Limitations of regional ventilation inhomogeneity indices in children with cystic fibrosis.

Background: S is a multiple breath washout (MBW) index that measures convection-dependent ventilation inhomogeneity (CDI) arising within conductive airways, but the calculation method is unreliable in subjects with advanced cystic fibrosis (CF) lung disease. A new CDI index, S *, has been proposed for use in adults with CF and moderate to severe ventilation inhomogeneity. We aimed to evaluate the most appropriate CDI index in children and adolescents with CF and various degrees of inhomogeneity, and from that the most appropriate diffusion-convection-interaction index (S or S *).

Methods: S , S  and the alternative indices, S *, and S * were retrospectively calculated in subjects with CF aged 3 to 18 years and age-matched controls, who underwent sulfur hexafluoride MBW between 2003 and 2015. The upper limit of normal was based on 95th percentile of the control population.

Results: One hundred and twenty-seven subjects with CF (44% male; mean age ± SD: 7.5 years ± 4.9) and 94 controls (53% male; 7.9 years ± 5.1) were included in the final analysis. All measures of ventilation inhomogeneity were significantly higher in children with CF. As predicted, S reached a maximum value at lung clearance index (LCI) values of approximately 9. In subjects with LCI ≥ 9 S * showed good correlation with LCI, whilst S had no relationship with LCI (Spearman rank correlation S */LCI, 0.49; P < .01; S /LCI, -0.068; P = .46). In subjects with mild disease (LCI < 9) S was more frequently abnormal than S * (37% vs 16%; P = .01).

Conclusions: S and S are sensitive indices of early regional inhomogeneity, but are of no value when LCI ≥ 9. In these subjects, S * & S * are potential alternatives.