Effects of cystic fibrosis lung disease on gas mixing indices derived from alveolar slope analysis

Scond and Sacin are derived from analysis of concentration-normalized phase III slopes (SnIII) of a multiple breath inert gas washout. Studies in healthy and COPD subjects suggest these reflect ventilation heterogeneity in conducting and acinar airway zones respectively, but similar studies in cystic fibrosis (CF) are lacking. Scond, Sacin and lung clearance index (LCI, a measure of overall gas mixing efficiency) were measured in 22 adults and 18 children with CF and 17 adult and 29 child controls. Plethysmography and gas transfer measurements were performed in adults, and spirometry in all subjects. Scond was elevated in almost all CF patients, including children with mild disease and normal LCI. However, Scond did not correlate with other measurements and appeared to reach a maximum; further increase in ventilation heterogeneity being restricted to Sacin. The nature and/or severity of CF lung disease may invalidate assumptions underlying the ability to separate phase III slope analysis of ventilation heterogeneity into proximal and peripheral components, and LCI may be a better indicator of gas mixing in this population.