High Flow Hacks, Part 1

High-flow nasal cannula is an oxygenation strategy for acute hypoxemic respiratory failure, not a substitute for ventilatory support. HFNC improves comfort, allows eating and speech, and can buy time safely when paired with reassessment tools like the ROX index.

High-Flow Nasal Cannula Basics

• Best-fit respiratory failure: HFNC fits acute hypoxemic respiratory failure, especially when patients need high FiO2 without the mask burden of traditional noninvasive ventilation.
• Device physiology and comfort: Heated humidified flow up to 50-60 L/min reduces air hunger and improves tolerance, while allowing patients to talk, eat, and clear secretions.
• Low-level PEEP effect: HFNC generates a modest PEEP effect at higher flows, most noticeable when the patient keeps the mouth closed, a bedside trick we emphasize in the episode.
• Ideal bedside candidates: Good candidates include septic hypoxemia, secretion-heavy or vomiting patients, claustrophobia with masks, and preoxygenation before intubation.
• Escalation mindset early: If you are approaching the ceiling of your device, think ahead to intubation rather than reflexively stepping to NIV unless the diagnosis has changed.

Choosing HFNC Versus NIPPV

• Oxygenation versus ventilation: HFNC is primarily an oxygenation tool, whereas BiPAP adds inspiratory support and unloads work of breathing in mechanical respiratory failure.
• COPD and CHF exception: BiPAP remains the preferred noninvasive strategy for COPD and cardiogenic pulmonary edema, where mortality benefit is already established.
• CPAP versus BiPAP distinction: CPAP supplies continuous pressure like PEEP, while BiPAP adds separate inspiratory pressure support that better assists the tiring patient.
• Tolerance rescue options: Mask intolerance should start with verbal coaching; if needed, tiny-dose ketamine or fentanyl can help, with nuances on patient selection we get into in the chapter.
• When to intubate: Worsening hypoxia, rising work of breathing, aspiration risk, declining mental status, or arrest are hard stops where noninvasive support has failed.

Predicting HFNC Failure

• ROX index role: The ROX index combines oxygenation with respiratory rate and is the most ED-friendly way to track whether HFNC is working or failing.
• Trend over time: A ROX index that stays low or worsens over serial checks signals likely failure; we walk through the timing checkpoints in the episode.
• P/F ratio limits: A falling P/F ratio also points to worsening lung injury, but repeated ABGs and longer observation make it less practical in the ED.
• Severity score signal: Higher or rising SOFA and APACHE scores correlate with HFNC failure, though they are usually too cumbersome for real-time emergency decisions.
• Delayed intubation risk: The real hazard is not HFNC itself but missing failure early, because delayed intubation in worsening hypoxemia is linked to worse outcomes.

References:

1. Roca O, et al. An Index Combining Respiratory Rate and Oxygenation to Predict Outcome of Nasal High-Flow Therapy. Am J Respir Crit Care Med. 2019;199(11):1368-1376. PMID: 30576221
2. Rochwerg B, et al. High flow nasal cannula compared with conventional oxygen therapy for acute hypoxemic respiratory failure: a systematic review and meta-analysis. Intensive Care Med. 2019;45(5):563-572. PMID: 30888444
3. Ou X,et al. Effect of high-flow nasal cannula oxygen therapy in adults with acute hypoxemic respiratory failure: a meta-analysis of randomized controlled trials. CMAJ. 2017;189(7):E260-E267. PMID: 28246239
4. Ferreyro BL, et al. Association of Noninvasive Oxygenation Strategies With All-Cause Mortality in Adults With Acute Hypoxemic Respiratory Failure: A Systematic Review and Meta-analysis. JAMA. 2020;324(1):57-67. PMID: 32496521
5. Li J,et al. High-flow nasal cannula for COVID-19 patients: low risk of bio-aerosol dispersion. Eur Respir J. 2020;55(5):2000892. Published 2020 May 14. PMID: 32299867
6. Mauri T, et al. Physiologic Effects of High-Flow Nasal Cannula in Acute Hypoxemic Respiratory Failure. Am J Respir Crit Care Med. 2017;195(9):1207-1215. PMID: 27997805
7. Kang BJ, et al. Failure of high-flow nasal cannula therapy may delay intubation and increase mortality. Intensive Care Med. 2015;41(4):623-632. PMID: 25691263
8. Mauri T, et al. Optimum support by high-flow nasal cannula in acute hypoxemic respiratory failure: effects of increasing flow rates. Intensive Care Med. 2017;43(10):1453-1463. PMID: 28762180
9. Kim SH, Kim CH, Kim SY, Song SH et al. Predicting factors for the failure of high flow nasal cannula therapy in patients with acute respiratory failure. European Resp Journal 2018;PA2297. DOI: 10.1183/13993003.congress-2018.PA2297