Pao2/Fio2 is used commonly for diagnosis of lung injury (acute respiratory distress syndrome and transfusion-related acute lung injury), for assessment of pulmonary disease course and therapy, and in pulmonary transplantation for evaluation of donor lungs and clinical outcome. It was developed for convenience, without formal mathematical and graphic assessment to validate its suitability for these purposes.
We examined, mathematically and graphically, the relationship of Pao2/Fio2 to Fio2 at constant normal and several degrees of increased intrapulmonary shunting (Qs/Qt), assessing the impact of intra- and extrapulmonary factors on the relationship and thus the reliability of Pao2/Fio2.
Measurements and Main Results:
The relationship of Pao2/Fio2 varies at all shunt fractions but most with Qs/Qt from 0.1 to 0.3 with Fio2 approximately greater than 0.4. At higher Qs/Qt, the relationship is more constant and changes less with Fio2 more than 0.4. Hemoglobin concentration and arterial-venous oxygen content difference have large effects that can confound interpretation of Pao2/Fio2. Barometric pressure has a substantial effect; Pco2, base excess, and respiratory quotient have small effects.
At high Qs/Qt with Fio2 more than 0.4, the relationship of Pao2/Fio2 to Fio2 is relatively constant. However, with Qs/Qt of 0.1–0.3, Pao2/Fio2 changes substantially with Fio2. Understanding the important effects of nonpulmonary factors (especially hemoglobin concentration and arterial-venous oxygen content difference) should enhance appropriate clinical use, interpretation of Pao2/Fio2, and interpretation of previous publications and future studies (especially those seeking to assess effects of anemia or transfusion on lung function). The ratio of Pao2/Fio2 is a good tool for some, but not many clinical circumstances, and is insufficiently robust for most research applications.