Sensitivity of phase III slope to FRC and breathing pattern in a single-path airway model

A numerical solution of the convection-diffusion equation with an alveolar source term in a single-path model (SPM) of airway gas transport simulates steady state CO₂ washout. The SPM is used to examine the effects of independent changes to adjustable model parameters of lung volume and breathing pattern on the slope of phase III of the single breath CO₂ washout curve. The parameters investigated include functional residual capacity (FRC), tidal volume ( V_T), and breathing frequency (f). Reduced tidal volume causes significant steepening of phase III with calculated slopes as high as 2.4%/1 compared to 0.76%/1 from a standard V _T of 750 ml. The tidal volume sensitivity of phase III slope is in good agreement with experimental data. Variations in FRC and f cause only slight changes in calculated slopes with values from 0.6 to 1.1%/1