Fig. 3

Trend over time in derived (from SpO2 from peripheral pulse oximetry) versus measured (from PaO2 in ABG) P/F ratio. The vertical black line denotes intubation time (Ti). Shaded areas indicate variability in measurements due to many missing measured PaO2 values relative to continually available SpO2 values. However, the direction of change over time is similar in derived and measured P/F values. The gap in derived and measured P/F during the first 24 h of mechanical ventilation likely represents a combination of the maximum SpO2 being 100% (as opposed to PaO2 which can be over 600) which sets an upper limit to the derived P/F from SpO2; and due to the shape of the oxygen dissociation curve wherein small changes in SpO2 correspond to larger changes in PaO2