MI Polkey, D Kyroussis, GH Mills, CH Hamnegard, SE Keilty, M Green and J Moxham
Respiratory Muscle Laboratories, Kings College and Royal Brompton Hospital, London, United Kingdom.

When patients with COPD walk to a state of intolerable dyspnea, there is excessive inspiratory muscle loading, as evidenced by slowing of the maximum relaxation rate of the inspiratory muscles, measured from esophageal pressure during a sniff (Sn Pes MRR). In this setting, inspiratory pressure support (IPS) delivered via an orofacial mask increases walking distance and reduces dyspnea, but the mechanism by which this benefit is achieved remains unclear. In this study we compared Sn Pes MRR after equidistant treadmill walking in six men with severe COPD (mean FEV1: 0.6 L, 22% predicted). After the free walk there was a mean slowing of Sn Pes MRR of 41% (p < 0.03). After the IPS- assisted walks, the slowing of Sn Pes MRR was 20% of baseline; this was significantly less than after the free walk (p < 0.05). Four subjects performed shorter walks; after free walks of one third and two thirds of maximum distance, the mean slowing of Sn Pes MRR was 23% and 28%, respectively. We conclude that when patients with COPD walk to exhaustion, IPS reduces slowing of inspiratory muscle MRR, and that this represents a considerable unloading of the inspiratory muscles. The magnitude of the reduction is approximately the same as reducing the distance walked by two thirds.

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