Oxygen exposure as quantified by time-weighted area under curve for arterial oxygen content is associated with mortality in mechanically ventilated critically ill patients

Background Oxygen is frequently administered to intensive care patients, for both treatment and prophylaxis. Arterial oxygen content (CaO2) represents the total amount of oxygen in arterial blood, both bound to hemoglobin and dissolved. CaO2 could be a useful marker of tissue oxygen levels and oxygen exposure. Aims We undertook this study to determine the relationship between CaO2 and mortality in mechanically ventilated critically ill patients. Settings and design A retrospective cohort study of all mechanically ventilated adult patients in the Multiparameter Intelligent Monitoring in Intensive Care III database was conducted. Patients with less than three arterial blood gases were excluded. The primary exposure variable was time-weighted CaO2 (TWCaO2) over the course of the entire ICU admission. The primary outcome was 6-month mortality. Multivariate logistic regression analysis was used to assess the relationship between CaO2 and mortality. Results A total of 7452 patients were identified who satisfied all inclusion and exclusion criteria. In the multivariate analysis, higher CaO2 was significantly associated with increased mortality. After adjustment for age, sex, transfusion, admission type, Elixhauser Comorbidity Index, Simplified Acute Physiology Score II, and time-weighted fraction of inspired oxygen, the highest quartile had an odds ratio (mortality) of 1.22 (95% confidence interval, 1.03–1.46; P=0.02). The second (TWCaO2, 12.2–13.3 ml/100 ml) and third (TWCaO2, 13.4–14.4 ml/100 ml) quartiles had odds ratio (mortality) of 1.19. Postestimation analysis revealed good model discrimination with a c-statistic of 0.80 for the final model. Conclusion In mechanically ventilated patients, after adjusting for disease severity and comorbidities, higher oxygen exposure as indicated by TWCaO2 over the entire ICU admission was associated with increased mortality