Decreased mechanical efficiency in clinically stable patients with COPD

Abstract

Background - It has recently been reported that total daily energy expenditure (TDE) is increased in patients with chronic obstructive pulmonary disease (COPD) and it was hypothesised that these patients may have a decreased mechanical efficiency during activities. The purpose of the present study was to measure the mechanical efficiency of submaximal leg exercise, and to characterise patients with a potentially low efficiency in terms of body composition, resting energy expenditure, lung function, and symptom limited exercise performance. Methods - Metabolic and ventilatory variables were measured breath by breath during submaximal cycle ergometry exercise performed at 50% of symptom limited achieved maximal load in 33 clinically stable patients with COPD (23 men) with forced expiratory volume in one second (FEV1) of 40 (12)% predicted. Net mechanical efficiency was calculated adjusting for resting energy expenditure (REE). Results - Median mechanical efficiency was 15.5% and ranged from 8.5% to 22.7%. Patients with an extremely low mechanical efficiency (<17%, n = 21) demonstrated an increased V̇O2/V̇E compared with those with a normal effciency (median difference 4.7ml/l, p = 0.005) during submaximal exercise. There was no difference between the groups differentiated by mechanical efficiency in blood gas tensions at rest, airflow obstruction, respiratory muscle strength, hyperinflation at rest, resting energy expenditure or body composition. There was a significant difference in total airways resistance (92% predicted, p = 0.03) between the groups differentiated by mechanical effciency. Conclusions - It is concluded that many patients with severe COPD have decreased mechanical efficiency. Furthermore, based on the results of this study it is hypothesised that an increased oxygen cost of breathing during exercise contributes to the decreased mechanical efficiency.