Contents lists available at ScienceDirect Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech High fow nasal cannula: Infuence of gas type and fow rate on airway pressure and CO 2 clearance in adult nasal airway replicas C.P. Moore a , I.M. Katz b , M. Pichelin b , G. Caillibotte b , W.H. Finlay a , A.R. Martin a, ⁎ a Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada b Air Liquide Santé International, Paris Innovation Campus, Les Loges en Josas, France ARTICLE INFO Keywords: High fow nasal cannula Respiratory support Nasal cannula Heliox PEEP Upper airways ABSTRACT Background: High fow nasal cannula therapy is a form of respiratory support which delivers high fow rates of heated, humidifed gas to the nares via specialized cannula. Two primary mechanisms of action attributed to the therapy are the provision of positive airway pressure as well as clearance of CO 2 -rich exhaled gas from the upper airways. Methods: Physiologically accurate nose-throat airway replicas were connected at the trachea to a lung simulator, where CO 2 was supplied to mimic the CO 2 content in exhaled gas. Cannula delivered either air, oxygen or heliox (80/20%volume helium/oxygen) to the replicas at fow rates ranging from 0 to 60 l/min. Five replicas and three cannulas were compared. Tracheal pressure and CO 2 concentration were continuously measured. The lung si- mulator provided breaths with tidal volume of 500 ml and frequency of 18 breaths/min. Additional clearance measurements were conducted for tidal volume and breathing frequency of 750 ml and 27 breaths/min, re- spectively. Findings: Cannula fow rate was the dominant factor governing CO 2 concentration. Average CO 2 concentration decreased with increasing cannula fow rate, but above 30 L/min this efect was less pronounced. Tracheal positive end-expiratory pressure increased with fow rate and was lower for heliox than for air or oxygen. A predictive correlation was developed and used to predict positive end-expiratory pressure for a given cannula size as a function of supplied fow rate and occlusion of the nares. Interpretation: Compared with administration of air or oxygen, administration of heliox is expected to result in similar CO 2 clearance from the upper airway, but markedly lower airway pressure. 1. Introduction High fow nasal cannula (HFNC) therapy is a form of non-invasive respiratory support that has been used to treat respiratory failure in adults (Dysart et al., 2009; Nishimura, 2015; Zhang et al., 2016). The therapy uses heated and humidifed gas, delivered through specialized nasal cannula to patients at high fow rates. Gas fows typically range in adults from 15 to 60 LPM, although higher fow rates have been con- sidered (Parke, 2015; Vargas et al., 2015). In adults, HFNC therapy is often used to treat a variety of respiratory disorders, such as acute re- spiratory failure in chronic obstructive pulmonary disorder (COPD)(Di mussi et al., 2018; Dysart et al., 2009; Zhang et al., 2016). When compared to conventional oxygen therapy, HFNC has shown reduced reintubation and improved secondary outcomes, and similar outcomes to other forms of respiratory support, such as non-invasive ventilation, in post-extubated patients (Di mussi et al., 2018; Hernández et al., 2016; Maggiore et al., 2014; Nishimura, 2015; Rittayamai et al., 2014). One of the primary mechanisms of action of HFNC, which distin- guishes it from other forms of respiratory support, is the upper airway deadspace washout (Dysart et al., 2009; Zhang et al., 2016). In vitro and in silico studies investigating the upper airway washout have found CO 2 clearance increases with fow rate, although this increase is less pro- nounced at higher fow rates (Moore et al., in press; Nielsen et al., 2018; Van Hove et al., 2016a). Recent in vitro experiments done in realistic adult nasal airway replicas found that faster gas clearance corresponded with higher fow rates and with smaller cannula outlet area (Miller et al., 2016; Moore et al., in press). A second mechanism of action widely attributed to HFNC therapy is https://doi.org/10.1016/j.clinbiomech.2019.04.004 Received 8 November 2018; Accepted 4 April 2019 ⁎ Corresponding author at: Department of Mechanical Engineering, University of Alberta, 10-324 Donadeo Innovation Centre for Engineering, Edmonton, AB T6G1H9, Canada. E-mail addresses: cpmoore@ualberta.ca (C.P. Moore), Ira.katz@airliquide.com (I.M. Katz), Marine.pichelin@airliquide.com (M. Pichelin), Georges.caillibotte@airliquide.com (G. Caillibotte), fnlay@ualberta.ca (W.H. Finlay), andrew.martin@ualberta.ca (A.R. Martin). Clinical Biomechanics 65 (2019) 73–80 0268-0033/ © 2019 Elsevier Ltd. All rights reserved. T