Byline: James. Harvey, Dayal. Jayawardena, Mahesh. Ramanan
Background Oxygen is frequently administered to intensive care patients, for both treatment and prophylaxis. Arterial oxygen content (CaO[sub]2) represents the total amount of oxygen in arterial blood, both bound to hemoglobin and dissolved. CaO[sub]2 could be a useful marker of tissue oxygen levels and oxygen exposure. Aims We undertook this study to determine the relationship between CaO[sub]2 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 CaO[sub]2 (TWCaO[sub]2) 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 CaO[sub]2 and mortality. Results A total of 7452 patients were identified who satisfied all inclusion and exclusion criteria. In the multivariate analysis, higher CaO[sub]2 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 (TWCaO[sub]2, 12.2-13.3 ml/100 ml) and third (TWCaO[sub]2, 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 TWCaO[sub]2 over the entire ICU admission was associated with increased mortality.
Oxygen is frequently administered to intensive care patients to treat or prevent hypoxemia ,. The goal of oxygen therapy is to prevent tissue hypoxia and thereby reduce the risk of organ injury or cell death . Oxygen delivery is proportional to cardiac output and arterial oxygen content (CaO[sub]2). The CaO[sub]2 is the total amount of oxygen in arterial blood both bound to hemoglobin and dissolved in blood. It can be calculated as follows: CaO[sub]2=(Hb×1.36×SaO[sub]2)+(PaO[sub]2×0.0031), where 0.0031 is the solubility coefficient of oxygen in human plasma . Each gram of hemoglobin is theoretically capable of carrying 1.39 ml of oxygen, though in vivo , this value is slightly reduced. As the solubility of oxygen in the blood is low, CaO[sub]2 is largely determined by the hemoglobin concentration and the percentage saturated with oxygen. Thus, when hemoglobin is fully saturated, additional oxygen only marginally increases CaO[sub]2 . The effects of oxygen therapy are frequently assessed by measuring arterial oxygen saturation (SaO[sub]2) and partial pressure of oxygen in arterial blood (PaO[sub]2) ,. Normal values vary with age and chronic respiratory illnesses, and explicit guidelines for PaO[sub]2 and SaO[sub]2 targets do not exist. Currently, guidelines from the British Thoracic Society recommend maintaining SaO[sub]2 94-98% for seriously ill, nonventilated patients . The harms of hypoxemia are well established. In addition to predicting inpatient mortality, lower PaO[sub]2 levels in acute respiratory distress syndrome have been identified as a risk factor...