A New Formula for Predicting the Fraction of Delivered Oxygen During Low-Flow Oxygen Therapy.

Duprez, Frédéric [UCL] Mashayekhi, Shahram Cuvelier, Gregory Legrand, Alexandre Reychler, Gregory [UCL]

BACKGROUND: During O2 therapy at low flow in patients who breathe spontaneously, the fraction of delivered O2 (FDO2 ) is unknown. In recent years, FDO2 prediction formulas have been proposed. However, they do not take into account the effect of inspiratory flow (V̇I) on the FDO2 . The aim of this study was to validate a new FDO2 prediction formula, which takes into account the V̇I and compares it with other FDO2 prediction formulas. METHODS: During a bench study, spontaneous breathing was generated with a mechanical test lung connected to a mechanical ventilator set to volume control mode. O2 flow from a wall-mounted tube was delivered through a heat-and-moisture exchanger filter. A flow sensor recorded each breath of the V̇I in ambient temperature and barometric pressure conditions. Three parameters [O2 flow at 2, 3, 4, 5, 6 L/min; minute ventilation at 5, 10, 15, 20 L/min; and ratio of the inspiratory time (TI) to the total breathing cycle time (Ttot) (TI/Ttot) of 0.33 (TI/Ttot value) and 0.50 (TI/Ttot value)] were modified to generate many ventilatory patterns. An O2 analyzer continuously examined the FDO2 . RESULTS: When the O2 flow and/or TI/Ttot increased, the FDO2 increased. When the minute ventilation increased, the FDO2 decreased. The results of the Bland-Altman method for the FDO2 , calculated by using our mathematical model and the measured FDO2 , showed that the mean ± SD bias value was equal to 1.49 ± 0.84%, and the limits of agreement ranged from -0.17% to 3.14%. The intraclass correlation coefficients were 0.991 for TI/Ttot = 0.33 and 0.994 for TI/Ttot = 0.50, and the coefficient of variation was 2.1% for TI/Ttot = 0.33 and 1.3% for TI/Ttot = 0.50. The results of the Bland-Altman method for the FDO2 calculated by using the Shapiro formula and the FDO2 measured on the bench indicated that the bias value was 0.075 ± 8.66% and the limits of agreement ranged from -16.89% to 17.04%. For the Vincent formula, the bias value was 3.08% ± 8.56% and the limits of agreement ranged from -13.69% to 19.84%. CONCLUSIONS: The V̇I has a major impact on FDO2 during O2 therapy at low flow. FDO2 comparisons between frequently used prediction formulas and FDO2 measured on the bench indicated greater differences. Uncritical use of these formulas should be used cautiously to predict FDO2 . In this study, our prediction formula indicated a good accuracy for predicting FDO2 during supplemental oxygenation through a heat-and-moisture exchanger in patients who breathe spontaneously.