The influence of lung volume on expiratory flow rates in diffuse interstitial lung disease

This study evaluated maximum expiratory flow rates with respect to lung volume and maximum recoil pressure in selected patients with diffuse interstitial lung disease who had normal large airway function by standard technique. Coefficient of retraction was normal or greater than normal in all. Peak flow varied directly with lung volume as in normals. At 50% vital capacity (VC) and 25% VC, the absolute flow rates varied from higher to lower than normal. However, when flow was adjusted to volume, the flow/volume ratio was normal or high in all. Flow/volume ratio at mid-lung volume appeared to increase with increase in coefficient of retraction. Patients with frequency dependence of compliance had lower flow/volume ratios at 25% VC than those without, although still within normal range. Thus, despite recognized wide variations in normals, the flow/volume ratio is pertinent to the evaluation of reduced air flow rates in in interstitial lung disease to distinguish abnormal upstream airway resistance from volume-dependent reduction of flow rate. An effort-independent flow rate that yields a supernormal flow/volume ratio suggests increased recoil properties of the respiratory system.