Detection of small airway dysfunction using specific airway conductance

Study objective: To assess the potential utility of specific airway conductance (sGaw) in detecting small airways dysfunction, the postlung- transplant bronchiolitis obliterans syndrome (BOS) was used as a model of small airways dysfunction. BOS is defined as an otherwise unexplained 20% reduction in FEV₁. We hypothesized that if sGaw is sensitive to small airways dysfunction, it should decrease before the decline in FEV₁. Design/methods: The pulmonary function test and sGaw measurements of patients who underwent heart-lung or bilateral lung transplantation between May 1981 and January 1993 were reviewed. Patients with and without BOS were identified. A significant decrease in sGaw was defined as a 20% fall from baseline. Results: Twenty-six BOS and 15 non-BOS patients had at least three sGaw measurements such that trends could be examined. Eleven of the 26 BOS patients (42%) had a significant decrease in sGaw before a 20% decrease in FEV₁, as compared to 2 of the 15 non-BOS patients (13%) (p=0.08). In comparison, 12 of the 26 BOS patients (46%) and 4 of the 15 non-BOS patients (27%) had a significant decrease in forced expiratory flow at 25 to 75% of the forced lung volume (FEF_25-75) (p=0.32), an accepted test of small airways dysfunction. Conclusion: sGaw tended to decrease before FEV₁ in BOS. The trend in sGaw was similar to the trend in FEE_25-75. We conclude that (1) small airways may contribute more to airway conductance than previously thought, and (2) further prospective studies are warranted to better define the relative contribution of small and large airways to sGaw.