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.

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.