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Use of high-flow humidified nasal cannulas to deliver continuous positive airway pressure in children is increasing. Data on the relationship between the flow values and the corresponding pressures are limited. The purpose of this experiment was to evaluate the relationship between the device, intraprong, and proximal airway pressures and the flow values in a neonatal/pediatric test lung model, using the Vapotherm 2000i and Fisher-Paykel humidified nasal cannulas devices.Using a pediatric size cannula (2-mm inner diameter), we measured the device, intraprong, and proximal airway pressures at random flow values between 0 L/min and 12 L/min with an FIO2 of 0.21 at a temperature of 37°C and 100% humidity. Measurements were repeated for both devices under simulated minimal and moderate nares-prong leak (leak). Effects of varying mouth leak were also studied.All three pressures generally increased with increasing flows with both devices, irrespective of leak. In case of minimal leak, the Fisher-Paykel device generated larger pressures than the Vapotherm device for flows of <8 L/min, whereas this trend was reversed at higher flows due principally to the pressure release feature of the Fisher-Paykel system. Under minimal leak, the intraprong pressure values varied between 22% and 27% and 20% and 32% of the corresponding device pressure value for Fisher-Paykel and Vapotherm, respectively. The proximal airway pressure was further reduced by about 20% to 30% relative to the intrapong pressure values with the two devices. The device pressure was essentially unaffected by nares-prong leaks or mouth leak. The intraprong pressure and particularly the proximal airway pressure were reduced substantially, as either nares or mouth leak increased.High flow humidified nasal cannulas systems may deliver uncontrolled continuous positive airway pressure to infants. This, along with the potentially large nares and mouth leak effects on any form of continuous positive airway pressure, renders the effective humidified nasal cannulas delivered continuous positive airway pressure particularly unpredictable.