Comparative study of intra-airway gas transport by alternative modes of ventilation

Abstract

The effectiveness of three alternative modes of ventilation [high-frequency ventilation (HFV), constant-flow ventilation (CFV), and high-frequency external vibration ventilation (HFVV)] was compared. Local intra-airway gas transport was measured with catheters placed in the distal trachea and in bronchi located 5.5, 9, and 11 cm from the carina. A new bolus dispersion method was devised to measure the local effective diffusivities (Deff) induced by these modes of ventilation and by cardiogenic oscillations relative to molecular diffusivity (Dmol). Mixing induced by cardiogenic oscillations was 7 +/- 2- to 26 +/- 4-fold greater than by molecular diffusion alone. Intra-airway transport by CFV, applied at three flow rates (0.3, 1.0, and 3.0 l.min-1.kg-1), was most effective in the trachea but fell sharply in the more peripheral airways. Local transport by HFVV, at a frequency of 22 Hz and a vertical amplitude of 0.4 cm, was most effective in the periphery (Deff = 793 x Dmol), whereas the effectiveness of transport by HFV, applied with 10 and 20 ml at 22 Hz, was evenly distributed. Doubling the HFV oscillatory volume caused a 4.5 +/- 2.7-fold increase in Deff/Dmol. Combining HFVV with CFV at 0.3 l.min-1.kg-1 induced transport rates that were 187- to 2,034-fold greater than by molecular diffusion alone in the bronchi and a higher relative transport (due to convection) in the trachea. We conclude that the combination of HFVV with low-flow CFV provides a high rate of intra-airway transport with minimal mechanical perturbations to the pulmonary system.