To the Editor:
I read the interesting study by Monteiro et al.1 However, I would like to point out the relevant findings by Cordoni et al.2 Monteiro et al reported 17 (37%) patients with COPD and substantial dynamic hyperinflation (DH) as defined by at least a 15% reduction from pre-exercise inspiratory capacity (IC). They concluded that the application of expiratory positive airway pressure (EPAP) reduced DH, as shown by lower operational lung volumes after submaximal exercise in COPD patients who previously manifested exercise DH.1
Cordoni et al evaluated 30 patients with COPD.2 Nineteen (63.30%) patients had significant reduction in IC and were classified in the DH group, and 11 showed no reduction in IC or even had an increase in IC and were therefore included in the no-DH group.2
Interestingly, although the DH group had a greater pre-exercise IC, this group had an abrupt decline in the rest-exercise transition period. Statistically significant correlations among the variations in IC, tolerance limited, and dyspnea score at the end of test were found only in the DH group.2
Cordoni et al presented more detailed analysis of behavior of operating lung volumes in the DH group, which revealed different patterns on decline of IC throughout the test. Among the 19 patients with DH group, the decrease in IC in the rest-exercise transition showed that at the 2 min mark 7 patients had progressive reduction in IC (progressive DH), and 12 had a stable pattern of reduction in the IC (stable DH). The subgroup of the DH group with progressive reduction in IC had more pulmonary function test impairment at rest, lower exercise tolerance, possibly greater perception of dyspnea, and more perception of leg fatigue.2
From a clinical standpoint, those with progressive DH would benefit the most from interventions that decrease muscle overload, such as the use of bronchodilators and EPAP.2 The valuable findings of Monteiro et al1 in pointing out the benefits of utilizing EPAP in patients with DH and decreased IC, should probably be reevaluated in view of the findings of Cordoni et al,2 and be applied to patients with documented progressive DH. Perhaps narrowing the application of EPAP to patients with COPD and progressive DH, since Cordoni et al found that the DH group had more clinical and laboratory findings of impairment, would result in better utilization of resources and augmentation of treatment outcome.
Footnotes
The author has disclosed no conflicts of interest.
- Copyright © 2013 by Daedalus Enterprises
References
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The Authors Respond:
We would like to thank Dr Alkhuja for his comments regarding our paper.1 The etiology of exercise intolerance in patients with COPD is considered to be multifactorial, with dynamic hyperinflation (DH) having been identified as a key component. Physiology-based strategies to reduce hyperinflation and improve exercise tolerance are important tools in the management of patients with COPD. Four main interventions that reduce hyperinflation during exercise have been identified: bronchodilators, supplemental oxygen, helium/oxygen inhalation, and exercise training reconditioning.2
We demonstrated in our study that the application of expiratory positive airway pressure reduces exercise-related DH.1 The kinetic behavior of DH seems to be more complex than previously thought, however, with some patients able to delay3 or even avoid4 its development during cycle ergometry.
Cordoni et al5 demonstrated DH patterns using ambulation, rather than cycling, in patients with moderate to severe COPD, showing that these patterns are not artifacts of cycling exercise. They further demonstrated that a pattern of progressively increasing operating lung volumes in DH patients has a greater impact on exercise tolerance than does a stable pattern. These differing patterns may explain why not all patients derive similar exercise tolerance benefits from DH-reduction strategies.6
We agree with Dr Alkhuja that patients with progressive DH would see the most benefit from interventions aimed at reducing DH. Although this issue was not directly investigated in our study, we believe that our patient population consisted mainly of patients with progressive DH. We limited our investigation to patients with previously demonstrated overt DH, defined as an inspiratory capacity reduction of at least 15% from rest. As demonstrated in Figure 2 of Cordoni et al, patients with progressive DH show greater decrements in inspiratory capacity with exercise.
The criterion used to classify DH is also relevant. Recently, Louvaris et al studied COPD patients with and without HD. In order to make this classification, they adopted the criterion of an inspiratory capacity decrease of greater than 4.5% of the baseline value, or greater than 150 mL.7 Cordonie et al considered DH to constitute any drop in inspiratory capacity, compared with the baseline value. This discrepancy in definition undermines the comparability of the findings as well as the ability of this classification to identify patients who will benefit from DH treatments.
In patients with DH, a pattern of progressive reduction in operative lung volumes has a greater impact on exercise tolerance than does a stable pattern.6 This reinforces the importance of constant monitoring of these variables during exercise, not just before and after activity.
Additional studies are needed to measure the impact of DH patterns on pharmacologic and nonpharmacologic interventions aimed at reducing DH and improving exercise tolerance in patients with COPD. As Dr Alkhuja points out, this would certainly result in better resource utilization and improved treatment outcomes.
Footnotes
The authors have disclosed no conflicts of interest.
References
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