Effect of Very-High-Flow Nasal Therapy on Airway Pressure and End-Expiratory Lung Impedance in Healthy Volunteers

Abstract

BACKGROUND: Previous research has demonstrated a positive linear correlation between flow delivered and airway pressure generated by high-flow nasal therapy. Current practice is to use flows over a range of 30 -60 L/min; however, it is technically possible to apply higher flows. In this study, airway pressure measurements and electrical impedance tomography were used to assess the relationship between flows of up to 100 L/min and changes in lung physiology. METHODS: Fifteen healthy volunteers were enrolled into this study. A high-flow nasal system capable of delivering a flow of 100 L/min was purpose-built using 2 Optiflow systems. Airway pressure was measured via the nasopharynx, and cumulative changes in end-expiratory lung impedance were recorded using the PulmoVista 500 system at gas flows of 30 -100 L/min in increments of 10 L/min. RESULTS: The mean age of study participants was 31 (range 22-44) y, the mean ؎ SD height was 171.8 ؎ 7.5 cm, the mean ؎ SD weight was 69.7 ؎ 10 kg, and 47% were males. Flows ranged from 30 to 100 L/min with resulting mean ؎ SD airway pressures of 2.7 ؎ 0.7 to 11.9 ؎ 2.7 cm H 2 O. A cumulative and linear increase in end-expiratory lung impedance was observed with increasing flows, as well as a decrease in breathing frequency. CONCLUSIONS: Measured airway pressure and lung impedance increased linearly with increased gas flow. Observed airway pressures were in the range used clinically with face-mask noninvasive ventilation. Developments in delivery systems may result in this therapy being an acceptable alternative to face-mask noninvasive ventilation.

A Skeptical Perspective on High-Flow Nasal Cannula in the Treatment of Acute Hypoxemic Respiratory Failure

“…Underscoring this point, we note that change in end-expiratory lung impedance (and thus end-expiratory lung volume) was widely variable, suggesting either a limitation of technique or that the relationship between flow and end-expiratory lung volume may be more complicated then assumed. The latter possibility appears to be supported by the analysis of the P aO 2 /F IO 2 data from Vargas et al 10 Applying the estimate (10 L/min flow ϭ 1.2 cm H 2 O) of Parke et al 11 to the subjects in Vargas' study with flow fixed at 60 L/min in all subjects, one would expect an airway pressure of between 5 and 7 cm H 2 O. If this were to correlate with PEEP at the alveolus, as has been implied in previous studies, it would not be unreasonable, using a conservative interpretation of such estimates, to expect a comparable improvement in oxygenation in both the 60 L/min HFNC group and the NIV group receiving 5 cm H 2 O CPAP.…”

“…Parke et al 12 had previously described a linear correlation between flow and airway pressure in HFNC up to the currently accepted maximum rate of 60 L/min. Here, 11 they increased flow up to as much as 100 L/min in healthy volunteers and quantified the correlation between flow rate and nasopharyngeal PEEP at about 1 cm H 2 O nasopharyngeal PEEP increment for each 10 L/ min of flow, up to a maximum of 8 -12 cm H 2 O at 100 L/ min.…”

“…Several theories as to how HFNC may alter respiratory mechanics include enhanced washout of dead space with increased carbon dioxide removal, improved humidification and its effect on airway secretions, and the generation of PEEP, a beneficial by-product of high flow delivery. 3 Two small studies in this issue from Parke et al 11 and Vargas et al 10 used complementary experimental approaches to demonstrate physiologic changes during HFNC use that could presumably improve oxygenation as well as symptoms typically associated with respiratory distress. Both studies reported a rapid, significant decrease in breathing frequency after the initiation of HFNC.…”

Efficacy and Safety of Using High-Flow Nasal Oxygenation in Patients Undergoing Rapid Sequence Intubation

“…(12), this fact may be related to both effective supply of oxygen and other mechanisms, such as apnoeic ventilation (19)(20)(21). In addition, during pre-oxygenation with HFNO, end expiratory lung volume and FRC increased and dead space decreased (18,(22)(23)(24)(25).…”

High-Flow Nasal Cannula Support Therapy: New Insights and Improving Performance

“…In fact, most patients included in studies have bilateral infiltrates [20,32]. The Berlin definition of ARDS [34] requires a minimum of 5 cmH 2 O of PEEP, and it has been shown that HFNC can provide a level of PEEP that is higher at peak expiratory pressure [7]. Moreover, ARDS does not begin at the time of mechanical ventilation onset.…”