Measurement of central and mixed venous-to-arterial carbon dioxide differences in cardiac surgery patients

Background: Measurement of central venous-to-arterial CO2 difference (p(cv-a)CO2) as an estimate of mixed venous-to-arterial CO2 difference (p(v-a)CO2) has been recommended as an supplementary parameter to identify the inadequacy of tissue oxygenation in septic and postoperative non-cardiac high risk surgery patients. This study investigates the agreement between p(cv-a)CO2 and p(v-a)CO2, and explores the relationship of p(v-a)CO2 with parameters of global and regional tissue oxygenation. Methods: Simultaneous measurements of p(cv-a)CO2 and p(v-a)CO2 were performed in postoperative cardiac surgery patients immediately before and after cardiopulmonary bypass (CPB), and up to 6 hours post CPB. In addition, parameters of global blood flow and tissue oxygenation, i.e. cardiac index, mixed venous oxygen saturation (SvO2), arterial lactate, and regional blood flow, i.e. gastric tonometry were assessed. Pooled data were used for Bland-Altman and correlation analysis, as appropriate. Results: Although significantly correlated, p(cv-a)CO2 and p(v-a)CO2 showed large limits of agreement (6.7 mmHg, percentage error of 115 %). Correlation analyses revealed no meaningful correlation between p(v-a)CO2 and CI, SvO2, arterial lactate, and p(g-a)CO2 (R 2: 0.013, 0.007, 0.000, 0.006, respectively, with p > 0.05 each). Conclusions: In cardiac surgery patients p(cv-a)CO2 cannot be used as an estimate of p(va)CO2 with acceptable accuracy. There is no evidence that measurements of p(cv-a)CO2 or p(v-a)CO2 could help diagnose global or regional tissue hypoxia in this patient group.