High-Flow Nasal Cannula — Mechanisms, Evidence & the ROX Index

Also known as High-flow nasal cannula · HFNC · HFNO · Optiflow · ROX index · FLORALI trial · Dead-space washout · Heated humidified high-flow

High-flow nasal cannula (HFNC) delivers up to 60 L/min of heated, humidified oxygen-enriched gas through large-bore nasal prongs. Its five physiological mechanisms are low-level PEEP, dead-space washout, heated humidification, reduced inspiratory resistance, and a precise titrated FiO2. The FLORALI trial (NEJM 2015) supports HFNC in acute hypoxaemic respiratory failure, with reduced intubation in pneumonia. The ROX index (SpO2/FiO2 divided by respiratory rate; Roca, J Crit Care 2016) predicts success and the need to intubate — at least 4.88 favours success, under 3.85 favours failure. HFNC is a bridge, not a destination: a failing patient must be intubated.

high7 referencesUpdated 26 July 202621 min readVerification in progress

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High-flow nasal cannula (HFNC) delivers up to 60 L/min of heated, humidified, titrated oxygen through large-bore nasal prongs. It works through five mechanisms — low-level PEEP, dead-space washout, heated humidification, reduced inspiratory resistance, and a precise FiO2 — and sits between standard oxygen and NIV. FLORALI (NEJM 2015) supports HFNC as first-line for acute hypoxaemic respiratory failure, especially in pneumonia (PaO2/FiO2 less than 150), where it reduced intubation and 90-day mortality. The ROX index — (SpO2/FiO2) divided by respiratory rate (Roca 2016) — is the bedside prediction tool: at least 4.88 favours success, under 3.85 favours failure, and the trend matters more than any single value. HFNC is a bridge, not a destination — a failing patient must be intubated.^[1]^[2]

Meet the patient

It is 2am in the emergency department. A 62-year-old man with community-acquired pneumonia — fever 39, RR 32, SpO2 90% on a non-rebreather at 15 L/min — is being weighed against NIV or intubation by the night registrar. He is conscious, distressed, working hard, but not yet tiring. You reach for the Airvo. The question you will be asked, in some form, every shift: does HFNC work here, and how will I know when to stop?^[1]

Hold two things and everything below slots into place. HFNC is a multi-mechanism support device, not just "lots of oxygen" — and HFNC failure must be declared early, before the patient crashes into a delayed, higher-risk intubation. The ROX index is the bedside number that lets you do both.^[2]

References7Show

[1]Frat JP, Thille AW, Mercat A, et al. High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure N Engl J Med, 2015.PMID 25981908
[2]Roca O, Messika J, Caralt B, et al. Predicting success of high-flow nasal cannula in pneumonia patients with hypoxemic respiratory failure: The utility of the ROX index J Crit Care, 2016.PMID 27481760
[3]Stéphan F, Barrucand B, Petit P, et al. High-Flow Nasal Oxygen vs Noninvasive Positive Airway Pressure in Hypoxemic Patients After Cardiothoracic Surgery: A Randomized Clinical Trial JAMA, 2015.PMID 25980660
[4]Hernández G, Vaquero C, Colinas L, et al. Effect of Postextubation High-Flow Nasal Cannula vs Noninvasive Ventilation on Reintubation and Postextubation Respiratory Failure in High-Risk Patients: A Randomized Clinical Trial JAMA, 2016.PMID 27706464
[5]Hernández G, Vaquero C, González P, et al. Effect of Postextubation High-Flow Nasal Cannula vs Conventional Oxygen Therapy on Reintubation in Low-Risk Patients: A Randomized Clinical Trial JAMA, 2016.PMID 26975498
[6]Mauri T, Turrini C, Eronia N, et al. Physiologic Effects of High-Flow Nasal Cannula in Acute Hypoxemic Respiratory Failure Am J Respir Crit Care Med, 2017.PMID 27997805
[7]Ou X, Hua Y, Liu J, Gong C, Zhao W. Effect of high-flow nasal cannula oxygen therapy in adults with acute hypoxemic respiratory failure: a meta-analysis of randomized controlled trials CMAJ, 2017.PMID 28246239