Influence of lung volume on the mixing defect for an inert gas

For quantitative assessment of how gas mixing in the lung compares to the ideal of perfect mixing, it is necessary to measure pre-inspiratory lung volume as well as to study the inhaled and exhaled gas. Normal young-adult persons in the seated position assumed various lung volumes and then took 1-L test breaths (11% Ar, 22% O2 in N2). Integration of concentration at the mouth against volume yielded amounts of Ar inhaled, exhaled and retained; un-exhaled concentration in the lung after the single breath was calculated from amount retained divided by lung volume (measured by rebreathing). Retention was 60% of the inhaled amount when pre-inspiratory volume was above FRC and less when it was below. Series dead space increased with lung volume. End-tidal Ar was the same as, or slightly above, the concentration predicted for perfect mixing and un-exhaled Ar was slightly below the perfect-mix concentration.

three separable influences decrease the transfer of inspirate to the resident gas. A dilution effect, predictable from the volumes involved, decreases retention at low lung volumes. An enlarged series dead space decreases retention at high volumes. In normal persons, a mixing defect that is attributable to poor distribution or incomplete gas-phase diffusion is small and can be ascribed almost completely to the part of the expirate that gives rise to the slope of the alveolar plateau, except when breaths are taken at very low lung volumes.