Abstract
Background
Lung clearance index (LCI) is a widely reported marker of ventilation heterogeneity (VH) derived from multiple breath inert gas washout.
Aims and hypotheses
We aimed to develop a model of gas mixing that could be used to measure the two components of increased LCI, namely (i) differences in specific ventilation between lung units and (ii) increased dead space, in patients with asthma and cystic fibrosis (CF). We hypothesised that:
i) A two-compartment model of gas mixing would accurately describe washout curves across the spectrum of VH severity.
ii) The VH observed in asthma and CF would be distinguished by differing contributions of specific ventilation inequality and dead space effects.
Methods
Washout data from sixty-six healthy control subjects, seventy-four patients with asthma, and forty-one patients with CF were fitted to a mathematical two-compartment model of gas mixing, and the parameters specific ventilation index (SVI) and alveolar dead space (Vdalv) were derived.
Results
The two-compartment model accurately fitted washout curves from healthy subjects, asthma patients and CF patients (median R2 = 0.998, 0.997 and 0.991 respectively). SVI and Vdalv were significantly increased in patients with CF compared to healthy controls, but not in patients with asthma, in whom phase III slope parameters were more discriminatory.
Conclusion
The parameters SVI and Vdalv are sufficient to describe washout curves across the entire spectrum of VH severity. Further studies are required to determine if these indices may be used to define clinical sub-phenotypes of airway diseases.
- © 2013 ERS