Fig. 1.

Schematic of the key elements of the computational model used for simulating multibreath nitrogen washout (MBNW) data. A parallel collection of lungs with identical functional residual capacities, V_i(0), and dead space volumes, V_d,i, connect at the airway opening where a flow-weighted sum of the contributions from each unit add to produce the mole fraction of nitrogen, F(t), measured at the mouth. The fractional contribution to expired flow from each unit, γ_i, determines the individual unit flows, ${\dot{V}}_i\left(t\right)$, according to the individual unit tidal volumes, V_T,i, such that units with high tidal volumes contribute fractional flows that decrease linearly as V(t) decreases throughout expiration, while units with low tidal volumes contribute fractional flows that increase linearly. This ensures the slope of Phase III is positive and that this slope increases as regional differences in ${\dot{V}}_i\left(t\right)$ increase.