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

During O₂ therapy at low flow in patients who breathe spontaneously, the fraction of delivered O₂ (F_DO ) is unknown. In recent years, F_DO prediction formulas have been proposed. However, they do not take into account the effect of inspiratory flow (V̇_I) on the F_DO . The aim of this study was to validate a new F_DO prediction formula, which takes into account the V̇_I and compares it with other F_DO prediction formulas.

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 V̇_I 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 (T_I) to the total breathing cycle time (T_tot) (T_I/T_tot) of 0.33 (T_I/T_tot value) and 0.50 (T_I/T_tot value)] were modified to generate many ventilatory patterns. An O₂ analyzer continuously examined the F_DO .

When the O₂ flow and/or T_I/T_tot increased, the F_DO increased. When the minute ventilation increased, the F_DO decreased. The results of the Bland-Altman method for the F_DO , calculated by using our mathematical model and the measured F_DO , 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 T_I/T_tot = 0.33 and 0.994 for T_I/T_tot = 0.50, and the coefficient of variation was 2.1% for T_I/T_tot = 0.33 and 1.3% for T_I/T_tot = 0.50. The results of the Bland-Altman method for the F_DO calculated by using the Shapiro formula and the F_DO 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%.

The V̇_I has a major impact on F_DO during O₂ therapy at low flow. F_DO comparisons between frequently used prediction formulas and F_DO measured on the bench indicated greater differences. Uncritical use of these formulas should be used cautiously to predict F_DO . In this study, our prediction formula indicated a good accuracy for predicting F_DO during supplemental oxygenation through a heat-and-moisture exchanger in patients who breathe spontaneously.