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

Oxygenation through High Flow Delivery Systems (HFO) is described as capable of delivering accurate F-i(O2). Meanwhile, peak inspiratory flow <(V) over dot>(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 <(V) over dot>(I) are available. We sought to evaluate factors affecting oxygenation while using HFO systems at high <(V) over dot>(I) in a bench study. Spontaneous breathing was generated with a mechanical test lung connected to a mechanical ventilator Servo-i (R), 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 (F-i(O2) 0.6, 0.8, and 1, <(V) over dot>(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 F-i(O2) in the lung does not equal set F-i(O2) on the device. The substance of this variation (Delta F-i(O2)) is tightly correlated to <(V) over dot>(I) (Pearson's coefficient of 0.94, p-value< 0.001). Additionally, set F-i(O2) and Flow at HFO device appear to significatively affect Delta F-i(O2) as well (p-values < 0.001, adjusted to <(V) over dot>(I)). The result of multivariate linear regression indicates predictors (<(V) over dot>(I), Flow and set F-i(O2)) to explain 92% of the variance of delta F-i(O2) through K-Fold Cross Validation. Moreover, adjunction of a dead space in the breathing circuit significantly decreased Delta F-i(O2) (p <0.01). The present bench study did expose a weakness of HFO devices in reliability of delivering accurate F-I(O2) at high <(V) over dot>(I) as well as, to a lesser extent, at <(V) over dot>(I) below equivalent set HFO Flows. Moreover, set HFO flow and set F-I(O2) 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.