Allison: High flow like a pro!!

Today we are excited to welcome back one of the smartest fellows we have ever graduated, Michael Allison, MD. Dr. Allison is a native New Yorker, training at SUNY Downstate College of Medicine before completing a rigorous EM/IM/CCM training program here at the University of Maryland. Since that time he has spent his days at St. Agnes Hospital located downtown in Baltimore, MD. Since his departure, Dr. Allison has spent countless hours researching the exact mechanisms of respiratory failure in an attempt to solve this problem before it ever develops. Today he takes us through his approach to using high flow nasal cannula for nearly every patient who walks into his ICU, and in doing so unlocks a few novel uses along the way. Trust me, this is a talk your patients cannot afford for you to miss!

Clinical Pearls (Thanks to Mike Allison, MD)

  • Most data regarding high-flow nasal cannula (HFNC) use has been recent (i.e. last 7 years)
    • Gradually more and more data is being released as time goes on
  • How does it work? 
    • Concept:
      • Air is heated and humidified
        • FiO2 is created by mixing 100% wall oxygen with room air
      • Flow can range from 20-60 lpm
    • O2 dilution
      • Traditionally taught that for each liter of oxygen, we get 4% FiO2, however this is FALSE as we dilute this oxygen with room air
      • High flow reduces air entrainment (unlike conventional nasal cannula, or NRB) and allows actual FiO2 values >90%
    • Under Pressure
      • Posterior oropharynx pressure in patients on HFNC ranges 2-6cm H2O with mouth closed and 0-1cm H20 with mouth open
        • We care about the pressure transmitted down the trachea, peaks at 2cm H2O
        • A modest amount of positive pressure
      • Amount of pressure is flow-dependent and peaks at end expiration
    • End-expiratory lung volume
      • HFNC can increased FRC by 25%
        • Theoretically: improves recuitment and physiology
    • Wash-out
      • Efficiency of breathing improves due to oxygen wash out of CO2
        • May be reason patient work of breathing improves with HFNC
    • Conditioning of air
      • 37º F at 100% relative humidity
        • A minute ventilation of 6L/min requires ~156 Kcal/min to condition that air
        • Preconditioned air = decreased work of breathing
      • If HFNC does this for the patient, may improve outcomes
        • No adult studies to support this concept
        • Pediatric studies children who were randomized to HF after birth had larger weight gains
  • When to use it?
    • Acute hypoxia:
      • Florali NEJM 2015 randomized 300 patients to HFNC vs standard O2 therapy vs NIPPV
        • Primary outcome was avoidance of intubation
          • Decreased in HFNC group but did not meet statistical significance (38% vs 47% in standard O2 and 50% in NIPPV, P=0.18)
          • Post-hoc: there was a decrease in intubations in patients with P:F<200
        • Secondary outcome was to eval an improvement in 90-day mortality
          • Just reached a statistically significant decrease in HF group (12% vs 28% in Standard O2 and 23% in NIPPV, P=0.05).
      • Consideration #1: 300 patients may not be large enough to properly power a three-way comparison study
      • Consideration #2: The majority of patients had community-acquired pneumonia and were excluded if they showed any signs of shock
    • Early vs late application:
      • Intensive care medicine 2015 study of 175 patients
        • Median mortality in early group (applied to patient <48 hours after admission) was 39% vs 67% in late group (applied to patient >48 hours after admission)
        • Late group had a better PaO2 to start
    • Predicting failure: most trials have shown 25-30% of patients will fail HFNC
      • Intensive Care Medicine 2011 pilot study showed 3 predictors of HFNC failure at 2 hours of application:
        1. Increased respiratory rate
        2. Thoraco-abdominal dyssynchrony
        3. Drop from baseline PaO2
      • Derivation studies have used a ROX index (SaO2/Fio2)/RR of <4.88 at 12 hours post HFNC application to predict failure
    • Post-extubation:
      • AJRCCM 2014 study compared HFNC vs venturi mask for the 48 hours post-extubation and found reintubation rates were significantly lower in HF group (8% vs 35%, p=0.001).
        • HFNC: Better P:F ratio at 24 hours with divergence over time + less desaturations + decreased RR
        • BUT the 35% re-intubation rate for venturi is REALLY high
      • JAMA 2016 study: low-risk patients (intubated for <7 days, <65 yo, APACHE<12, no CHF/COPD, underwent simple weaning, had no hypercarbia)
        • Significantly lower reintubation rates for 72 hours in group using HFNC for 24 hours after extubation (5% vs 12%, p=0.04)
        • So… is this saying that EVERY patient should be extubated to HFNC?
  • Practical tips:
    1. Start at 60 lpm and titrate down
      • Patients tolerate this flow well with little adverse effects
    2. Titrate FiO2 to Sp02 90-94%
      • Titrating to SpO2 100% leads to O2 toxicity
    3. 2-12 hours after HFNC application, reassess for intubation:
      • Respiratory rate
      • P:F ratio
      • Pattern of breathing
      • ROX index
    4. COPD, pulmonary edema, or patients in shock = NIPPV (these were the majority of the patients who failed HFNC in the trials), NOT HFNC
      • Future data may show benefit in these conditions but none currently exist
    5. Beware of “60lpm/100%FiO2”
      1. When patients who are failing HFNC are left on these maximal settings for too long, they tend to desaturate during intubation

