Abstract
BACKGROUND:
Limited data are available to describe the CPAP effects that can be expected when using high flow with a traditional nasal cannula.
OBJECTIVE:
To describe the relationship between the pressure generated at the airway opening and flow through a nasal cannula using a simulated infant model. We hypothesized that positive pressure generated by a standard cannula at flows > 2 L/min would be minimal and clinically unimportant.
METHODS:
Nares were simulated with holes drilled in a plastic fixture. A nares template for CPAP prongs served as a sizing template for the holes. Small, medium, and large nares fixtures were constructed and connected to a lung simulator that simulated spontaneous breathing. Respiratory muscle pressure was simulated by setting a waveform and adjusting the amplitude to deliver a range of tidal volumes (VT) from 3 mL to 12 mL. Lung compliance and resistance were set at 0.5 mL/cm H2O and 125 cm H2O/L/s, respectively. Nasal cannulas were inserted in the model nares. We assured that the prong occlusion of the nares did not exceed 50%. Cannula flow was adjusted from 2–6 L/min in 1-L/min increments. Data were averaged over 20 breaths. Mean airway pressure and percent change in VT were recorded.
RESULTS:
The greatest effect on VT (mean ± SD 0.16 ± 0.10 mL) and pressure change (mean ± SD 0.7 ± 0.5 cm H2O) occurred with the premature cannula. The least effect on pressure (mean ± SD 0.3 ± 0.22 cm H2O) and VT change (mean ± SD 0.01 ± 0.02 mL) occurred with the infant cannula.
CONCLUSIONS:
Clinically important pressures were not generated by high flows with a standard nasal cannula. The differences in spontaneous VT across all flows were negligible.
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