Fig. 1

Oxygenation thresholds for initiating invasive ventilation. This figure explains how the saturation-to-inspired oxygen ratio (SF) threshold work. We begin with a patient on non-invasive oxygen support (left). Every threshold is tested on this patient (three gray arrows). The patient has the same underlying progression of SF ratios, independent of the choice of threshold (each line graph of SF versus time is identical). Top (SF < 110): At the first observation of SF < 110 (hour 4, red), the patient is intubated. Subsequent SF are not observed (grey), Middle (SF < 98 ): SF ≥98 until hour 12 (red), at which point the patient is intubated. Subsequent SF are not observed (grey). Bottom (SF < 88): SF ratio remains 88 or greater, so the patient remains on non-invasive oxygen support. All SF are observed