Measurement of CT lung density in patients with chronic asthma.

Evidence for the presence of emphysema in patients with asthma is controversial. We have previously shown that decreased lung density, measured by computed tomographic (CT) scanning, preoperatively, correlates with morphometric measurements of microscopic emphysema in subsequently resected lungs. The aim of this study was to compare CT lung density, in 17 patients with chronic asthma (forced expiratory volume in one second (FEV1) 1.98 (SD 0.77) L), 17 patients with chronic bronchitis and emphysema (FEV1 0.97 (0.56) L) and seven normal subjects (FEV1 3.5 (034) L). All subjects underwent CT scanning of the lungs and respiratory function testing within 2 days of each other. In five of the asthmatic patients a CT scan was performed on two occasions before and after treatment with nebulized bronchodilator. In a different group of five asthmatics these measurements were performed at the end of and 6 weeks after an exacerbation. The mean value of the lowest fifth percentile of the CT lung density in the patients with chronic obstructive pulmonary disease (COPD) was -942 (SD 36) Hounsfield units (HU), in the 17 asthmatic patients was -912 (34) HU, and in normal subjects was -880 (13). Despite a significant increase in peak expiratory flow rate from 266 (SD 102) to 406 (83) L x s(-1) (p<0.02) following nebulized beta2-agonist in five patients with chronic asthma, there was no significant change in CT lung density (p>0.05) Our study indicates that at least some patients with chronic, stable asthma develop a reduction in computed tomography lung density, similar to that in patients with emphysema.