Mechanical properties of the lung in diffuse interstitial lung disease

To define the mechanical properties of the lungs in diffuse interstitial lung disease (DILD) the static deflation volume-pressure (VP) and maximum expiratory flow-volume characteristics were measured in 18 patients with DILD including 14 with biopsy confirmation. Elastic recoil, defined by the position of the VP curve, was increased in 12, normal in four and decreased in two patients. The distensibility of inflatable lung, defined by the constant K of a single exponential fitted to the VP data, was decreased in 10 of the 12 with increased recoil and increased in the two with decreased recoil. Those with a normal or increased pulmonary distensibility smoked more than those with decreased distensibility (p less than 0.05). Maximum expiratory flow rates (MEFR) were decreased at any elastic recoil pressure in 12 patients; when corrected for lung size MEFR were decreased in seven patients, six of whom were smokers. These results show that in DILD a decreased pulmonary distensibility is common but not invariable, that an increased elastic recoil reflects both loss of lung units and decreased distensibility of inflatable units and that an increased flow resistance is common. Co-existent emphysema could explain a normal or increased pulmonary distensibility in DILD. The increased flow resistance is due in part to fibrosis and loss of lung units with functional loss of the attendant airways. Decreased MEFR, despite correction for lung size, in seven subjects suggests that structural narrowing of airways, due to cigarette smoking and/or bronchiolar involvement by the disease, also contributes to an increased flow resistance in some patients with DILD.