Convective mixing mechanisms in high frequency intermittent jet ventilation

Abstract

A liquid flow visualization technique was used to identify the location of neutrally buoyant bead clouds injected into airway models during flows simulating high frequency intermittent jet ventilation (HFIJV) in neonatal lungs. The motions of these bead clouds show that the convective or bulk mixing that occurs during HFIJV is made up of two parts; a turbulent convective exchange with the atmosphere caused by the jet in the trachea and a streaming motion along the airways driven by an interaction between the jet and the expansion and contraction of the airways due to their compliance. These convective streaming motions combine with molecular diffusion to produce augmented diffusion which transports O2 and CO2 between the trachea and the peripheral alveoli. Optimizing HFIJV (as well as other forms of HFV) depends on maximizing these airway convective streaming flows which depend on many more lung and fluid mechanical parameters than are necessary to describe conventional mechanical ventilation.

Jetventilation und Anästhesie für diagnostische und therapeutische Eingriffe an den Atemwegen

“…Diese beiden Arten von Diffusion bewirken den Abbau der longitudinalen Konzentrationsgradienten für O 2 von außen in Richtung Alveolen und für CO 2 von den Alveolen nach außen [2,29]. Nahezu gleichzeitig entweicht ein beträchtlicher Teil des Exspirationsvolumens entlang der Querschnittsperipherie nach außen.…”

Jet Ventilation

“…11 High-frequency jet ventilation results in inhomogeneous ventilation. 12 Regional variation in gas concentration, air space volumes, and pressures are observed.…”

“…7 This likely is due to unequal inspiratory and expiratory impedances, distribution of oscillatory flow, and expiratory flow limitation. 11,21 In addition, high-velocity gas streams as generated during high-frequency ventilation preferentially follow straight pathways. Because of the geometry of the central airway, this may result in regional differences, with an increased tendency of the lung base to be overinflated, compared with the apex.…”

High Frequency Ventilation and Regional Compliance

“…At frequencies less than 7 Hz and in conventional respiration, regional alveolar ventilation has to be assumed to depend solely on segmental compliance and airway resistance. Only at higher exciting frequencies, beyond 10 Hz, does ventilation become independent of compliance [15][16][17][18][19].…”

“…(iv, v) Two further mechanisms discussed widely in the literature [15,18,26,27] (Taylor dispersion and asymmetrical flow profiles) are endobronchial flow and mixing processes. Investigation of these phenomena is hampered by lack of measuring methods which can be used within the airway system.…”