Summary: The Relationship between high flow nasal cannula flow rate and effort of breathing in children.

Topic: Flow Rates

Weiler T, Kamerkar A, Hotz J, et al. The Relationship between high flow nasal cannula flow rate and effort of breathing in children. The Journal of Pediatrics. 2017 October;189:66-71. 

In 2017, Weiler and colleagues studied and published “The Relationship between High Flow Nasal Cannula Flow Rate and Effort of Breathing in Children” in the Journal of Pediatrics.  The single-center prospective trial was conducted in a 24-bed pediatric intensive care unit on children <3 years of age receiving high flow nasal cannula (HFNC) therapies. The study design used esophageal manometry as an objective metric of effort of breathing to determine optimal HFNC rates on children <3 years of age.  

The authors used esophageal manometry to measure the percent change in pressure rate product (PRP) as a function of weight-indexed flow rates of 0.5, 1.0, 1.5, and 2.0 L/kg/minute with Fisher & Paykel and Vapotherm’s high velocity therapy delivery systems. The flow levels were trialed in a random order, each for a 5-minute period and measurements were collected during quiet tidal breathing with up to 2 daily measurements at each of the stated flow rates up to 5 days while the patient was receiving HFNC therapy.  

The study enrolled twenty-one patients with the most common diagnoses of bronchiolitis and pneumonia. The authors analyzed 49 titration episodes for the enrolled infants which showed a significant difference in the percent change in PRP from baseline (of 0.5 L/kg/minute) with increasing flow rates for the entire cohort (P < .001) with the largest change at 2.0 L/kg/min (-21%). 

To assess the relationship between patient size and dose response of HFNC flow rate, the researchers compared subgroups stratified by weight (patients ≤8 kg and >8 kg). For patients ≤8 kg (12 patients, 20 episodes), there was a significant dose-dependent relationship between increasing flow rate and greater percent change in PRP from baseline (P = .001) with a maximum reduction at flow rates of 1.5 L/kg/minute. The largest effect was seen in lighter children <5 kg.   

The authors note that sustained periods of high effort of breathing lead to respiratory muscle fatigue. The therapeutic use of optimal flow HFNC rates to reduce effort of breathing may prevent the progression to respiratory failure. The authors, however, note a plateau effect of clinical improvement from 1.5LPM – 2LPM and recommend that if a patient continues to have high effort of breathing on HFNC flow rates of 2 L/kg/minute, practitioners should consider alternative methods of respiratory support as further increases in HFNC flow rate are unlikely to be beneficial. 

The study concluded that the optimal HFNC flow rate to reduce effort of breathing in infants and young children is approximately 1.5-2.0 L/kg/minute with more benefit seen in children ≤8 kg. 

The authors note there are a few limitations of the study. First, this is a single-center trial with a relatively small sample size (although adequately powered). The investigators used measured body weight to calculate weight-indexed flow rates and not ideal body weight (IBW). Upon retrospective analysis, the patient population had a median weight slightly less than the IBW for this aged cohort. It may be that IBW would be more suitable to use in obese patients, which were not prevalent in this patient population but could be of minor importance here. Furthermore, obtaining effort of breathing measurements during quiet tidal breathing can be challenging in some infants and young children. Despite best efforts to assure calm periods of breathing for accurate measurements while filtering artifact, the dynamic nature of effort of breathing can vary during observation periods.  The investigators did not randomize the order of Fisher & Paykel and Vapotherm for patients that had a head-to head comparison. However, they noted that physiologic effects of HFNC have rapid onset and resolution, making it unlikely that the previous HFNC delivery system has a long-lasting impact on effort of breathing 

Finally, it is possible that the presence of the esophageal pressure probe may have an impact upon the airway physiology. It could increase inspiratory and expiratory resistance, cause discomfort, decrease the total cross-sectional area of the nares, or increase nasopharyngeal pressure by limiting the space for gas efflux. This could have attenuated the efficacy of HFNC therapy and may affect the magnitude of the flow vs effort of breathing dose-dependence relationship. 

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