Optimizing Tracheal Oxygen Tension and Diffusion Ratio When Choosing High-Flow Oxygen Therapy or CPAP for the Treatment of Hypoxemic Respiratory Failure: Insights from Ex Vivo Physiologic Modelling

Abstract

This article is a review of the physiological and technological processes underpinning high-flow nasal therapy with oxygen (HFNT or HFOT) for the treatment of hypoxemic respiratory failure. A mathematical model was carefully built to represent the relationships between the settings on the HFNT device and the resultant diffusion of oxygen into hypoxemic, arterial blood. The analysis was used to recommend a strategy for setting the flow rate at or above the patient’s peak inspiratory flow when HFNT is used with a blender and equal to the patient’s peak inspiratory rate when bleed-in oxygen is used. The analysis also teaches how to titrate the settings to achieve a desired fraction of inhaled oxygen, (FiO2), in the trachea using a simple ratio when bleed-in oxygen is used. The model was used to compare HFNT as a method to improve oxygen diffusion efficacy with other forms of oxygen therapy. The analysis in this article relates the efficacy of HFOT/HFNT to that of CPAP with supplemental oxygen by computing the diffusion ratio of oxygen therapy versus breathing room air. We predicted that in non-atelectatic lungs, when considering oxygenation, HFNT can be equally effective as CPAP with supplemental oxygen therapy for treating hypoxemic respiratory failure.

Non-Invasive Respiratory Support: How to Get It Right in Clinical Medicine

“…In particular, Vega Pittao et al [ 2 ] described the mechanisms of HFNC in fields that have still not been fully investigated like chronic hypercapnic respiratory failure, while in a sophisticated ex vivo modelling, Truschel and Polkey [ 3 ] demonstrated that HFNC is as effective for promoting oxygen diffusion as CPAP when oxygen is entrained at the mask and is superior to CPAP where oxygen is entrained at the back of the device. Specifically, the benefit from alveolar expansion from CPAP pressure or increased end expiratory lung volume (EELV) provides no significant benefit to oxygenation in non-atelectatic lungs.…”