Correlation of high flow nasal cannula outlet area with gas clearance and pressure in adult upper airway replicas
Introduction
Acute respiratory failure is a cause of approximately 30% of ICU admissions, and symptoms may persist despite treatment (Nagata et al., 2015). Origins of such respiratory failure are numerous; acute lung injury (ALI) or acute respiratory distress syndrome (ARDS), related to community- and hospital-acquired pneumonias, sepsis, chest or head injury, aspiration, or other causes, can rapidly progress to acute respiratory failure requiring noninvasive or invasive mechanical ventilation (Frat et al., 2015, Matthay et al., 2012, Sztrymf et al., 2012).
An increasingly common intervention for acute respiratory failure is the use of high flow nasal cannula (HFNC) therapy to support patient breathing (Cuquemelle et al., 2012, Frat et al., 2015, Maggiore et al., 2014, Roca et al., 2010, Sztrymf et al., 2012, Vargas et al., 2015). HFNC delivers heated and humidified mixtures of oxygen and air through specialized nasal cannulas (Vargas et al., 2015). HFNC typically delivers gas at up to 60 liters per minute (LPM), although higher flow rates, up to 100 LPM, have been investigated (Parke, 2015). Oxygen provided by HFNC has been shown to improve oxygenation and reduce ventilation requirements in critical care subjects (Vargas et al., 2015). In a recent large, randomized clinical trial enrolling patients with non-hypercapnic acute hypoxemic respiratory failure, HFNC oxygen therapy resulted in reduced mortality in the ICU and at 90 days, as compared with standard oxygen therapy or noninvasive ventilation (Frat et al., 2015).
While data for hypoxemic patients using HFNC is promising, existing reports do not conclusively characterize the underlying mechanisms of action, especially for patients on palliative care, or at risk of respiratory failure due to muscle fatigue (Roberts and Oeckler, 2015, Spoletini et al., 2015). Delivery of high flows in excess of patient inspiratory flow rates allows the fraction of inhaled oxygen (FiO2) to be tightly controlled by adjusting the oxygen concentration in the delivered gas mixture. Additionally, the provision of high flow incurs a small positive distending pressure, which may improve ventilatory mechanics and maintain alveolar patency similar to the maintenance of continuous positive airway pressure (CPAP) (Dysart et al., 2009). However, computational fluid dynamics (CFD) simulations suggest pressures equivalent to those of continuous positive airway pressure (CPAP) above 6 cmH2O are not typical (Van Hove et al., 2016).
Another primary benefit that is proposed of HFNC is the ability to washout exhaled carbon dioxide (CO2) from the nasal cavity, such that a higher concentration of oxygen can be inspired, especially during weak or injured breathing (Dysart et al., 2009, Van Hove et al., 2016). For this mechanism, the nasopharyngeal dead space becomes a reservoir for fresh gas, which ensures that during the beginning of inspiration the inhaled oxygen volume is maximized, while washing out CO2, thus increasing alveolar efficiency. It has been suggested that a primary advantage of HFNC therapy over conventional CPAP is this continuous washout, or dead space clearance, mechanism (Dysart et al., 2009).
The nasopharyngeal space is a complex geometry, making dead space clearance challenging to model analytically. Experimental results reported by Moller et al. in a static, geometrically realistic nasopharyngeal replica indicated that the use of high flow has the capacity to increase the rate at which dead space gas is replaced (Möller et al., 2015), for therapy flows of up to 45 LPM. As the flow rate received from the cannula increased from 15 to 45 LPM, a statistically significant decrease in clearance half times was measured (Möller et al., 2015). Similarly, Van Hove et al. (2016) experimentally evaluated CO2 clearance in a realistic nasal cavity replica to confirm CFD simulations done in the same geometry, concluding that CO2 clearance increased with delivered flow rates up to 60 LPM. Previously, studies of aerosol deposition during HFNC have demonstrated significant backflow through the nasal passages, which may indicate increased clearance (Golshahi et al., 2014a, Golshahi et al., 2014b). Additionally, Gardner et al. (2016) demonstrated experimentally CO2 clearance of upper airways in accurate replicas of infant upper airways.
In the present study, we examined the influence of HFNC flow rate on clearance times and tracheal pressures in vitro in five adult nasal airway replicas. Use of multiple geometries permitted insight into intersubject variability in gas clearance and airway pressure arising from variation in nasopharyngeal geometry. In addition, the potential influence of nasal cannula size was investigated by repeating experiments for two HFNC used with commercial high flow delivery systems, and a standard cannula commonly used for low-flow oxygen administration.
Section snippets
Nasal airway replicas
Nasal airway replicas were constructed in acrylic using rapid prototyping (Invision SR 3-D printer) based on magnetic resonance imaging (MRI) scans of healthy adult subjects as described elsewhere (Golshahi et al., 2011). The subjects were originally selected in Golshahi et al. (2011) from a larger set of subjects to represent a wide range of pressure drops from the nares to the trachea of the replicas (Golshahi et al., 2011). The nasopharynx and larynx were constructed separately and then
Clearance time
Average tracheal and laryngeal clearance times for the five replicas are shown in Fig. 4. Clearance time is defined as the time measured between 25% and 50%, 75% or 95% oxygen at the sample point. Clearance time decreases from the larger to the smaller cannula for all flow rates and at both the trachea and the larynx. Additionally Fig. 4 shows that as oxygen flow rate increases, the clearance time decreases. This trend is consistent with the theory for fully mixed volumes where the time
Cannula clearance performance
Cannula selection is a major factor in determining clearance time. Average clearance times, shown in Fig. 4, show that the small cannula consistently has shorter clearance times than the larger cannulas.
The effect of both supply flow rate and cannula outlet diameter on clearance time is captured by the Reynolds number, as . The clearance time is plotted as a function of Reynolds number for each replica in Fig. 6, where gas properties correspond to 100% oxygen. For a given replica
Conclusions
Both upper airway clearance rate and tracheal pressure increased with flow rate in our high flow therapy measurements. The clearance rate and tracheal pressure also increased with decreasing cannula outlet diameter. The dependence upon outlet velocity and nares velocity of tracheal pressure follows that expected from a mechanical energy balance. As both increased flow rate and tracheal pressure are generally associated with improved clinical outcomes, it may be that a smaller cannula outlet
Abbreviations
- ALI
acute lung injury
- ARDS
acute respiratory distress syndrome
- CFD
computational fluid dynamics
- CPAP
continuous positive airway pressure
- HFNC
high flow nasal cannula
- HFOT
high flow oxygen therapy
- LPM
liters per minute
- PEEP
positive end expiratory pressure
- SLPM
standard liters per minute
Acknowledgements
Not applicable.
Ethics approval and consent to participate
No human participants, or human tissue are included in this study. MRI images were obtained under the approval of the University of Alberta Health Research Ethics Board, as previously reported (19).
No other ethical concerns relating to human data are applicable for this study.
Competing interests
IK and CG are currently employees of Air Liquide at the time of this study, and AM has previously been employed by Air Liquide. Air Liquide markets HFNC devices in several countries.
Funding
This work was funded by Air Liquide (RES0025295) and by Alberta Economic Development and Trade (0119-20160318-UALBERTA), and benefited from an equipment grant from the Canadian Natural Sciences and Engineering Research Council.
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