Structure-function in smokers: when a small airways test really reflects the small airways.

If multiple-breath washout (MBW)-derived acinar ventilation heterogeneity (Sacin) really represents peripheral units, the N 2 phase-III of the first MBW exhalation should be curvilinear. This is essentially due to the superposed effect of gas diffusion and convection resulting in an equilibration of N 2 concentrations between neighboring lung units throughout exhalation. We investigated this in smokers with computed tomography (CT)-proven functional small airway disease. Instantaneous N 2 -slopes were computed over 40-ms intervals throughout phase-III and normalized by mean phase-III N 2 concentration. N 2 phase-III (concave) curvilinearity was quantified as the rate at which the instantaneous N 2 -slope decreases past the phase-II peak over a 1-s interval; for a linear N 2 phase-III unaffected by diffusion, this rate would amount to 0 L -1 /s. N 2 phase-III curvilinearity was obtained on the experimental curves and on existing model simulations of N 2 curves from a normal peripheral lung model and one with missing terminal bronchioles (either 50% or 30% TB left). In 46 smokers [66 (±8) yr; 49 (±26) pack·yr] with CT-based evidence of peripheral lung destruction, instantaneous N 2 -slope decrease was compared between those with (f SAD +f Emphys ) > 20% [-0.26 ± 0.14 (SD) L -1 /s; n = 24] and those with (f SAD +f Emphys ) < 20% [-0.16 ± 0.12 (SD) L -1 /s; n = 22] ( P = 0.014). Experimental values fell in the range predicted by a realistic peripheral lung model with progressive reduction of terminal bronchioles: values of instantaneous N 2 -slope decrease obtained from model simulations were -0.09 L -1 /s (normal lung; 100% TB left), -0.17 L -1 /s (normal lung 50% TB left), and -0.29 L -1 /s (30% TB left). In smokers with CT-based evidence of functional small airway alterations, it is possible to demonstrate that Sacin really does represent the most peripheral airspaces. NEW & NOTEWORTHY In smokers with computed tomography-based evidence of functional small airway alterations by parametric response mapping, it is possible to demonstrate that the multiple-breath washout-derived Sacin, an index of acinar ventilation heterogeneity, actually does represent the most peripheral airspaces. This is done by verifying on experimental N 2 washout curves of the first breath, N 2 phase-III concavity predicted by the diffusion-convection interdependence model.