P0466 - Forcing Oscillation by Temperature Variation with High Flow Nasal Cannula

Background/Purpose: We have previously demonstrated the use of high-speed, high flow nasal cannula in the generation of a milieu that supports the potential production of high-frequency ventilation. The use of high-frequency ventilation in Pediatric and Neonatal applications supplied by way of non-invasive devices has gained in popularity. These devices vary and may transmit high-frequency ventilation more or less well depending on the ability of the high-frequency waveform to pass successfully through the interface. Specific devices may have an effect of retarding or extinguishing the bulk waveform. Turbulent flow has both the potential to disrupt and produce a high-frequency ventilation waveform. The Reynold’s number (NR) indicates circumstances that may cause the development of turbulence. Numbers higher than 2000 are associated with disruption of normal laminar flow. We asked if the potential production of high-frequency ventilation could be altered by temperature gating between waveforms.

Methods: For this study, we assumed cannula diameters of 1.0-4.0 mm and nasal cannula flow rates of 1-8 LPM through a high flow apparatus. For purposes of clarity, a single prong (e.g., SOLO) and cannula length of 1cm were assumed. For the first setting, air dynamic density and viscosity, a temperature of 35 degrees C and 1 atm were assumed (i.e., 1.895 x10-5 kg/m-s and 1.145 kg/m^3). For the second setting air dynamic density and viscosity, a temperature of 40 degrees C and 1 atm were assumed (i.e., 1.918 x10-5 kg/m-s and 1.127 kg/m^3)

Results: Solving for the Reynolds number using R=2pvr/n for both temperatures yields consistently different values across the range of settings (p<0.001)

Discussion: Calculation of the Reynolds number is critically essential in demonstrating the loss of laminar flow. In the model, gating different temperatures across the different cannula diameters and flow models produces changes in the calculated Reynolds number that may be sufficient to produce changes from laminar to turbulent flow. The alternation of these flow states may allow the use of this flow pattern as an oscillator. Although the exact Reynold’s numbers at which turbulence occurs may not be defined, numbers of 2000-3000 are associated with turbulence. Gating between different temperatures in an extremely rapid fashion may result in enough change in the Reynolds number to cause an alternation in this pattern.

Conclusion: High flow nasal cannula can produce turbulent flow. Gating using a temperature model may be sufficient to create shifts in the laminar flow pattern and turbulence without changing flow or other cannula parameters. Whether temperature gradients could be introduced in a rapid enough fashion to produce these perturbations is speculative; however, if workable, this would allow the delivery of consistent flows and the entrainment of an oscillatory waveform without the need for a ventilator upstream.