Abstract
BackgroundAlthough clinical studies of the high-flow nasal cannula (HFNC) and its effect on positive end-expiratory pressure (PEEP) have been done, the washout effect has not been well evaluated. Therefore, we made an experimental respiratory model to evaluate the respiratory physiological effect of HFNC.MethodsAn airway model was made by a 3D printer using the craniocervical 3D-CT data of a healthy 32-year-old male. CO2 was infused into four respiratory lung models (normal-lung, open- and closed-mouth models; restrictive- and obstructive-lung, open-mouth models) to maintain the partial pressure of end-tidal CO2 (PETCO2) at 40 mmHg. HFNC flow was changed from 10 to 60 L/min. Capnograms were recorded at the upper pharynx, oral cavity, subglottic, and inlet sites of each lung model.ResultsWith the normal-lung, open-mouth model, 10 L/min of HFNC flow decreased the subglottic PETCO2 to 30 mmHg. Increasing the HFNC flow did not further decrease the subglottic PETCO2. With the normal-lung, closed-mouth model, HFNC flow of 40 L/min was required to decrease the PETCO2 at all sites. Subglottic PETCO2 reached 30 mmHg with an HFNC flow of 60 L/min. In the obstructive-lung, open-mouth model, PETCO2 at all sites had the same trend as in the normal-lung, open-mouth model. In the restrictive-lung, open-mouth model, 20 L/min of HFNC flow decreased the subglottic PETCO2 to 25 mmHg, and it did not decrease further. As HFNC flow was increased, PEEP up to 7 cmH2O was gradually generated in the open-mouth models and up to 17 cmH2O in the normal-lung, closed-mouth model.ConclusionsThe washout effect of the HFNC was effective with relatively low flow in the open-mouth models. The closed-mouth model needed more flow to generate a washout effect. Therefore, HFNC flow should be considered based on the need for the washout effect or PEEP.
Highlights
Clinical studies of the high-flow nasal cannula (HFNC) and its effect on positive end-expiratory pressure (PEEP) have been done, the washout effect has not been well evaluated
Normal-lung, open-mouth model Capnograms recorded at the subglottic area and the inlet of the lung model had flat phase 3 waveforms, which demonstrated that the gas delivered by HFNC does not reach and wash out the dead space below the subglottic area
The washout effect does not need a higher flow of HFNC, but PEEP requires a higher flow; a higher flow may increase the work of breathing (WOB)
Summary
Clinical studies of the high-flow nasal cannula (HFNC) and its effect on positive end-expiratory pressure (PEEP) have been done, the washout effect has not been well evaluated. The initial assumptions about HFNC for adults were that it allowed humidification and accurate FIO2 management; HFNC’s respiratory physiological effects did not become well understood. Those respiratory physiological effects are estimated to include the following: washout of the anatomical dead space of the respiratory tract, a positive end-expiratory pressure (PEEP) effect, and reduction of the metabolic cost of gas conditioning by humidification. The mechanism has been surmised either from the study of tracheal gas insufflation (TGI) [9, 10], which is quite a different system from HFNC with “high flow” less than 8 L/min [10], or from a simple respiratory model, which had a larger dead space than the actual value in humans [11]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
