Clinical utility of work-of-breathing measurements in COPD

The measurement of the mechanical work of breathing (WOB) has been used in clinical research for over four decades and has provided valuable insights into the pathophysiological derangements that exist in patients with chronic obstructive pulmonary disease (COPD). These WOB calculations are based on the principle that work is performed when pressure changes the volume of the respiratory system. In 1958, E.J.M. Campbell introduced a graphic analysis of the oesophageal pressure (Poes)/tidal volume (VT) plots over the respiratory cycle, which allowed work to be separated into several components [1]. These components include: inspiratory, expiratory and total resistive work rates in joules (J·min-1); inspiratory elastic work rates (J·min-1); and total inspiratory work (J·I-1). A detailed discussion of the technical aspects of WOB measurement is beyond the scope of this abstract and excellent reviews are available elsewhere [2]. WOB is increased in flow-limited COPD patients at rest, mainly because of the increased flow resistive and elastic loads on the inspiratory muscles. During exercise, flow resistive work increases as ventilation increases, elastic/threshold loads increase because of the effects of dynamic hyperinflation, and dynamic lung compliance is reduced below resting levels because of the effects of exercise tachypnoea. For these reasons, WOB at any given ventilation is often markedly increased in COPD, as compared with controls, particularly during exercise.