The Effect of Bronchodilator Inhalation on the Diffusion Capacity in Asthma and Emphysema

Abstract

Crapo et al reported (Chest 1984;86:336) that inhaled bronchodilator (BD) had no effect on lung diffusion (D_L) in normal subjects or patients with reversible or irreversible airways obstruction. However, we had observed that on occasion some asthmatics had higher D_L values post-BD than pre-BD; this seemed to be related to the overall BD regimen. To investigate the difference between Crapo et al's findings and our own observations, we studied 57 patients with asthma and 27 with emphysema, as well as 3 normal subjects. Methods: We measured DL and specific DL. (DL/VA) before and after bronchodilatation was achieved with isoetharine diluted in normal saline and administered as an aerosol. In the 3 normal subjects, 13 of the asthmatics, and 5 patients with emphysema, we also studied maldistribution of intrapulmonic gases by administering the single-breath nitrogen washout for slope of Phase III (SLP3). Results: In 3/57 asthmatics (5%), D_L was low before BD (mean, 53% of predicted) but it corrected after BD (change significant at P<0.01). In the remaining 54/57 asthmatics (95%), the pre-BD D_L was in the normal range and there was no significant difference post-BD. The same 3 asthmatics who had low pre-BD D_L also had a low mean DL/VA before bronchodilator, and it increased significantly (P < 0.01) post-BD. The other 54 asthmatics had a normal DL/VA pre- and post-BD, with no significant change. In 20/27 emphysema patients (74%), the DL decreased significantly (P < 0.01) post-BD, as did the DL/VA. In the other 7 emphysema patients, the mean DL increased slightly post-BD (not significant), and the DL/VA remained unchanged post-BD. Intrapulmonic gas mixing was markedly abnormal in the emphysema patients and did not change significantly post-BD. In the asthmatics, the gas mixing problem was less severe pre-BD than in those with emphysema, and a small drop in the value post-BD was not significant. In the normal subjects, there were no significant changes in DL, DL/VA, or SLP3. In summary, the large majority of asthmatics had no change in D_L post-BD, while a small minority had moderately reduced D_L pre-BD and an improvement to normal post-BD. In contrast, most emphysema patients demonstrated a significant reduction after BD in an already low DL. Conclusions: We propose that D_L be measured pre-BD for convenience but that those patients found to have obstructive airways disease and reduced D_L should also have post-BD D_L testing in order to avoid confusion in differential diagnosis.