A high-flow nasal cannula system with relatively low flow effectively washes out CO(2) from the anatomical dead space in a sophisticated respiratory model made by a 3D printer

BACKGROUND: 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. Therefore, we made an experimental respiratory model to evaluate the respiratory physiological effect of HFNC. METHODS: An airway model was made by a 3D printer using the craniocervical 3D-CT data of a healthy 32-year-old male. 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(ET)CO(2)) 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. RESULTS: With the normal-lung, open-mouth model, 10 L/min of HFNC flow decreased the subglottic P(ET)CO(2) to 30 mmHg. Increasing the HFNC flow did not further decrease the subglottic P(ET)CO(2). With the normal-lung, closed-mouth model, HFNC flow of 40 L/min was required to decrease the P(ET)CO(2) at all sites. Subglottic P(ET)CO(2) reached 30 mmHg with an HFNC flow of 60 L/min. In the obstructive-lung, open-mouth model, P(ET)CO(2) 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 P(ET)CO(2) to 25 mmHg, and it did not decrease further. As HFNC flow was increased, PEEP up to 7 cmH(2)O was gradually generated in the open-mouth models and up to 17 cmH(2)O in the normal-lung, closed-mouth model. CONCLUSIONS: The 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.