The Influence of Hypercapnia and Atmospheric Pressure on the PaO2/FIO2 Ratio-Pathophysiologic Considerations, a Case Series, and Introduction of a Clinical Tool

OBJECTIVES: The ratio between PaO2 and FIO2 is used as a marker for impaired oxygenation and acute respiratory distress syndrome classification. However, any discrepancy between FIO2 and O2 fraction in the alveolus affects the PaO2/FIO2 ratio. Correcting the PaO2/FIO2 ratios using the alveolar gas equation may result in an improved reflection of the pulmonary situation. This study investigates the difference between standard and corrected PaO2/FIO2 in magnitude, its correlation with the mortality of acute respiratory distress syndrome classification, and trends over time.

DESIGN: A register and a retrospective study combined with the development of a mathematical model to determine the difference between standard and corrected PaO2/FIO2 ratio for various levels of PaCO2 and atmospheric pressure.

SETTING: ICU in a secondary hospital in the Netherlands.

PATIENTS: Patients admitted to the ICU for pneumonia or acute respiratory distress syndrome. Register cohort: January 1, 2010, till March 1, 2020 (n = 1008). Retrospective cohort: March 1, 2020, till June 1, 2020 (n = 34).

MEASUREMENTS AND MAIN RESULTS: The register was used to determine the 7-day ICU mortality per acute respiratory distress syndrome classification based on the standard and corrected PaO2/FIO2 ratio. The retrospective dataset correlated the PaCO2 with PaO2/FIO2 ratio over time in patients with assumed stable oxygenation. The model demonstrated an increased difference between the standard and corrected PaO2/FIO2 ratios by a lower FIO2 and atmospheric pressure and higher PaO2 and PaCO2. Reclassification of severe acute respiratory distress syndrome resulted in an increase in mortality from 28.1% for standard PaO2/FIO2 to 30.6% for corrected PaO2/FIO2 ratios. Acute Physiology and Chronic Health Evaluation scores correlated better with 7-day ICU-mortality when corrected PaO2/FIO2 ratio was used for classification. For patients with FIO2 less than 50% (n = 55), change in PaCO2 correlated with change in PaO2/FIO2 ratio (r = -0.388; p = 0.003).

INTERVENTIONS: A corrected PaO2/FIO2 ratio was calculated.

CONCLUSIONS: Correcting the PaO2/FIO2 ratio for the alveolar gas equation predominantly affects patients with high ratios between PaO2 and FIO2 and PaCO2 and at low atmospheric pressure. Using the corrected PaO2/FIO2 ratio for acute respiratory distress syndrome classification results in improved correlation with the 7-day ICU mortality and increases generalization among acute respiratory distress syndrome studies. The authors provide a free, web-based tool.