Allison: High flow like a pro!!

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