Clinical relevance of the PaO2/FiO2 ratio

We read with interest the report by Karbing and coworkers [1] in which they assess the clinical relevance of variation in the arterial oxygen tension (PaO2)/fractional inspired oxygen (FiO2) ratio, a widely used oxygenation index, alongside changes in FiO2. In mechanically ventilated and spontaneously breathing patients, they showed that the clinical utility of PaO2/FiO2 ratio is doubtful unless the FiO2 level at which the PaO2/FiO2 ratio is measured is specified. They included data from 28 mechanically ventilated patients and from an additional eight mechanically ventilated patients at one or two different positive end-expiratory pressure (PEEP) settings.