Central venous-arterial carbon dioxide difference as an indicator of cardiac index

Objective: The mixed venous-arterial (v-a) pCO₂ difference has been shown to be inversely correlated with the cardiac index (CI). A central venous pCO₂, which is easier to obtain, may provide similar information. The purpose of this study was to examine the correlation between the central venous-arterial pCO₂ difference and CI. Design: Prospective, cohort study. Setting: Intensive care unit of an urban tertiary care hospital. Patients and participants: Eighty-three consecutive intensive care unit patients. Measurements: Simultaneous blood gases from the arterial, pulmonary artery (PA), and central venous (CV) catheters were obtained. At the same time point, cardiac indices were measured by the thermodilution technique (an average of three measurements). The cardiac indices obtained by the venous-arterial differences were compared with those determined by thermodilution. Results: The correlation (R²) between the mixed venous-arterial pCO₂ difference and cardiac index was 0.903 ( p <0.0001), and the correlation between the central venous-arterial pCO₂ difference and cardiac index was 0.892 ( p <0.0001). The regression equations for these relationships were natural log (CI)=1.837−0.159 (v-a) CO₂ for the PA and natural log (CI)=1.787−0.151 (v-a) CO₂ for the CV ( p <0.0001 for both). The root-mean-squared error for the PA and CV regression equations were 0.095 and 0.101, respectively. Conclusion: Venous-arterial pCO₂ differences obtained from both the PA and CV circulations inversely correlate with the cardiac index. Substitution of a central for a mixed venous-arterial pCO₂ difference provides an accurate alternative method for calculation of cardiac output.