A graphic approach for prediction of arterial oxygen tension at different concentrations of inspired oxygen.

A rapid, simple method for predicting the arterial oxygen tension (PaO2) at any fractional concentration of inspired oxygen (FIO2) is presented. The method is based on a nomogram interrelating PaO2, arterial carbon dioxide tension (PaCO2) and FIO2, allowing rapid determination of the arterial to alveolar oxygen tension ratio a/A PO2. The applicability of the nomogram and its reliability for predicting the PaO2 at any FIO2 were studied prospectively in nine consecutive patients requiring mechanical ventilation (16 trials). On each occasion, a desired PaO2 was chosen and from the nomogram an FIO2 expected to yield this PaO2 was administered. The PaO2 was then measured. Actual PaO2 differed from desired PaO2 by a mean of 2.6 mm Hg (range 0 to 10 mm Hg, standard deviation (SD) 4.8). Retrospectively, we analyzed the nomogram's ability to predict the PaO2 +/- 9.6 mm Hg (2 SD) at varying FIO2. The overall sensitivity of this method was 85 percent (126 predictions out of 149 trials). We conclude that appropriate levels of supplemental oxygen can be rapidly and reliably chosen using the nomogram in selected patients.