The Effect of Simulated Mouth Opening on CO₂ Removal During High Flow Nasal Cannula Therapy

High-flow nasal cannula (HFNC) therapy was originally considered only for its targeted O₂ delivery but has been shown to also assist in the removal of end-expiratory CO₂. Variables that influence the capability of this modality to reduce end-expiratory CO₂ are being proposed and investigated. Two types of HFNC therapy were precisely modeled at numerous mouth openings to investigate the effect that mouth open percent (MOP) had on airway CO₂ reduction (flush) and upper airway pressure generation for each therapy.

Computational fluid dynamics models were run at MOPs of 0%, 2%, 5%, 10%, 20%, 30%, 40%, and 100%, and each was administered therapy via a large-bore cannula, small-bore cannula, or no therapy. All models were initially run with less accurate, but cheaper, modeling parameters to establish the range of MOPs to be tested with higher-accuracy modeling parameters. Higher-accuracy models were run for MOPs of 5%, 10%, 20%, 30%, and 100%. CO₂ and pressure in the upper airway were analyzed to provide information on the effect of MOP, therapy type, and therapy flow.

The distinction between cannulas peaked at an MOP of 20%, with large-bore having ∼3.5 mg more CO₂ entrained at end-exhale than small-bore. Increasing MOP and increasing therapy flow each decreased CO₂ retention in the airway. Therapy with a large-bore cannula resulted in less CO₂ flush than with a small-bore cannula, if all other factors were identical. Decreasing MOP and increasing therapy flow each increased distending pressure generation, while small-bore generated higher pressures than large-bore when all other factors were the same.

The results collected using the lower-cost models highlighted the necessity of high-accuracy modeling methodology. The CO₂ and pressure results from the higher-accuracy models can inform clinicians as to the benefits of certain HFNC modalities at intermediate mouth openings (pursed-lip breathing).