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Volume 2 Supplement 1


0232. Evaluation of HFNC'S wash out effect; a comparison of open- and closed-mouth models


Although clinical studies of the high-flow nasal cannula (HFNC) and its effect on positive end-expiratory pressure (PEEP) have been performed, the mechanism of the washout effect and its relation with HFNC flow have not been well evaluated. Therefore, we made a respiratory model that can exhale with controllable end-tidal PCO2 (PETCO2) to evaluate the washout effect of HFNC.Objective. To evaluate the quantitative results of HFNC's washout effect comparing open- and closed-mouth models.


OptiflowTM (Fisher and Paykel Healthcare, Auckland, NZ) was used as the HFNC system. The artificial respiratory model consisted of a lung model (the Dual Adult Training and Test Lung, Michigan Instruments Inc., Grand Rapids, MI, USA) and a ventilator (Puritan Bennett™ 840, Covidien, Dublin, Ireland). The HFNC and the respiratory model were connected by the airway model (Endotracheal Intubation Training Model LM-059, Koken Co., Ltd., Tokyo, Japan). Respiratory settings were as follows: respiratory rate, 16 breaths/min; inspiratory time, 1 second; and tidal volume (VT), 300, 500, or 800 mL. CO2 was infused into a distal site of the lung model to maintain PETCO2, measured just below the glottis, at 40 mmHg at each VT setting without HFNC. HFNC flow was changed from 10-60 L/min in each VT setting, and the change of PETCO2 was measured in the open- and closed-mouth models.


With any VT setting in the open-mouth model, PETCO2 quickly decreased to 20-25 mmHg as HFNC started at 10 L/min. Thereafter, PETCO2 did not change with an increasing HFNC flow (Figure: solid lines). With the closed-mouth model, PETCO2 gradually decreased as the HFNC flow was increased. The VT settings of 300 and 500 mL had the same trends and reached the bottom level of 22 mmHg with HFNC flow over 50 L/min. The VT setting of 800 mL had a smaller decrease in PETCO2 to 28 mmHg (Figure 1: dotted lines).


Generation of PEEP by HFNC needs high flow as 35 L/min to generate PEEP of 3 cmH2O1). In this study, it was demonstrated that HFNC's washout of the dead space is effective with relatively low flow as low as 10 L/min in open-mouth model. HFNC flow of 10 L/min can deliver gas of 166 mL/min, and this amount of gas delivery was thought to be enough to wash out the dead space during the exhalation time. The effect was weaker in the closed-mouth model, but by increasing the HFNC flow produced an adequate effect. In this closed-mouth model, more gas leaked from the nostril instead of the mouth, and therefore, less gas washed out the dead space, which caused a need for more HFNC flow to lower the PETCO2.

Figure 1
figure 1

Reduction of PETCO2


We concluded that the washout effect depends on HFNC flow especially with closed-mouth breathing while it may reach maximum with a relatively low flow of 10 L/min with open-mouth breathing.


  1. Parke R: Nasal high-flow therapy delivers low level positive airway pressure. Br J Anaesth 2009, 103: 886–890. 10.1093/bja/aep280

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Nakamura, N., Kurota, M., Watanabe, T. et al. 0232. Evaluation of HFNC'S wash out effect; a comparison of open- and closed-mouth models. ICMx 2 (Suppl 1), P16 (2014).

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