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

Background: During O therapy at low flow in patients who breathe spontaneously, the fraction of delivered O (F ) is unknown. In recent years, F prediction formulas have been proposed. However, they do not take into account the effect of inspiratory flow (V̇) on the F . The aim of this study was to validate a new F prediction formula, which takes into account the V̇ and compares it with other F 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. O flow from a wall-mounted tube was delivered through a heat-and-moisture exchanger filter. A flow sensor recorded each breath of the V̇ in ambient temperature and barometric pressure conditions. Three parameters [O 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) to the total breathing cycle time (T) (T/T) of 0.33 (T/T value) and 0.50 (T/T value)] were modified to generate many ventilatory patterns. An O analyzer continuously examined the F .

Results: When the O flow and/or T/T increased, the F increased. When the minute ventilation increased, the F decreased. The results of the Bland-Altman method for the F , calculated by using our mathematical model and the measured F , 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/T = 0.33 and 0.994 for T/T = 0.50, and the coefficient of variation was 2.1% for T/T = 0.33 and 1.3% for T/T = 0.50. The results of the Bland-Altman method for the F calculated by using the Shapiro formula and the F 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 V̇ has a major impact on F during O therapy at low flow. F comparisons between frequently used prediction formulas and F measured on the bench indicated greater differences. Uncritical use of these formulas should be used cautiously to predict F . In this study, our prediction formula indicated a good accuracy for predicting F during supplemental oxygenation through a heat-and-moisture exchanger in patients who breathe spontaneously.