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Although clinical studies on the effect of the high-flow nasal cannula (HFNC) system on washout have been performed, the mechanism has only been estimated from studies of transtracheal oxygen therapy, which is quite a different system; thus, the mechanism for the washout effect remains to be clarified. We used an artificial respiratory system model to evaluate the washout effect of the HFNC system. The experimental system consisted of the Optiflow instrument (Fisher and Paykel Healthcare, Auckland, New Zealand), which was used as the HFNC system and was connected to the Dual Adult Training and Test Lung (TTL [lung model]: tidal volume, 500 ml; respiratory rate, 16 /min; inspiration time, 1.0 sec; Michigan Instruments Inc., Grand Rapids, MI, USA) via the LM-059 Koken Airway Management Model (airway model: upper airway volume, 200 ml) (Koken Co., Ltd., Tokyo, Japan). CO2 was infused into a respiratory lung model to maintain the end-tidal PCO2 (PETCO2) of 40 mmHg at the subglottic area. PETCO2 was measured by the Capnomac Ultima (Datex-Ohmeda GE Healthcare, Fairfield, CT, USA). The HFNC gas flow rate was varied from 10 to 60 L/min, and changes in PETCO2 were measured using open- and closed-mouth airway models.
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