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.