The role of small airways in obstructive airway diseases Eur Respir Rev vol20, no. 119 023-033

RV/TLC remains the most interesting marker of small airways closure in clinical practice or in large multicentre trials.
FEF_25–75% is the spirometric variable most commonly cited as an indicator of small airways obstruction
FEF_25–75% correlates highly with FEV₁/FVC ratio (as % pred) but nonlinearly, 
Closing volume (i.e. the volume at which small airway closure occurs) is useful to measure premature airway closure that correlates well with RV/TLC 
High-resolution CT (HRCT) allows direct assessment of large and medium airways (diameter >2–2.5 mm), but also indirect assessment of small airways.
changes in mean lung density and the relative lung area with low attenuation were related to the change in FEV₁ and RV
limitations to the use of CT scans to monitor small airways disease:
1.ionized radiation__malignancy potential
2.standardisation of measurements of lung attenuation and air trapping
3.a decrease in lung attenuation may be related to reduction in ventilation but also to reduction in lung perfusion