Nasal highflow eliminates CO 2 from lower airways.

INTRODUCTION: Nasal highflow (NHF) has a growing evidence in treatment of hypoxemic respiratory failure. There are preliminary data available about use in acute and chronic type-II-respiratory failure. Unfortunately underlying mechanisms of NHF are not well understood. Increase in airway pressure seems too small to explain the observed reduction in hypercapnia. Most interesting effects are wash-out of upper airways and reduction of functional dead space. There are no data available about a wash-out of lower airways.

METHODS: We established a sheep lung model to evaluate CO2 -wash-out in lower airways. Therefore we placed measuring and insufflation catheter in maximal expanded lung. The lung was not ventilated in order to minimize influence of CO2 rebreathing and increase in airway pressure. Airway pressure and CO2 values were measured in lower airways and in tracheal space.

RESULTS: CO2 was decreased by NHF in lower airways and in tracheal space. Changes in CO2 were flow dependent. There was also an increase in airway pressure in these settings.

CONCLUSIONS: NHF is able to decrease CO2 in lower airways in a flow-dependent manner. This effect is independent of an increase in airway pressure and CO2 -rebreathing. So wash-out is an important reason for efficiency of NHF in decreasing hypercapnia.