Effect of resident gas density on CO2 elimination during high-frequency oscillation: a model study

In order to throw more light on the mechanisms governing the efficiency of intrapulmonary gas mixing during high-frequency oscillatory ventilation, an experimental, and theoretical, study was carried out on a model casting of the airways of a human lung that closely resembled the respiratory tract. The experiments were carried out under various conditions during high-frequency oscillation (HFO), by using alveolor resident gas mixtures of different densities. The efficiency of gas mixing was assessed by measuring the time constants of the CO2 alveolar washout which were compared to those obtained from simulations on a theoretical model based on a turbulent diffusional resistance concept. Our results showed that the decay in CO2 concentration was highly dependent on both frequency (f) and tidal volume (VT). Tidal volume was found to have a greater effect on efficiency of gas mixing than frequency. Moreover, even though there were statistically significant differences in the time courses of CO2 washout between N2 and He, N2 and SF6 or between He and SF6, this could not imply that gas mixing was limited by diffusion. Agreement between the experimental time constants of CO2 elimination during HFO and the predicted mixing time constants was satisfactory. It is concluded that turbulent augmented diffusion is the main factor responsible for effective gas transport during high-frequency oscillatory ventilation.