Plasma ammonia response to incremental cycling and walking tests in COPD

It is well documented that plasma ammonia accumulates during exercise under conditions of metabolic stress. Metabolic stress (when skeletal muscle ATP supply fails to meet demand) occurs at low work rates during cycling in patients with COPD, but not been described during walking. Walking is an important activity for many patients with COPD and is commonly prescribed in pragmatic outpatient pulmonary rehabilitation programmes. In this study we explored whether metabolic stress occurs during incremental walking at the low work rates these patients achieve.

Twenty-nine subjects with stable COPD [mean(SD) age 68(7)years, FEV(1) 50(19)% predicted] performed maximal cardiopulmonary exercise tests on a cycle ergometer and treadmill. Plasma ammonia concentration was measured at rest, 1 and 2min of exercise, peak exercise and 2min recovery.

Subjects achieved mean(SD) cycle work rate of 57(20)W with VO(2max) 15.5(4.6)ml/min per kg, and treadmill distance 284(175)m with VO(2peak) 16.8(4.2)ml/min per kg. Plasma ammonia concentration rose significantly (p<0.001) with walking [mean(SEM) change 24.7(3.8)micromol/l] and cycling [mean(SEM) change 35.2(4.3)micromol/l], but peak exercise ammonia was lower in walking (p<0.01). In a subgroup of subjects (n=7) plasma ammonia did not rise during either cycling or walking despite similar lactate rise and peak exercise indices.

Our data indicate that failure of muscle ATP re-synthesis to meet demand and development of metabolic stress can occur during walking in COPD patients at the low work rates these patients achieve. This may therefore be a factor contributing to exercise limitation independent of ventilatory limitation.