Noninvasive mechanical ventilation with high pressure strategy remains a “double edged sword”?

Abstract

Dear editor We read with great interest the original work by Murphy et al analyzing the effects of two treatment strategies for delivery of noninvasive mechanical ventilation in hypercapnic patients with chronic obstructive pulmonary disease.1 High pressure and high intensity noninvasive mechanical ventilation were compared in a short-term crossover trial to assess whether high intensity noninvasive mechanical ventilation (inspiratory pressure > 25 cm H2O associated with a high backup ventilator rate) may improve adherence, physiological, and subjective outcomes when compared with delivery of high pressure noninvasive mechanical ventilation (without elevated backup respiratory rate). The authors concluded that both strategies are equivalent in all the recorded outcomes, showing thus that driving pressure, but not backup respiratory rate, is essential to gain physiological and clinical benefits in this population when in a chronic stable condition. Despite previous randomized studies showing the potential benefits of long-term noninvasive mechanical ventilation in hypercapnic patients with chronic obstructive pulmonary disease, current research has still not clearly indicated the best strategy to improve the patient’s adherence with treatment.2,3 Overall, dropout during noninvasive mechanical ventilation remains a serious clinical problem.4 This study provides valuable information in this regard, suggesting that sufficiently high-pressure delivery is enough to achieve useful clinical and physiological goals. This notwithstanding, we believe that some of the expectations following the adoption of these different noninvasive mechanical ventilation strategies have not been adequately addressed in the present study. Therefore, we consider that it would be useful, from a practical point of view, to underline some points in this regard. First, the authors did not determine what effects the highest respiratory backup rate used in their study may have had. Although there have been no major studies published on application of high levels of backup that have proved to be useful in patients with severe chronic obstructive pulmonary disease, this is the best indication for hypoventilation syndromes, ie, obesity and overlap syndromes. In fact, we cannot exclude that addition of a high backup respiratory rate may help to resolve “overlap” when present at a subclinical level in patients with chronic obstructive pulmonary disease, or that it has not been adequately assessed before. However, it seems that the authors selected backup respiratory rate levels on a clinical basis without any physiological assessment in their study population. Despite patients in the present study not appearing to show any abnormal increase in their body mass index, the extrapolated conclusion of a lack of additional benefit from a well assessed strategy, including adequate backup respiratory rate, cannot be firmly excluded in such “extreme” cases.5 Second, there was a lack of complementary tests in this study that might have helped in analysis of the data. Indeed, the authors selected patients with a FEV1 (forced expiratory volume in one second) that could worsen with high backup and pressure, especially with the auto-positive end-expiratory pressure mechanism. It is not clear how selection of expiratory-positive airway pressure was made in the study population. Similarly, the authors did not take into account any potential auto-positive end-expiratory pressure effects during the 6-week period of observation. Third, the authors arbitrarily selected a population of hypercapnic patients with chronic obstructive pulmonary disease (daytime PaCO2 > 6 kPa) which would not be universally recognized as the most appropriate in terms of risk of frequency of exacerbations and clinical instability, and it is not clear whether any other additional clinical factors behind cardiac dysfunction may have interfered at admission or over the study period.6 Indeed, three of the five patients who withdrew did so because of factors other than mere mask/pressure intolerance (see Table E1).1 Final, there was no analysis of potential implications of air leakage in the observed results. No mention was made of measurement or monitoring of leakage during application of noninvasive mechanical ventilation. This aspect could have been potentially relevant and interfered with the results, especially during application of such high-pressure delivery, which is known to increase mask leakages.4 To conclude, we recognize that the paper by Murphy et al1 will add information to the complex process of setting and titration of noninvasive mechanical ventilation in the population of stable hypercapnic patients with chronic obstructive pulmonary disease. However, given the observations discussed, we are convinced that further studies of longer duration and including larger numbers of patients are needed to determine which physiological effects should be assessed and expected during application of both strategies. Currently, high-pressure strategies remain a “double edged sword” in daily practice.