Why Is PaO2 Not Enough? Arterial Oxygen Content as a Prognostic Indicator in COPD Patients

Background: Chronic hypoxemia in patients with COPD is associated with increased morbidity and mortality. Although arterial partial pressure of oxygen (PaO2) is widely used, it does not adequately reflect systemic oxygen transport. Arterial oxygen content (CaO2) may provide a more comprehensive assessment. Objective: This study aimed to evaluate whether or not CaO2 is a better predictor of mortality than PaO2 in patients with COPD. Methods: This retrospective observational cohort study included 147 COPD patients aged ≥45 years. Patients were categorized according to CaO2 levels (low, normal, high). Mortality at 1, 3, and 5 years was analyzed. Statistical methods included ROC curves, Kaplan–Meier survival analysis, and Cox regression models. Results: A total of 66 deaths (45.2%) occurred in the study cohort. Mortality was highest in the low CaO2 group. The CaO2 demonstrated better predictive performance than PaO2 (AUC 0.73 versus 0.53, respectively). Low CaO2 was associated with a 2.5-fold increased risk of mortality. Despite improvements in PaO2 after long-term oxygen therapy, CaO2 did not significantly change. Conclusions: The CaO2 is a more informative marker of oxygen transport and mortality risk than PaO2 in COPD patients. It should be considered a complementary parameter in clinical assessment.