The relationship between the arteriovenous carbon dioxide gradient and cardiac index

Abstract

It has been reported that under normal conditions, mixed venous blood gases have approximated arterial samples; however, during cardiac arrest or severe cardiogenic shock, marked differences between arterial and venous blood gases have been noted. To further assess the relationships between arterial and mixed venous blood gases and cardiac index, a study population was chosen consisting of patients with less severe states of cardiac impairment. The differences between arterial and mixed venous P CO 2S and pHs were compared with cardiac indexes (CI) of 44 patients in an intensive care unit with arterial lines and Swan-Ganz catheters in place. Twenty-six patients with normal Cis (2.6 to 4.1 L/min/m 2) had a mean difference in mixed venous-arterial P CO 2 (ΔP CO 2) of 4.88± 0.40 mm Hg. In patients with low Cis (< 2.6), the ΔP CO 2 was 7.44 ± 0.63 mm Hg ( P = .001). The difference of mixed venous and arterial pH (ΔpH) was 0.027 ± 0.004 pH units for patients with normal Cis and 0.04 ± 0.003 pH units for those with low Cis ( P < .002). When the Cls of all patients were plotted against the ΔP CO 2s, there was an inverse linear relationship wherein ΔP CO 2 increased as Cl decreased ( r = −.47, P = .0011). There is an inverse relationship between ΔP CO 2 and Cl that has not been previously described. An elevated ΔP CO 2 may be a marker of a low cardiac index.