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Veno-arterial carbon dioxide gradient at the early stage of septic shock
© BioMed Central Ltd. 2010
Published: 1 March 2010
The measurement of cardiac output and its adequacy to tissue needs are essential for hemodynamic evaluation. In a recent review it was stated that the venous-to-arterial carbon dioxide difference (dPCO2) could be considered a marker of adequacy of venous blood flow to remove the total CO2 produced by the peripheral tissues. Several studies have already shown that dPCO2 and the cardiac index (CI) are inversely correlated in critically ill patients. However, the dPCO2 can be influenced not only by CI, but also by other factors governing CO2 production and CO2 elimination. The aim of this work was to study the behaviour of dPCO2, measured from central venous blood, and its evolution during the early stage of septic shock.
Forty-six patients with septic shock were prospectively included. dPCO2 was calculated by the difference between the arterial PCO2 and the PCO2 from central venous blood. A value of dPCO2 >6 mmHg was considered high. The CI was measured by transpulmonary thermodilution. ScvO2 and serum lactate were obtained. Patients were separated into a normal dPCO2 group and a high dPCO2 group. The results were compared by Student t test or Mann-Whitney test. P < 0.05 was chosen as significance.
At inclusion 24 patients (52%) had a high dPCO2. These patients had a lower CI (3.5 ± 1.1 vs 4.2 ± 0.99 l/min/m2, P = 0.04) and lower ScvO2 (57 ± 17 vs 71 ± 8%, P < 0.001) than patients with normal dPCO2. No difference was found in PaCO2 or PaO2/FiO2 levels between both groups, suggesting a similar CO2 elimination. Thirteen patients had a decrease in dPCO2 from above to below 6 mmHg (from 8 ± 2 to 5 ± 1 mmHg, P = 0.001) associated with an increase in CI (from 3.7 ± 1.3 to 4.2 ± 1.3 l/min/m2, P = 0.003). Conversely, 11 patients had an increase in dPCO2 from below to above 6 mmHg (from 3.8 ± 2.4 to 7.7 ± 2.6 mmHg, P = 0.002). In these patients, CI decreased (from 4.3 ± 1.3 to 3.5 ± 1 l/min/m2, P = 0.013) equally. A negative correlation was established between dPCO2 and CI (r2 = 0.40, P < 0.001) and between changes in dPCO2 and change in CI (r2 = 0.16, P = 0.008) during the study. The lactate level was similar in both groups. The ScvO2 was correlated with CI (r2 = 0.44, P < 0.001).
dPCO2 patients with septic shock seem to be related principally to CI. dPCO2 might be a marker of tissue perfusion adequacy to patient's metabolism and could be a resuscitation target for management of septic shock patients.