The closed-mouth model needed more flow to generate a washout effect. The washout effect of the HFNC was effective with relatively low flow in the open-mouth models. As HFNC flow was increased, PEEP up to 7 cmH 2O was gradually generated in the open-mouth models and up to 17 cmH 2O in the normal-lung, closed-mouth model. In the restrictive-lung, open-mouth model, 20 L/min of HFNC flow decreased the subglottic P ETCO 2 to 25 mmHg, and it did not decrease further. In the obstructive-lung, open-mouth model, P ETCO 2 at all sites had the same trend as in the normal-lung, open-mouth model. Subglottic P ETCO 2 reached 30 mmHg with an HFNC flow of 60 L/min. With the normal-lung, closed-mouth model, HFNC flow of 40 L/min was required to decrease the P ETCO 2 at all sites. Increasing the HFNC flow did not further decrease the subglottic P ETCO 2. With the normal-lung, open-mouth model, 10 L/min of HFNC flow decreased the subglottic P ETCO 2 to 30 mmHg. Capnograms were recorded at the upper pharynx, oral cavity, subglottic, and inlet sites of each lung model. HFNC flow was changed from 10 to 60 L/min. CO 2 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 CO 2 (P ETCO 2) at 40 mmHg. MethodsĪn airway model was made by a 3D printer using the craniocervical 3D-CT data of a healthy 32-year-old male. Therefore, we made an experimental respiratory model to evaluate the respiratory physiological effect of HFNC. Although 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.
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