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 differenceof Grs between functional residual capacity and total lung capacity to the relevant changes in lung volume (ΔGrs/ΔVL) or transpulmonary pressure (ΔGrs/ΔPtp). Similar analysis was applied to Xrs to estimate lung volume recruitment (ΔXrs/ΔVL or ΔXrs/ΔPtp). The extent of emphysema in COPD subjects was estimated from the percentage of low attenuation area (LAA) at high-resolution computed tomography. At baseline, ΔGrs/ΔVL and ΔXrs/ΔVL were significantly less in COPD than control subjects, indicating less distensibility and volume recruitment in the former. In COPD, ΔGrs/ΔPtp and Δrs/ΔPtp were uncorrelated with LAA but correlated with 1-s forced expiratory volume and with each other. After albuterol, both ΔGrs/ΔPtp and ΔGrs/ΔVL became significantly and negatively correlated with LAA, while ΔXrs/ΔPtp and ΔXrs/ΔVL decreased significantly independently of LAA. Moreover, ΔGrs/ΔPtp and ΔXrs/ΔPtp 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.