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 (V(T)) from 3 mL to 12 mL. Lung compliance and resistance were 
set at 0.5 mL/cm H(2)O and 125 cm H(2)O/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 V(T) were recorded.
RESULTS: The greatest effect on V(T) (mean ± SD 0.16 ± 0.10 mL) and pressure 
change (mean ± SD 0.7 ± 0.5 cm H(2)O) occurred with the premature cannula. The 
least effect on pressure (mean ± SD 0.3 ± 0.22 cm H(2)O) and V(T) 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 V(T) across all 
flows were negligible.