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Abstract

Introduction: There is increased interest in using noninvasive respiratory support modalities for treatment of pediatric patients with acute hypoxemic respiratory failure. The PaO2/FiO2 (PF) ratio is required to identify patients with ARDS by the Berlin definition. Although the Berlin definition of ARDS includes patients treated with CPAP ≥ 5 cm H2O there may be disparity between the preset fraction of inhaled oxygen (FiO2) and the fraction of delivered oxygen (FDO2) in patients treated with nasal modes of positive pressure support. There have been no published reports that have described the FDO2 across a range of patient minute ventilation and disease states during nasal noninvasive support. In order to determine the incidence of ARDS, we must be able to accurately and reliably predict oxygen delivery during noninvasive support. Methods: Neonatal, infant, and child face/nasal airway models were constructed with a 3D printer using data from a newborn, 7 month old, and 5 year old, respectively. All had simulated oral leaks and were affixed to a spontaneously breathing test lung model (ASL 5000; Ingmar, PA) configured for healthy, restrictive, and obstructive lung mechanics. Simulated conditions were programmed as resistance, compliance, pleural pressure and respiratory rate to achieve appropriate minute ventilation. The lung model was supported at 0.40 FiO2, using high flow nasal cannula (HFNC; non-occlusive prong), continuous positive airway pressure (CPAP; occlusive prong), and noninvasive ventilation (NIV; occlusive prong). FDO2 was measured distal to the oral leak with an oxygen analyzer during HFNC (0.5–32 L/min), nasal CPAP (4–8 cm H20), and NIV. We instituted a priori acceptable FDO2 measurements within a 0.5% margin to account for any error related to the accuracy of the oxygen analyzer. After a 2 minute stabilization of the analyzer, FDO2 measurements were obtained sequentially (N=10) for each model. Results: FDO2/FiO2 approximates 1 in neonates with healthy and obstructive disease at flow ≥ 4L/min, and in restrictive disease at ≥ 2L/min. The FDO2/FiO2 approximates 1 in all disease conditions at 8L/min and 24L/min in infants and children, respectively. Linear relationships with R2 >95% (neonate), >89% (infant) and >84% (child) were evident at lower flows where FDO2/FiO2 did not yet approach 1. In all disease states for the infant model, FDO2/FiO2 decreased linearly during HFNC between flows of 8L/min and 24L/min. Measured FDO2/FiO2 was 1 in Neonatal NIV, infant and child BiPAP during all levels of support tested. FDO2/FiO2 = 1 with all CPAP conditions, with the exception of the infant healthy model at CPAP 4. Conclusions: The major finding of this study is that there is a linear relationship between FDO2/FiO2 with HFNC until a critical point at which the delivered flow exceeds the minute ventilation and leak, resulting in less air entrainment. At this point, the preset FiO2 and FDO2 equilibrate, effectively providing FDO2 = FiO2. In respiratory devices that are intended to deliver pressure, there appears to be no loss of oxygen from the preset value to what is delivered, regardless of nasal/oral leak, pressure and disease state. This may be due, in part, to the occlusive nature of the nasal airway interface and airway pressurization at the level of the oral leak. These data suggest that FiO2 delivery can be reliably predicted in children treated with nasal modes of noninvasive support. This has clinical and research implications to identify patients with and at risk for ARDS.