Suggested Reading

  1. Kang BJ, Koh Y, Lim CM, Huh JW, Baek S, Han M, Seo HS, Suh HJ, Seo GJ, Kim EY, Hong SB. Failure of high-flow nasal cannula therapy may delay intubation and increase mortality. Intensive Care Med. 2015 Apr;41(4):623-32. doi: 10.1007/s00134-015-3693-5. Epub 2015 Feb 18. [PubMed Link]
  2. Sztrymf B, Messika J, Bertrand F, Hurel D, Leon R, Dreyfuss D, Ricard JD. Beneficial effects of humidified high flow nasal oxygen in critical care patients: a prospective pilot study. Intensive Care Med. 2011 Nov;37(11):1780-6. [PubMed Link]
  3. Maggiore SM, Idone FA, Vaschetto R, Festa R, Cataldo A, Antonicelli F, Montini L, De Gaetano A, Navalesi P, Antonelli M. Nasal high-flow versus Venturi mask oxygen therapy after extubation. Effects on oxygenation, comfort, and clinical outcome. Am J Respir Crit Care Med. 2014 Aug 1;190(3):282-8. [PubMed Link]
  4. Hernández G, Vaquero C, González P, Subira C, Frutos-Vivar F, Rialp G, Laborda C, Colinas L1, Cuena R, Fernández R. Effect of Postextubation High-Flow Nasal Cannula vs Conventional Oxygen Therapy on Reintubation in Low-Risk Patients: A Randomized Clinical Trial. JAMA. 2016 Apr 5;315(13):1354-61. [PubMed Link]
  5. Frat JP, Thille AW, Mercat A, Girault C, Ragot S, Perbet S, Prat G, Boulain T, Morawiec E, Cottereau A, Devaquet J, Nseir S, Razazi K, Mira JP, Argaud L, Chakarian JC, Ricard JD, Wittebole X, Chevalier S, Herbland A, Fartoukh M, Constantin JM, Tonnelier JM, Pierrot M, Mathonnet A, Béduneau G, Delétage-Métreau C, Richard JC, Brochard L, Robert R; FLORALI Study Group; REVA Network. High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure. N Engl J Med. 2015 Jun 4;372(23):2185-96. [EMCrit Link]
About the Author

Jim Lantry

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Just your average critical care doc: Wandering the ED and ICUs for the USAF down in the San Antonio Military Medical Center, traveling the globe to cannulate for ECLS wherever the need arises, and trying to keep up with great minds of today. E:

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