Early evolution of arterial oxygenation in severe community-acquired pneumonia: A prospective observational study

Abstract

Purpose Acute respiratory failure requiring mechanical ventilation in severe community-acquired pneumonia has been shown to be a significant negative prognostic factor. We analyzed the early evolution of the Pa o 2/F io 2 ratio and evaluated its clinical value as an outcome predictor. Materials and methods This is a prospective study conducted in a tertiary referral hospital. In 62 adult patients requiring early mechanical ventilation due to severe community-acquired pneumonia, we measured serial changes in Pa o 2/F io 2 ratio and other clinical variables within the first 48 hours of mechanical ventilation and compared the difference between survivors and nonsurvivors. Results The initial Pa o 2/F io 2 ratio was lower in nonsurvivors (n = 27) than in survivors (n = 35) (158.0 ± 55.8 vs 117.9 ± 50.6, P = .025). Over the next 48 hours, the ratio increased significantly in survivors but not in nonsurvivors (analysis of variance, P < .001). An increase in Pa o 2/F io 2 ratio greater than 56 mm Hg had a sensitivity of 75% and a specificity of 81% of survival. A definite causative pathogen was identified in 36 patients (58%) and the 3 most commonly isolated pathogens were Streptococcus pneumoniae, Staphylococcus aureus, and Klebsiella pneumoniae. Ten patients received inadequate initial empirical antimicrobial therapy, in which the Pa o 2/F io 2 ratio change was significantly less than those who were adequately treated (analysis of variance, P < .001). Mortality was much higher (86% [6/7]) in patients who received inadequate antibiotics and where Pa o 2/F io 2 ratio change was less than 56 mm Hg. On multivariate analysis, trend changes in Pa o 2/F io 2 ratio over 48 hours, shock, and Acute Physiology and Chronic Health Evaluation II score were documented to be independent predictors of mortality. Conclusions A progressive improvement of Pa o 2/F io 2 ratio during the first 48 hours of mechanical ventilation indicates favorable outcome. Serial measurement of this ratio should be considered in decision making for therapeutic strategy.