation or the height of the work of breathing.
To conclude, while the review of Kolodziej et al. 1 is timely and a major contribution, we feel the strength of the conclusions is overstated. With this review in hand, some of the gaps in our knowledge are carefully uncovered, which should lead to well-designed randomised controlled trials of sufficient power. Some are undoubtedly underway.
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