Mechanisms for isolated volume response to a bronchodilator in patients with COPD

We hypothesized that an altered effect of lung inflation on airway caliber may in part explain the isolated volume response to bronchodilators, i.e., an increase of forced vital capacity (FVC) without change in 1-s forced exp iratory volume (FEV1). Small-airway caliber was measured by high-resolution computed tomography at functional residual capacity and total lung capacit y in five chronic obstructive pulmonary disease patients with an isolated i ncrease of FVC (FVC responders) and five with an increase of both FVC and F EV1 (FVC-FEV1 responders) after inhalation of salbutamol. In FVC-FEV1 respo nders, the airway diameter increased with the cube root of increase in lung volume but was unchanged or even decreased in four of five FVC responders. FVC responders had more severe emphysema, as inferred from lung function a nd imaging studies, than FVC-FEV1 responders. We speculate that longitudina l traction or space competition (Verbeken EK, Cauberghs M, and Van de Woest ijne KP, J Appl Physiol 81: 2468-2480, 1996) are possible underlying mechan isms. We conclude that the isolated volume response to bronchodilators is a ssociated with severe emphysema and likely results from an altered effect o f lung inflation on airway caliber.