By J. Brady Scott, PhD, RRT, RRT-ACCS, AE-C, FAARC, FCCP
Associate Professor, Rush University, Chicago
The Special Feature in the July 2020 issue of Critical Care Alert entitled “High-Flow Nasal Cannula Oxygen Therapy in Adult Acute Care” discussed the physiologic effects of high-flow nasal cannula (HFNC) (positive pressure, adequate heat and humidity, stable fraction of inspired oxygen [FIO2] delivery, and anatomic dead space washout) and evidence to support the use of the modality.1 Notably absent from that article was a section regarding flow settings, because the impact was quite uncertain at the time. Since then, additional evidence has emerged providing more clarity on the topic.2 This article intends to examine the impact of flow settings in adult patients for various clinical conditions.
Comfort
A purported benefit of HFNC oxygen therapy is comfort and patient tolerability. When evaluating the effect of temperature and flow on patient comfort, Mauri et al noted that patient comfort was lower at 37°C compared to 31°C but not different between 30 L/min and 60 L/min flows.3 While their study suggested that comfort was highest at the lowest temperature and high flows in a subgroup of patients needing ≥ 0.45 FIO2, it was limited by the short (20 minute) period of assessing tolerance.2 Comfort regarding temperature probably differs from individual to individual, and starting at 37°C (in an effort to provide optimum humidification) and adjusting to patient comfort is perhaps the best approach.
Studies evaluating the impact of flow settings on comfort show that individuals differ in their reports of comfort. Butt et al noted that comfort was highest at flows between 30 L/min to 40 L/min among post-extubation patients but admitted variability existed between individuals.4 A study evaluating flows of > 60 L/min noted that higher flows resulted in lower respiratory rates, increased lung homogeneity, and had more of a positive pressure effect; however, comfort was worse at the higher flows tested (~100 L/min).5 Past studies have demonstrated that higher flows most profoundly impact the known physiologic effects of HFNC, so starting with a flow of 40 L/min and titrating based on patient tolerance and other measures (oxygenation, ratio of oxygen saturation [ROX] index, respiratory rate [RR], and tolerance) has been suggested.2,6
Stable FIO2 Delivery, Positive Pressure, Anatomic Dead Space Washout, and Lung Impedance
When HFNC flow is lower than a patient’s peak tidal inspiratory flow (PTIF), tracheal FIO2 is lower than the FIO2 set on the device.2 The variability in tracheal FIO2 from lower HFNC flows is from room air (FIO2 = 0.21) entrainment, which dilutes the delivered amount. Thus, to create a “more stable” delivery of FIO2 from an HFNC device, flows must be set high enough to meet and/or exceed inspiratory demand.2
Some studies show that when PTIF is exceeded, positive pressure is generated. This effect is thought to be advantageous because it may improve alveolar recruitment. The challenge of generating pressure via HFNC is that multiple factors (open vs. closed mouth, nasal prong size, gas type, and lung compliance) affect the degree of positive pressure created.2,7,8 According to Li et al, mouth status affects positive pressure generation the most.2 When the mouth is opened, positive pressure levels are low, likely eliminating any clinically meaningful pressure levels. Regarding anatomic dead space washout, studies have shown that higher HFNC flows clear carbon dioxide better than lower flows at the same RR.2,9,10
Increasing flows improves global end-expiratory lung impedance (EELI) in healthy subjects and patients with acute hypoxemic respiratory failure. As flows increase, so do end-expiratory lung volume and positive end-expiratory pressure.11 However, studies have shown that high flows can cause overdistension in non-dependent lung regions.5,12 Those studies suggested that HFNC flows of 60 L/min to 65 L/min lead to more alveolar recruitment than overdistension, and higher flows, like 100 L/min, may lead to overdistension in non-dependent lung regions.2,5,12 It is important to acknowledge that there was a large variation between subjects during these studies. Some improvements in EELI were noted at 30 L/min or 45 L/min, but not at 60 L/min. Improvements in oxygenation from HFNC oxygen therapy, at least in part, depend on the balance of alveolar recruitment and overdistension.2
Other Effects of Flow
In 2017, Mauri et al noted that inspiratory effort was reduced by higher HFNC flows, which was attributed to alveolar recruitment, anatomic dead space washout, a reduction in nasal airway resistance, better secretion clearance, and an improvement in dynamic lung compliance.2,12 That said, they noted some patients appeared to work harder to breathe when flows were increased from 30 L/min or 45 L/min to 60 L/min. The increased work of breathing might have resulted from worsening lung compliance due to alveolar overdistension from the higher flows. Because higher flows may contribute to lung injury in patients without recruitment from higher HFNC flows, some have suggested that inspiratory effort be used to titrate flows.2,13
