Venous-to-Arterial Carbon Dioxide Gradient and Its Ratio to Arterial-central Venous Oxygen Content Difference in Assessing Oxygen Metabolism and Tissue Perfusion in Septic Shock: A Prospective Observational Study

BACKGROUND: Traditional biomarkers such as lactate and central venous oxygen saturation (ScvO₂) have limitations in accurately reflecting tissue perfusion and oxygen metabolism in septic shock. This study investigates the prognostic value of central venous-to-arterial carbon dioxide difference (Pcv-aCO₂) and its ratio to arterial-central venous oxygen content difference (Pcv-aCO₂/Ca-cvO₂) in evaluating oxygen metabolism and guiding resuscitation in septic shock. METHODS: A prospective observational cohort study was conducted in a tertiary university intensive care unit (ICU) from October 2020 to December 2022. Ninety adult patients diagnosed with septic shock within 24 hours of ICU admission were enrolled and stratified into four groups based on Pcv-aCO₂ (cutoff: 6 mmHg) and Pcv-aCO₂/Ca-cvO₂ ratio (cutoff: 1.8). Arterial and central venous blood gas analyses were performed on ICU days 1, 2, 3, and 5. Dynamic changes in ScvO₂, lactate, Pcv-aCO₂, and Pcv-aCO₂/Ca-cvO₂ were recorded. The primary outcome was 28-day mortality. Logistic regression and ROC analysis were used to identify independent prognostic indicators. RESULTS: Only Group A (Pcv-aCO₂≤6 mmHg and Pcv-aCO₂/Ca-cvO₂≤1.8) showed significant reductions in lactate levels over time (P = 0.003). Elevated Pcv-aCO₂ and Pcv-aCO₂/Ca-cvO₂ were associated with lower ScvO₂ and persistently high lactate levels, indicating inadequate tissue oxygenation. On day 2 of ICU admission, Pcv-aCO₂ positively correlated with fluid intake (r = 0.259, P = 0.016), highlighting the importance of restrictive fluid resuscitation. Pcv-aCO₂ on day 3 (cutoff: 7.64 mmHg; AUC: 0.667) and APACHE II score (cutoff: 27.5; AUC: 0.725) independently predicted 28-day mortality. Their combination achieved the highest predictive value (AUC: 0.972, P = 0.040). CONCLUSION: In patients with septic shock, elevated Pcv-aCO₂ and Pcv-aCO₂/Ca-cvO₂ ratios are associated with impaired oxygen metabolism and worse outcomes, even when conventional markers appear normal. These parameters may serve as more sensitive indicators of tissue hypoperfusion in guiding early resuscitation efforts.