Clinical question
Is there an optimal rate of flow for high-flow nasal cannula in respiratory distress?
Background
High-flow nasal cannula (HFNC) has been increasingly used to treat children with moderate to severe bronchiolitis, both in intensive care unit (ICU) settings and on inpatient wards. Studies have shown it may allow children with bronchiolitis to avoid ICU admission and intubation. In preterm infants it has been shown to decrease work of breathing. No prior studies, however, have examined optimizing the rate of flow for individual patients, and considerable heterogeneity exists in choosing initial HFNC flow rates.
Dr. Samuel C. Stubblefield
Reliably measuring effort of breathing has proved challenging. Placing a manometer in the esophagus allows measurement of the pressure-rate product (PRP), a previously validated measure of effort of breathing computed by multiplying the difference between maximum and minimum esophageal pressures by the respiratory rate.1 An increasing PRP indicates increasing effort of breathing. The authors chose systems from Fisher & Paykel and Vapotherm for their testing.
Study design
Single-center prospective observational trial.
Setting
24-bed pediatric intensive care unit in a 347-bed urban free-standing children’s hospital.
Synopsis
A single center recruited patients aged 37 weeks corrected gestational age to 3 years who were admitted to the ICU with respiratory distress. Fifty-four patients met inclusion criteria and 21 were enrolled and completed the study. Prior data suggested a sample size of 20 would be sufficient to identify a clinically significant effect size. Median age was 6 months.
Thirteen patients had bronchiolitis, three had pneumonia, and five had other respiratory illnesses. Each patient received HFNC delivered by both systems in sequence with flow rates of 0.5, 1, 1.5, and 2 L/kg per minute to a maximum of 30 L/min. Following the trials, patients remained on HFNC as per usual care with twice-daily PRP measurements until weaned off HFNC.
A dose-dependent relationship existed between flow and change in PRP, with the greatest reduction in PRP at 2 L/kg per minute flow (P less than .001) and a slightly smaller but similar reduction in PRP at 1.5 L/kg per minute. When stratifying the subjects by weight, this effect was not statistically significant for patients heavier than 8 kg (P = .38), with all significant changes being in patients less than 8 kg (P less than .001) with a median drop in PRP of 25%. Further examining these younger and lighter patients, the greatest reduction in PRP was in the lightest patients (less than 5 kg).
Given the similarity in drop in PRP at 1.5 L/kg per minute and 2 L/kg per minute, the authors suggest this flow rate yields a plateau effect and minimal further improvement would be seen with increasing flow rates. A rate of 2 L/kg per minute was chosen as a maximum a priori as it was judged the highest level of HFNC patients could tolerate without worsening agitation or air leak. There was no difference seen between the two HFNC systems in the study. The authors did not report the fraction of inspired oxygen settings used, the size of HFNC cannulas, or how PRP changed over several days as HFNC was weaned.
Bottom line
The optimal HFNC rate to decrease effort of breathing for children less than 3 years old is between 1.5 and 2 L/kg/min with the greatest improvement expected in children under 5 kg.
Citation
Weiler T et al. The Relationship between High Flow Nasal Cannula Flow Rate and Effort of Breathing in Children. The Journal of Pediatrics. October 2017. doi: 10.1016/j.jpeds.2017.06.006.
Reference
1. Argent AC, Newth CJL, Klein M. The mechanics of breathing in children with acute severe croup. Intensive Care Med. 2008;34(2):324-32. doi: 10.1007/s00134-007-0910-x.
Dr. Stubblefield is a pediatric hospitalist at Nemours/Alfred I. duPont Hospital for Children in Wilmington, Del., and clinical assistant professor of pediatrics at Jefferson Medical College in Philadelphia.
