Gas mixing techniques have been shown to be sensitive parameters to define abnormalities in patients with cystic fibrosis, but it is unclear how useful this technique is to detect changes after a therapeutic intervention. Abnormalities in gas clearance from the lung are largely due to retention of inhaled gases due to mucous obstruction in the lower airways and can be assessed with the lung clearance index (LCI). Interventions that improve mucous accumulation are expected to improve lung clearance as assessed by this technique. RhDNase has been demonstrated to improve lung function and reduce pulmonary exacerbations in patients with cystic fibrosis due to improved mucus clearance.