HFNC
Basics:
HFNC is generally well tolerated
Flow ranges from 1-40L/min
Gas can be humidified and warmed
Decreases WOB and CO2
Increases O2 sat
Proposed Mechanism of Action:
Washes out CO2 rich gas from anatomical dead space (areas that are ventilated but not perfused, such as the area between the mouth to bronchi)
Provides positive airway pressure to stent open airways and decreases inspiratory effort
Since HFNC air is warmed and humidified, the body expends less energy optimizing the gas
2012 - Predictors of Failure in Infants with Viral Bronchiolitis Treated with HFNC (Peds Critical Care Med)
Methods
Retrospective study done in Ohio (booo)
Inclusion
= or <12 months of age requiring HFNC for viral bronchiolitis in the PICU
HFNC 3-8L/min used to maintain O2 sat >92%
Compared responders (avoided intubation) vs. non-responders (got intubated) to HFNC
Results/Discussion
113 pts met criteria
19% failed, 81% succeeded
General
Non-responders were intubated by ~14 hrs in avg
Non-responders had significantly lower pH (7.26 vs. 7.30) and higher PCO2 (67 vs. 56) prior to initiation of HFNC
Non-responders had significantly lower RR (44 vs. 54) prior to initiation of HFNC
Pre HFNC to 1 hr post HFNC
Non-responders had no change in RR pre/post HFNC while responders had a decrease of ~8 in their RR
Responders had decreased episodes of desats while non-responders were likely to have persistent desats
Most likely predictors of not responding to HFNC:
High PCO2 = poor ventilation
Low RR = less tachypnea = pts getting fatigued and could no longer increase their min ventilation
2013 - Is HFNC Effective in Acute Viral Bronchiolitis? (Intensive Care Med)
Methods
Prospective study done in France
Inclusion
<6 months of age with RSV bronchiolitis requiring HFNC in the PICU
Measured pharyngeal pressure (estimate of PEEP) at different flow rates
Results/Discussion
21 pts met criteria
Flows = or >6L/min generated positive pressure during both inspiration and expiration
Flows = or >2L/kg/min associated w/ pharyngeal pressure = or >4cmH2O
Increasing from 1 to 7L/min resulted in significant reduction in RR and 50% reduction in WOB score
No complications from HFNC use
None of the 21 pts required intubation
To maximize benefit from HFNC, put pacifier in pt's mouth and use largest prongs that fit into nostrils (minimize positive pressure leaks)
2014 - Effort of Breathing in Children Receiving HFNC (Peds Critical Care Med)
Methods
Prospective study done in California
Inclusion
<18 yrs of age receiving HFNC or pts planned to extubate to HFNC w/in 72hrs (mean age of this cohort ~7 yrs)
Esophageal pressure (estimate of pleural pressure; negative pressure in the pleural = chest expands (inhalation) because the lung gets pulled against the chest wall; positive pressure = expiration because the lung gets pushed away from the chest wall) measured during CPAP 4-5 and HFNC 2-8L/min
WOB, determined by PRP (pressure rate product; change in pleural pressure X RR; lower number means less WOB), calculated for CPAP and HFNC
Results/Discussion
25 pts met criteria (20 pts initially intubated)
PRP collected on HFNC for 25 pts, CPAP for 18 pts, and NC for 20 pts
PRP significantly lower on HFNC 8L/min vs. HFNC 2-5L/min (even significantly different at HFNC 5L/min vs. HFNC 2L/min)
PRP of CPAP not significantly different from HFNC 5-8L/min but significantly lower compared to HFNC 2L/min
PRP decreased by 25% when flow rate of HFNC increased from 2 to 8L/min
Esophageal pressure at end exhalation (estimate of PEEP) increased when flow rate of HFNC increased from 2L/min to 8L/min
No significant difference in esophageal pressure at end exhalation between CPAP and HFNC 8L/min but significantly different with HFNC 2-5L/min
No adverse events with HFNC