Ventilation inhomogeneities and mixed venous blood N-2 in multibreath N-2 washout

The single path model of airway gas transport, with and without a distributed blood source term, was used to simulate multiple-breath N-2 washout by breathing pure O-2 in two lung models: a single-region lung model (SRLM) which produces series inhomogeneity, and a seven-region lung model (7RLM) incorporating both series and parallel inhomogeneities. Normalized phase III slopes (Sn) from N-2 expirograms were computed for each breath and compared with published human experimental data obtained under similar conditions. The 7RLM predicts well the trend of experimental Sn N-2 changes and is superior to the SRLM in the first part (the unsteady state), implying that this part of the curve is mostly due to convective mixing of the seven parallel flow streams. In the quasi-steady state, the 7RLM is not obviously superior to the SRLM. Functional residual capacity and pulmonary perfusion are shown to strongly affect the number of breaths required to reach the quasi-steady state. The anatomical dimensions that appear to be critical in SRLM are not as important in the 7RLM. (C) 1997 Elsevier Science B.V.