Nasal highflow eliminates CO₂ from lower airways

Highlights

• In our study we investigate influence of wash-out effect by using NHF in a sheep lung model.

• We found a pressure-independent CO₂-wash-out of lower airways.

• So we can conclude that NHF is able to eliminate CO₂.

Abstract

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.

Introduction

Nasal highflow (NHF) is becoming increasingly important in clinical settings. It has shown effective in patients with hypoxemic respiratory failure due to pneumonia. When suffering from severe acute respiratory failure these patients show a decrease in intubation rates compared with oxygen treatment alone or moderate non-invasive ventilation (NIV) (Frat et al., 2015). Other indications are currently being discussed and have to be further evaluated in future studies (La Combe et al., 2016, Maggiore et al., 2014, Réminiac et al., 2016).

Physiological and clinical studies show a decrease in breathing rates, an increase in tidal volume as well as a decrease in breathing effort (Bräunlich et al., 2016). In rapidly breathing patients, NHF appears to be able to better maintain stable oxygenation levels compared with other oxygen application devices (Wagstaff and Soni, 2007). Furthermore, early data show a decrease in partial pressure of carbon dioxide (PCO₂) in patients with chronic obstructive pulmonary disease (COPD) and stable chronic hypercapnia (Bräunlich et al., 2013, Bräunlich et al., 2015, Bräunlich et al., 2016).

It has not yet been fully elucidated, which aspects of NHF are actually most important. Some authors accentuate that the main effect is an increase in mean airway pressure. However, said increases seem to be too small as to fully explain the effects of NHF. Additionally, washing-out the upper airways, reducing CO₂-rebreathing and functional dead space reduction contribute to the mode of action. The wash-out effect has already been proven using a model as well as in newborn piglets (Frizzola et al., 2011, Möller et al., 2015). This study aims to illustrate the wash-out mechanism to be effective even in the lower airways by means of an animal model.