The role of spirometry in evaluating therapeutic responses in advanced COPD

Traditional spirometry, while of unquestionable diagnostic utility, provides imprecise information about the nature and extent of physiological impairment or the resultant clinical consequences in any given patient with chronic obstructive pulmonary disease (COPD). The corollary is that exclusive reliance on spirometric forced expiratory flow rates as the primary outcome measure for the evaluation of therapeutic efficacy can lead to significant underestimation of clinical benefit. Recognition of the limitations of routinely used physiological parameters has prompted a search for additional simple and reliable tests for use in clinical trials. Among these, the spirometric inspiratory capacity (IC) shows early promise as a useful, clinically relevant outcome measure that complements traditional expiratory flow measurements. Consistent improvements in IC after bronchodilator therapy signify reduction in lung hyperinflation and can occur in the setting of minimal or no change in maximal expiratory flow rates, particularly in patients with more severe disease. Moreover, improved IC has been shown to correlate well with improvement in important clinical outcomes such as dyspnea and exercise endurance in patients with moderate to severe COPD. This review charts the evolving experience with this novel parameter in the clinical trial setting.