Collateral ventilation and the sloping alveolar plateaus of He and SF6: a model study

In case of intralobular airway obstructions in dog lungs and in human lungs, ventilation of the regions distal to the obstructions appears to be surprisingly effective. In general, this is attributed to collateral ventilation which takes place between obstructed and adjacent, non-obstructed air spaces. In this paper we investigate the contribution of collateral ventilation to the distribution of gases in the lung and to the slopes of the alveolar plateaus (S) of the expirograms of He and SF6. To that end, we have simulated single-breath washout experiments with a mathematical lung model in which a fraction of the total acinar space was obstructed. The obstructed part was collaterally ventilated via the non-obstructed part by convection where both parts distended and retracted isotropically during the breathing maneuver. Collateral diffusion was assumed to be negligible during inspiration and expiration, but was taken into account for periods of post-inspiratory apnea (tA). With 20% of the total acinar space obstructed and tA = 0 sec, S(He) = 3.3%.L-1 and SSF6 = 4.7%.L-1. For both gases S increased with increasing degree of airway obstruction, and decreased with increasing tA and with increasing degree of collateral diffusion. For all simulated maneuvers the alveolar plateau was rectilinearly shaped. We conclude that in case of airway obstructions collateral ventilation may substantially contribute to the slope of the alveolar plateaus of the expirograms of He and SF6.