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

Objective: The mixed venous-arterial ( v-a) pCO(2) difference has been shown to be inversely correlated with the cardiac index (CI). A central venous pCO(2), which is easier to obtain, may provide similar information. The purpose of this study was to examine the correlation between the central venous-arterial pCO(2) 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 (R-2) between the mixed venous-arterial pCO(2) difference and cardiac index was 0.903 ( p < 0.0001), and the correlation between the central venous-arterial pCO(2) 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 pCO(2) differences obtained from both the PA and CV circulations inversely correlate with the cardiac index. Substitution of a central for a mixed venous-arterial pCO(2) difference provides an accurate alternative method for calculation of cardiac output.