Gas mixing in the small airways, described by old and new models

This is a review of models built to explain the inefficiency of intrapulmonary gas mixing observed during multibreath tests, or to explain the slope of the alveolar plateau of the expiratory concentration-volume curve during single breath tests. The question, raised in 1917, whether diffusion was the limiting factor, received a negative answer in 1946. It then became necessary to postulate parallel unequal ventilation in combination with asynchronism. After 1966 diffusion was held responsible for stratification, a form of series inequality. However, a quantitative agreement with experimental data could only be achieved after the introduction of asymmetrical branching into the models.