Estimating the best fraction of inspired oxygen for calculation of PaO2/FiO2 ratio in acute respiratory distress syndrome due to COVID?19 pneumonia
Article Type:
Research/Original Article (دارای رتبه معتبر)

The ratio of partial pressure of oxygen in arterial blood (PaO2) to the fraction of inspiratory oxygen concentration (FiO2) is an indicator of pulmonary shunt fraction. PaO2/FiO2 (P/F) ratio is used to classify severity of acute respiratory distress syndrome (ARDS). With the same shunt fraction, P/F ratio decreases with increases in FiO2 which may lead to errors in classifying severity of ARDS. The effect of FiO2 on P/F ratio has not been investigated in COVID?19 pneumonia. In this study, we estimated the best FiO2 for the calculation of P/F ratio in a sample of patients with ARDS due to COVID?19 pneumonia.

Materials and Methods

Blood gas and ventilatory data of 108 COVID?19 ARDS patients were analyzed in a cross?sectional observational study. Using Oxygen Status Algorithm the calculated shunt fraction served a basis for calculating P/F ratio for different FiO2. The severity of ARDS determined by P/F ratios at each FiO2s was compared with the shunt?based severity to find the optimum FiO2 for calculation of P/F ratio so the resulting classification has the best match with the reference classification.


A FiO2 of 1.0 for calculation of P/F ratio and ARDS classification showed the best match with shunt?based ARDS classification. A regression model was obtained with the PaO2, patient’s original FiO2, Hemoglobin concentration, and SaO2 as the independent predictors of the P/F ratio for the FiO2 of 1.0.


This study shows a FiO2 of 1.0 as the best value for correct calculation of P/F ratio and proper classification of ARDS.

Journal of Research in Medical Sciences, Volume:27 Issue: 5, May 2022
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