CO2-derived parameters to assess hemodynamics in critical-ill patients

Abstract

Carbon dioxide (CO2) is an end product of cell metabolism. Several CO2 gradients, obtained by measuring the partial pressure of CO2 (PCO2) in different compartments, are used to assess the circulatory status of critically-ill patients. During shock, the increase in venous and tissue PCO2 is mainly the result of tissue hypoperfusion and not of hypoxemia, thus reflecting an alteration of macro and / or microcirculation. A PCO2 veno-arterial gradient (Pv-aCO2) ≥ 6 mmHg may suggest an inadequate cardiac output. At the local scale, a high tissue-arterial PCO2 gradient (Pt-aCO2) despite a normal Pv-aCO2 (<6 mmHg) may suggest an alteration in the local microcirculation. Finally, a combination of oxygen and CO2 derived parameters, such as the ratio between Pv-aCO2 and the difference in arterial and venous oxygen contents (Pv-aCO2 / CaO2 - CvO2) may suggest tissue hypoxia when it exceeds 1.4. In critically-ill patients with circulatory failure, the use of these parameters derived from CO2 must however be integrated into a multi-parameter approach including clinical examination, biological markers, echocardiography and other hemodynamic tools.