Airway distensibility and volume recruitment with lung inflation in COPD

The effects of full lung inflation on respiratory conductance (Grs) and reactance (Xrs) were measured in 15 subjects with moderate to severe chronic obstructive pulmonary disease (COPD) and 11 matched healthy control subjects. Airway distensibility was estimated from the ratio of the difference of Grs between functional residual capacity and total lung capacity to the relevant changes in lung volume (Delta Grs/Delta VL) or transpulmonary pressure (Delta Grs/Delta Ptp). Similar analysis was applied to Xrs to estimate lung volume recruitment (Delta Xrs/Delta VL or Delta Xrs/Delta Ptp). The extent of emphysema in COPD subjects was estimated from the percentage of low attenuation area (LAA) at high-resolution computed tomography. At baseline, Delta Grs/Delta VL and Delta Xrs/Delta VL were significantly less in COPD than control subjects, indicating less distensibility and volume recruitment in the former. In COPD, Delta Grs/Delta Ptp and Delta Xrs/Delta Ptp were uncorrelated with LAA but correlated with 1-s forced expiratory volume and with each other. After albuterol, both Delta Grs/Delta Ptp and Delta Grs/Delta VL became significantly and negatively correlated with LAA, while Delta Xrs/Delta Ptp and Delta Xrs/Delta VL decreased significantly independently of LAA. Moreover, Delta Grs/Delta Ptp and Delta Xrs/Delta Ptp with lung inflation were no longer correlated with each other, suggesting that airway distensibility and volume recruitment were affected differently by airway smooth muscle tone. Assuming that Grs mainly reflects airway caliber and Xrs the number of ventilated lung units, we conclude that airway smooth muscle contributes to airway stiffness and ventilation inhomogeneities in COPD subjects with prevailing bronchitis but only to the latter in those with more emphysema. We suggest that changes of airway distensibility and volume recruitment with a bronchodilator may be useful for disease phenotyping.