Accuracy of PaO2 /FiO2 calculated from SpO2 for severity assessment in ED patients with pneumonia.

Background and objectiveAssessment of oxygenation in patients with community-acquired pneumonia is critical for treatment. The accuracy of percutaneous oxygen saturation (SpO(2)) determined by pulse oximetry is uncertain, and it has limited value in patients receiving supplemental oxygen. We hypothesized that calculation of partial arterial oxygen concentration/inspired oxygen faction (PaO2/FiO(2)) from SpO(2) by the Ellis or Rice equations might adequately correlate with PaO2/FiO(2) measured by arterial blood gases. MethodsWe studied 1004 patients with pneumonia in the emergency department with simultaneous measurement of SpO(2) and PaO2 from two cohorts from Valencia, Spain and Utah, USA. We compared SpO(2) with measured SaO(2), compared the equations' accuracy in calculating PaO2/FiO(2) and determined how often patients would be misclassified at clinically important thresholds. We compared estimated PaO2/FiO(2) to measured PaO2/FiO(2) using the Spearman correlation. ResultsPairwise correlation of SpO(2) with SaO(2) was moderate (rho=0.66; P<0.01). Both equations performed similarly among patients with lower PaO2/FiO(2) ratios. The Ellis equation estimated PaO2/FiO(2) from SpO(2) more accurately than the Rice equation in patients with PaO2/FiO(2) 200. Simple agreement between calculated and measured P/F was 91% and 92%, respectively. ConclusionsThe Ellis equation was more accurate than the Rice equation for estimating PaO2/FiO(2), especially at higher levels of P/F ratio. Estimation of PaO2/FiO(2) from SpO(2) is accurate enough for initial oxygenation assessment. Ellis and Rice equations could misclassify 20% and 30% of patients, respectively, at higher levels of PaO2/FiO(2). For patients with abnormal oxygenation falling near thresholds for clinical decision making, arterial blood gas measurement preferably on room air is more accurate. Different equations for PaO2/FiO(2) calculation have been described for patients with acute respiratory distress syndrome; we publish data to validate these in emergency room patients with pneumonia. The use of PaO2/FiO(2) calculated from non-invasive techniques (SpO(2)) is accurate in the assessment of oxygenation in emergency room patients with community-acquired pneumonia.