Veno-arterial carbon dioxide gradient at the early stage of septic shock

Introduction

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.

Methods

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.

Results

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).

Conclusions

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.