The relation between pulmonary distensibility, lung volume, and elastic recoil pressure was examined in 45 patients (38 men) with cryptogenic fibrosing alveolitis (mean age 61 (SD 14) years). Exponential analysis of static pressure-volume data obtained during deflation of the lungs gave the exponent K, an index of the distensibility of the remaining inflatable lung tissue. Total lung capacity (TLC) was measured in a body plethysmograph or by nitrogen washout. The results were compared with values obtained in 147 healthy subjects (95 men), of mean age 39 (SD 16) years. Fibrosing alveolitis decreased K by 0.62 (SEM 0.04) kPa-1. This decrease was approximately equal to 2 SD of the regression of log K on age in healthy subjects. TLC was decreased to a mean of 70% (SD 14%) predicted in the patients with fibrosing alveolitis. Lung recoil pressure at maximum inspiration was about twice the expected value and regression analysis showed that most of this increase was related to the decreased K rather than to the decreased TLC. In the men with fibrosing alveolitis the regression of height standardised TLC (TLC/Ht3) on K was significant (p less than 0.02); the regression slope was similar to that for 95 healthy men, but was displaced to a smaller lung volume. The dependence of TLC/Ht3 on K is consistent with the close relation between K and peripheral airspace size found in normal lungs. In fibrosing alveolitis decreased pulmonary distensibility probably reflects a decrease in airspace size, whereas most of the decrease in lung volume reflects the loss of inflatable tissue in the fibrotic process.
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