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.