Abstract
Background: There is little guidance on optimal flow settings during therapy with high-flow nasal cannula (HFNC) based on breathing patterns in patients with acute hypoxemic respiratory failure (AHRF).
Methods: After approval by the ethic committee in Rush University (No. 18102503-IRB01) and registration on clinicaltrials.gov (NCT03738345), we conducted a prospective clinical study that included adult patients with hypoxemia treated by HFNC with a fraction of inspired oxygen (FIO2) ≥ 0.4. Peak tidal inspiratory flow (PTIF) was measured on each study participant, and HFNC flows were set to match their PTIF and then increased by 10 L/min up to a maximum flow of 60 L/min. FIO2 was titrated to maintain pulse oximetry (SpO2) of 90-97% at each flow setting. SpO2/ FIO2, respiratory rate (RR), ROX index ([SpO2/FIO2]/RR), and patient comfort were recorded after 5–10 min on each HFNC setting. We also conducted an in-vitro study replicating the breathing pattern acquired from the clinical study to explore the relationship between the HFNC flow settings and the tracheal FIO2, peak inspiratory and expiratory pressures.
Results: Forty-nine hypoxemic patients (25 had COVID-19) age 58.0 (SD 14.1) y were enrolled. Mean PTIF (34.1 [8.5] L/min) was similar in COVID-19 and non-COVID-19 patients. As HFNC flow increased, SpO2/ FIO2 and ROX index significantly improved (P < 0.05) (Figure 1). In the in-vitro study, when the HFNC flow was set >PTIF, tracheal peak inspiratory and expiratory pressures increased as HFNC flow increased but the FIO2 did not change (Figure 2).
Conclusions: Hypoxemic patients, with and without COVID-19, present similar PTIF values of ~30 to 40 L/min. We observed improvement in oxygenation with HFNC flows set above the patient’s PTIF. Thus, a pragmatic approach to set optimal flows in patients with AHRF would be to initiate HFNC flow at ~40 L/min and titrate the flow based on improvement in oxygenation and patient tolerance.
Fig 1. The correlation between SpO2/FIO2 ratio and flow ratio
Fig 2. The correlation between flow ratio and FIO2, peak inspiratory and expiratory pressure at trachea in the in-vitro study.
Footnotes
Commercial Relationships: Dr. Li declares to receive research funding from Fisher & Paykel Healthcare Ltd, Aerogen Ltd, and Rice Foundation and lecture honorarium from American Association for Respiratory Care and Fisher & Paykel Healthcare Ltd outside the submitted work. Dr. Li is the section editor for respiratory care journal. Dr. Scott declares to receive research funding from Teleflex. Dr. Fink is Chief Science Officer for Aerogen Pharma Corp. Dr. Dhand reports remuneration from GSK Pharmaceuticals, Boehringer-Ingelheim, Bayer, Mylan, Teva, and Astra-Zeneca Pharmaceuticals outside the submitted work. Dr. Roca discloses a research grant from Hamilton Medical and speaker fees from Hamilton Medical, Ambu, Aerogen Ltd, and Fisher&Paykel, and non-financial research support from Timpel and Masimo Corporation. His institution received fees for consultancy from Hamilton Medical. None of the companies/institutions had a role in the study design, data collection, analysis, preparation of the manuscript, or the decision to publish the findings. Ms. Reed has no conflicts to disclose.
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