The Effect of Simulated Mouth Opening on CO 2 Removal During High Flow Nasal Cannula Therapy

Abstract

Background:

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.

Methods:

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.

Results:

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.

Conclusions:

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).