Lung hyperinflation in COPD: applying physiology to clinical practice

In chronic obstructive pulmonary disease (COPD), worsening expiratory flow limitation together with alteration in the elastic properties of the lung are associated with progressive lung hyperinflation and gradual decline in the resting inspiratory capacity over time. Dynamic hyperinflation (DH) refers to the variable increase in end-expiratory lung volume (EELV) above the relaxation volume (VR) of the respiratory system that occurs when expiratory flow limitation is amplified (e.g., during bronchoconstriction and acute exacerbations) or when ventilation is increased in the setting of expiratory flow limitation. During exercise, the combined factors of worsening expiratory flow limitation, increasing respiratory neural drive and breathing pattern alterations dictate the pattern and extent of DH. Acute-on-chronic hyperinflation increases the intrinsic loads on the inspiratory muscles which become functionally weakened. The combined effects of compromised respiratory and integrated cardio-circulatory function due to lung hyperinflation contribute to exercise limitation. In COPD, the resting inspiratory capacity, which indirectly reflects the extent of lung hyperinflation, dictates the limits of tidal volume expansion and thus, peak ventilatory capacity during activity. Moreover, the growing disparity between increased respiratory neural drive and the blunted respiratory muscular/mechanical response due to lung hyperinflation is mechanistically linked to dyspnea during exercise in COPD. From a clinical standpoint, measurement of lung hyperinflation is integral to the assessment of physiological impairment in individuals with COPD and can effectively be targeted for treatment. Moreover, it is now well established that lung volume reduction (deflation) provides a solid mechanistic rationale for observed improvements in dyspnea and exercise tolerance in patients with COPD following bronchodilator therapy.