While most providers believe an oral diet is safe with HFNC oxygen therapy for stable patients who are not in danger of being intubated, the impact of flow in this situation is not clearly understood.14 Some studies using healthy volunteers suggest that HFNC might improve swallow function, while another noted choking was observed when flows were ≥ 40 L/min.15-17 The effect of various HFNC flows on swallow function deserves more research, especially clinical studies, so that clinicians can have confidence in feeding patients who require the modality. Until more is known, oral feeding should be monitored closely in patients requiring HFNC oxygen therapy support.2
Current Limitations and the Path Forward
The known physiologic effects of HFNC oxygen therapy are dependent upon flow.2 It appears that exceeding PTIF maximizes the benefits of HFNC oxygen therapy unless flows contribute to patient discomfort and alveolar overdistension. At the time of this writing, clinicians face the conundrum that there is no commercially available device to measure PTIF. Additionally, patient and disease conditions create variability in PTIF, further complicating the issue. Until clinicians have a device or tool to measure PTIF, they must rely on clinical indicators to set and titrate patient flows. In a systematic review by Li et al assessing the effect of flow settings, the authors suggested that clinical findings such as patient comfort, ROX index, RR, and oxygenation be used to individualize flow settings, at least for now.2
Summary
Studies focused on the impact of HFNC flow settings suggest significant variability between patients and conditions. It appears that higher flows are advantageous as long as patients can tolerate the device and lung injury is not occurring. Further research is needed to inform clinicians how to best initiate and titrate HFNC flow settings. Until then, clinicians can use clinical findings to individualize patient settings, recognizing that flows affect each patient differently.
REFERENCES
- Scott JB. High-flow nasal cannula oxygen therapy in adult acute care. Crit Care Alert 2020;28:25-29.
- Li J, Albuainain FA, Tan W, et al. The effects of flow settings during high-flow nasal cannula support for adult subjects: A systematic review. Crit Care 2023;27:78.
- Mauri T, Galazzi A, Binda F, et al. Impact of flow and temperature on patient comfort during respiratory support by high-flow nasal cannula. Crit Care 2018;22:120.
- Butt S, Pistidda L, Floris L, et al. Initial setting of high-flow nasal oxygen post extubation based on mean inspiratory flow during a spontaneous breathing trial. J Crit Care 2021;63:40-44.
- Basile MC, Mauri T, Spinelli E, et al. Nasal high flow higher than 60 L/min in patients with acute hypoxemic respiratory failure: A physiological study. Crit Care 2020;24:654.
- Li J, Scott JB, Fink JB, et al. Optimizing high-flow nasal cannula flow settings in adult hypoxemic patients based on peak inspiratory flow during tidal breathing. Ann Intensive Care 2021;11:164.
- Thille AW, Richard JC, Brochard L. The decision to extubate in the intensive care unit. Am J Respir Crit Care Med 2013;187:1294-1302.
- Drake MG. High-flow nasal cannula oxygen in adults: An evidence-based assessment. Ann Am Thorac Soc 2018;15:145-155.
- Moore CP, Katz IM, Pichelin M, et al. High flow nasal cannula: Influence of gas type and flow rate on airway pressure and CO2 clearance in adult nasal airway replicas. Clin Biomech 2019;65:73-80.
- Parke RL, McGuinness SP. Pressures delivered by nasal high flow oxygen during all phases of the respiratory cycle. Respir Care 2013;58:1621-1624.
- Onodera Y, Akimoto R, Suzuki H, et al. A high-flow nasal cannula system with relatively low flow effectively washes out CO2 from the anatomical dead space in a sophisticated respiratory model made by a 3D printer. Intensive Care Med Exp 2018;6:7.
- Mauri T, Alban L, Turrini C, et al. Optimum support by high-flow nasal cannula in acute hypoxemic respiratory failure: Effects of increasing flow rates. Intensive Care Med 2017;43:1453-1463.
- Zhang R, He H, Yun L, et al. Effect of postextubation high-flow nasal cannula therapy on lung recruitment and overdistension in high-risk patient. Crit Care 2020;24:82.
- Carlton ME, Peterson SJ, LaGorio L, et al. A survey of feeding practices with the use of high flow nasal cannula oxygen therapy. Respir Care 2022;67(Suppl 10):3774874.
- Sanuki T, Mishima G, Kiriishi K, et al. Effect of nasal high-flow oxygen therapy on the swallowing reflex: An in vivo volunteer study. Clin Oral Invest 2016;21:915-920.
- Allen K, Galek K. The influence of airflow via high-flow nasal cannula on duration of laryngeal vestibule closure. Dysphagia 2020;36:729-735.
- Arizono S, Oomagari M, Tawara Y, et al. Effects of different high-flow nasal cannula flow rates on swallowing function. Clin Biomech 2021;89:105477.
