Longitudinal dispersion of Xenon-133 bolus in the oropharynx and the larynx

The study aims at confirming that the upper airways (oropharynx) cannot be considered as a simple dead space playing no specific role in the mixing and to establish the transfer function of these airways during inspiration and expiration. The experiments are performed on five healthy subjects. A 133Xe bolus is injected in front of the lips while the subject breathes air at a constant flow rate. The radioactivity is measured during inspiration and expiration by two counter probes in front of the lips and at the level of the upper opening of the trachea. The experimental outputs, under the glottis for inspiration and in front of the lips for expiration, are compared with the responses of a mathematical model which takes into account the longitudinal mixing of gas in the oropharynx and the larynx. This method mades it possible to determine the parameters which the transfer function depends on. Our results suggest that the relationships between the longitudinal dispersion coefficient and the product of the mean axial velocity u by the diameter d or the length L of the upper airways, is linear. The transfer function is the same for the five subjects but the inspiratory and expiratory transfer functions are different