High-flow nasal cannula therapy, factors affecting effective inspired oxygen fraction: an experimental adult bench model

Abstract

Oxygenation through High Flow Delivery Systems (HFO) is described as capable of delivering accurate FiO2. Meanwhile, peak inspiratory flow dot{V}_{{text{I}}} ) of patients with acute hypoxemic respiratory failure can reach up to 120 L/min, largely exceeding HFO flow. Currently, very few data on the reliability of HFO devices at these high dot{V}_{{text{I}}} are available. We sought to evaluate factors affecting oxygenation while using HFO systems at high dot{V}_{{text{I}}} in a bench study. Spontaneous breathing was generated with a mechanical test lung connected to a mechanical ventilator Servo-i®, set to volume control mode. Gas flow from a HFO device was delivered to the test lung. The influence on effective inspired oxygen fraction of three parameters (FiO2 0.6, 0.8, and 1, dot{V}_{{text{I}}} from 28 to 98.1 L/min, and HFO Gas Flows from 40 to 60 L/min) were analyzed and are reported. The present bench study demonstrates that during HFO treatment, measured FiO2 in the lung does not equal set FiO2 on the device. The substance of this variation (ΔFiO2) is tightly correlated to dot{V}_{{text{I}}} (Pearson’s coefficient of 0.94, p-value < 0.001). Additionally, set FiO2 and Flow at HFO device appear to significatively affect ΔFiO2 as well (p-values < 0.001, adjusted to dot{V}_{{text{I}}} ). The result of multivariate linear regression indicates predictors (dot{V}_{{text{I}}} , Flow and set FiO2) to explain 92% of the variance of delta FiO2 through K-Fold Cross Validation. Moreover, adjunction of a dead space in the breathing circuit significantly decreased ΔFiO2 (p < 0.01). The present bench study did expose a weakness of HFO devices in reliability of delivering accurate FIO2 at high dot{V}_{{text{I}}} as well as, to a lesser extent, at dot{V}_{{text{I}}} below equivalent set HFO Flows. Moreover, set HFO flow and set FIO2 did influence the variability of effective inspired oxygen fraction. The adjunction of a dead space in the experimental set-up significantly amended this variability and should thus be further studied in order to improve success rate of HFO therapy.