Rate of increase in pulmonary distensibility in a longitudinal study of smokers

To examine the hypothesis that an abnormally rapid increase in pulmonary distensibility occurs in cigarette smokers, 39 adult smokers (24 men), mean age 47 (SD 8) years, who were not disabled were studied on two occasions over a mean interval of 3.5 (SD 0.5) years. Exponential analysis of static pressure-volume data obtained during deflation of the lungs gave the exponent K, an index of distensibility. Total lung capacity (TLC) was measured in a body plethysmograph. At entry into the longitudinal study means values for K and static recoil pressure in the 39 smokers available for follow up were similar to those obtained in the original group of 101 smokers (73 men), mean age 42 (SD 11) years, in the cross sectional study. Over the interval of the study, ln K and TLC increased and FEV1 decreased at rates greater than those found in a previous longitudinal study of 34 non-smokers (24 men), mean age 42 (SD 15) years. In the longitudinal study of smokers the observed changes in K and in recoil pressure over the interval of study were greater than the values obtained from the regression slopes found in the cross sectional study of smokers. On the basis of the regression model used previously in the longitudinal study of non-smokers, the age coefficient for ln K was greater than that found in the non-smokers (p less than 0.01). The regression model also showed that the slope of ln K on age increased in older subjects. Because K is related to peripheral airspace size, a rapid rate of increase in K identifies smokers in whom airspace size is increasing abnormally rapidly. In this study the rate of increase in K and the variation between subjects was sufficient to explain the magnitude of the increased pulmonary distensibility found in cigarette smokers who present with emphysema.