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 CO ₂ -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 CO ₂ rebreathing and increase in airway pressure. Airway pressure and CO ₂ values were measured in lower airways and in tracheal space.
Results: CO ₂ was decreased by NHF in lower airways and in tracheal space. Changes in CO ₂ were flow dependent. There was also an increase in airway pressure in these settings.
Conclusions: NHF is able to decrease CO ₂ in lower airways in a flow-dependent manner. This effect is independent of an increase in airway pressure and CO ₂ -rebreathing. So wash-out is an important reason for efficiency of NHF in decreasing hypercapnia.
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