Introduction: High-flow nasal canula (HFNC) is a frequently used mode of support in pediatric critical care. It is widely accepted that HFNC washes out upper airway dead space, but there is some evidence that suggests that HFNC is able to generate positive end expiratory pressure (PEEP). This study aims to quantify the amount of PEEP generated at different flows in 3D printed tracheo-bronchial pediatric models, as well as assessing for significant differences in pressures measured at each of the five third-generation bronchi. Methods: Anatomically accurate pediatric airway models from trachea to third generation bronchi were developed using high resolution CT lung scans and 3D modeling software for a 4-6 month old, a 4-6 year old, and a 11-14 year old with no lung pathology. The models were printed in resin that would mimic the tensile strength of bronchial cartilage and connected to a test lung with a digital manometer attached. Pressure measurements were taken at each terminal bronchus at flow rates of 1, 1.5, and 2 L/kg/min for the 4-6 month old and 4-6 year old, and 25, 40, and 50 L/min for the 11-14 year old. 50th percentiles of weight-for-age were used to calculate the flow values to test at. Pressure measurements were also taken with a continuous positive airway pressure (CPAP) machine providing PEEP values of 5, 6, and 8 cmH2O for comparison. Results: HFNC at 1, 1.5, and 2 L/Kg/min was only able to generate a fraction of the PEEP that the CPAP machine was able to generate. In the 4-6 month old, 1 L/Kg/min of flow produced a PEEP of 0.16 cmH2O, 1.5 L/Kg/Min produced 0.33 cmH2O, and 2 L/Kg/min produced a PEEP of 0.53 cmH2O. In the 4-6 year old, 1 L/Kg/min correlated to 0.62 cmH2O, 1.5 L/Kg/min averaged 1.18 cmH2O, and 2 L/kg/min produced 1.85 cmH2O. In the 11-14 year old, 25 L/min generated 0.87 cmH2O, 40 L/min generated 1.33 cmH2O, and 50 L/min generated 2.02 cmH2O. In comparison, the CPAP machine set at 5, 6, and 8 cmH2O generated 4,5, and 7 cmH2O at the terminal bronchi. Conclusions: Physical testing showed that HFNC flow rates produced equal pressures in all third generation bronchi, but were significantly lower than the pressures generated by the CPAP machine. Consequently, the flows necessary to generate comparable PEEP to CPAP are far greater than a patient would be able to tolerate.
Reddy et al. (Sun,) studied this question.