Modelling of gas transport in an apneic airway undergoing nasal high flow therapy

Abstract

Nasal high flow therapy (NHF) has been used during anaesthetic procedures to extend the duration of apnea. It has been observed clinically that the application of the therapy during apnea results in blood O2 and CO2 concentrations which remain within a tolerable range for apneic periods lasting up to 14 minutes. However in the absence of ventilation it is unclear how gas transport occurs through the airway during this procedure. A computational fluid dynamics model of the air and CO2 within the human airway was constructed in order to investigate the transport of gas species during apnea. Flow induced by the beating of the heart was found to be important in the transport and removal of CO2 from the apneic airway. Application of NHF therapy enhanced the ability of this cardiogenic flow to eliminate CO2 from the airway. The therapy converts the upper portion of the anatomical dead-space into a supply of fresh gas with low CO2 concentration and high levels of turbulent kinetic energy. Models which took into account the peripheral airways, using impedance boundary conditions, imposed a lower adverse pressure gradient on the therapy flow. This allows the therapy to washout a greater portion of the anatomical dead-space.