In Reply:
We thank Ms Couture for her interest and for commenting on our work.1 We agree that PaO2/FIO2 could not be accurately calculated in our study, because supplemental oxygen was delivered through a port in the mask, and the noninvasive ventilator that we used did not provide accurate FIO2.
According to the protocols of Mehta et al2 and Bellone et al3 to determine PaO2/FIO2 ratios while subjects are receiving noninvasive ventilation, FIO2 is calculated using the following conversion factor: (21% + [3% × oxygen flow in L/min of supplemental oxygen]). This conversion factor is influenced by minute ventilation, breathing patterns, and air leakage. We agree that SpO2/FIO2, as an alternative to PaO2/FIO2, may be a novel and noninvasive way to rapidly assess respiratory failure. However, the determination of FIO2 remains inaccurate and is influenced by minute ventilation, breathing patterns, and air leakage.
Ms Couture found that a patient with acute hypoxemic respiratory failure had an SpO2 of 89% on 6 L/min of supplemental oxygen, and SpO2 remained unchanged when the supplemental oxygen was increased to 20 L/min. Two possible reasons deserve attention. First, the increment of FIO2 was indeed less than the increment of supplemental oxygen. Second, if acute hypoxemic respiratory failure is caused by serious intrapulmonary shunt, it might be difficult to improve SpO2 by increasing FIO2.
Finally, the main purpose of the present study was to verify whether, compared with pressure-controlled mode, volume-controlled mode would be more effective in correcting hypercapnia, hence reducing the need for intubation and improving survival in subjects with acute hypercapnic respiratory failure. PaO2/FIO2 was not the primary end point of the present study and was not the primary issue in our subjects. Hence, despite the inaccurate determination, the PaO2/FIO2 values from the 2 groups remained comparable, and changes in their values could be used to demonstrate changes in subjects' oxygenation, because FIO2 was determined using an identical formulation.
Footnotes
The authors have disclosed no conflicts of interest.
- Copyright © 2017 by Daedalus Enterprises