The Influence of Hypercapnia and Atmospheric Pressure on the Pao 2/Fio 2 Ratio—Pathophysiologic Considerations, a Case Series, and Introduction of a Clinical Tool*

Abstract

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.

Índice de PaO₂/FAO₂: ¿un mejor indicador de oxigenación para valorar la hipoxemia? Resultados de un análisis de concordancia con el índice de PaO₂/FiO₂

“…Para el cálculo del índice PaO 2 /FAO 2 se utilizaron las siguientes ecuaciones: 7 1. Presión alveolar de oxígeno:…”

“…Este estudio reveló que es frecuente una diferencia de hasta 50 unidades entre la PaO 2 /FiO 2 y la PaO 2 /FAO 2 debido a incrementos de la PaCO 2 . 7,8 Estos resultados concuerdan con los hallazgos observados en nuestro estudio, donde las diferencias podían ser hasta de 54 unidades.…”

Is the Pao 2/Fio 2 Ratio in Acute Respiratory Distress Syndrome Another Source of Heterogeneity?*

“…In this issue of Critical Care Medicine , Gilissen et al (11) conducted a very interesting study that tried to shed some light on that issue. The authors analyzed whether using a modified ratio (obtained by replacing the F io 2 with alveolar oxygen level) would correlate better with outcomes.…”