The effect of cannula and oral aperture size on airway pressure generation during high-flow nasal cannula therapy
Robert P. Kacinski, Wayne S. Strasser, Jonathan B. WaughBackground
High-flow nasal cannula (HFNC) therapy can deliver ideally conditioned (body temperature, pressure, and water vapor saturated) gas at high flows with variable oxygen concentrations. Two proposed benefits are the washout of CO 2 from the upper airways and the production of a coincident positive airway pressure. The effects of cannula geometry and various degrees of mouth opening on the positive airway pressure generated by HFNC were tested using a computerized mathematical model based upon a scan of a human airway.
Methods
Computational fluid dynamics models of the human upper airway were used to collect pressure information throughout the airway. Mouth openings of 5, 10, 20, 30, and 100% were tested. Small-bore and large-bore cannula geometries were tested. All other airway components and therapy settings remained unchanged across tests.
Results
The small-bore cannula geometry produced higher distending pressures than the large-bore cannula geometry. Smaller mouth openings generated higher pressures than larger mouth openings. Mean positive airway pressure during expiration (P aw,exp ) ranged from 1.20 cm H 2 O to 3.97 cm H 2 O over the scenarios tested. Mean positive airway pressure during inspiration (P aw,insp ) ranged from −0.002 cm H 2 O to 0.734 cm H 2 O over the scenarios tested. Positive airway pressure at end expiration (P aw,EE ) ranged from 0.512 cm H 2 O to 2.24 cm H 2 O over the scenarios tested. Maximum pressures were generated using a small-bore cannula at a mouth opening of 5%, while minimum pressures were generated using a large-bore cannula at a mouth opening of 100%.
Conclusions
Mouth opening size and cannula prong size had a measurable effect on positive airway pressure resulting from HFNC. The smaller the mouth opening, the greater the resulting pressure. The small-bore cannula geometry produced higher pressures than the large-bore geometry at the same settings, indicating that a lower therapy flow using a small-bore cannula could provide an equivalent pressure to that of therapy using a large-bore geometry.