A combined parallel and series distribution model of inspired inert gases

The distribution within the lungs of inspired gas has been demonstrated to be uneven by the technique of the external counting of inspired radioactive gas (Milic-Emili et al. (1966) J. Appl. Physiol. 21: 749-759). The contribution of this regional distribution to the slope of the alveolar plateau observed at the lips from an inspired gas marker has been debated, particularly the part played by incomplete diffusive mixing (Sikand et al. (1966) J. Appl. Physiol. 21: 1331-1337). We have repeated the experiments of Sikand, obtaining similar results, by inspiring 1.9 L of 79% argon and 21% oxygen from functional residual capacity, with a subsequent expiration to residual volume, after various breath-holding times. The opposing views of the above authors (parallel versus series inhomogeneities) are here used to develop a model in which the lung is divided into seven regions from apex to base, each region being allocated a compliance curve (polynomial equation of third order) after that of Milic-Emili. Each model region then receives a volume of inspired gas according to its compliance and its regional dead space. This dead space has been allocated on the basis of increasing path lengths of inspired gas from the apex to the base. Beyond the front of this dead space, the mixing of gas is taken to be exponential with respect to expired volume and a curve is then allocated to this alveolar region. The model thus contains both parallel (inter-regional) and series (intra-regional) components. Following a simulated expiration of these seven regions, the model expired curve so obtained is in close agreement with the experimental data, both in respect of shape and of the quantity of tracer contained within it in the range of 0.75-4.5 L of expired gas. We therefore conclude that inter-regional factors are the principal determinant of the last 2.5 L of the expired gas tracer curve and that intra-regional components play a significant role in the first 1.25 L. The model is also applicable to any other inhaled inert gas.