Diffusion-convection equation solved in parallel regions of the lung

The single path model of airway gas transport was incorporated into each of the seven parallel regions model of Cruz (Cruz, J. C. Respir. Physiol. 86: 1-14, 1991). Thus, the effect of time on the predicted gas fractions in and out of the lung could be evaluated. Two experimental maneuvers were simula ted: (1) fast inhalation of an argon-oxygen mixture from a functional resid ual capacity and fast exhalation to residual volume, including inspiratory breath holdings of 5-20 s, and (2) the standard single-breath nitrogen wash out test. Expired argon and nitrogen are predicted within a +/-3% error of the experimental data with no breath holding. Breath holding predictions we re at variance with experimental results because the solution of the diffus ion-convection equation produced even mixing in the alveoli at the end of i nspiration. The minimum square of the difference between the experimental d ata (standard single-breath nitrogen washout test) and those provided by th e model was 0.0016. This model is capable of generating a nitrogen expirogr am with four phases when a vital capacity of oxygen is inhaled. However, th e model failed to produce a sharp distinction between phase 3 and phase 4. Thus, we conclude that uneven emptying of parallel regions generates any ex pirogram (a fast or slow expiratory maneuver). The alveolar gas stratificat ion that is created during inspiration disappears at the end of the inspira tory maneuver. As a result, breath holding maneuvers cannot be predicted in the anatomical model used. (C) 2000 Biomedical Engineering Society.