Use of venous-to-arterial carbon dioxide tension difference to guide resuscitation therapy in septic shock

Abstract

The mixed venous-to-arterial carbon dioxide (CO₂) tension difference [P (v-a) CO₂] is the difference between carbon dioxide tension (PCO₂) in mixed venous blood (sampled from a pulmonary artery catheter) and the PCO₂ in arterial blood. P (v-a) CO₂ depends on the cardiac output and the global CO₂ production, and on the complex relationship between PCO₂ and CO₂ content. Experimental and clinical studies support the evidence that P (v-a) CO₂ cannot serve as an indicator of tissue hypoxia, and should be regarded as an indicator of the adequacy of venous blood to wash out the total CO₂ generated by the peripheral tissues. P (v-a) CO₂ can be replaced by the central venous-to-arterial CO₂ difference (ΔPCO₂), which is calculated from simultaneous sampling of central venous blood from a central vein catheter and arterial blood and, therefore, more easy to obtain at the bedside. Determining the ΔPCO₂ during the resuscitation of septic shock patients might be useful when deciding when to continue resuscitation despite a central venous oxygen saturation (ScvO₂) > 70% associated with elevated blood lactate levels. Because high blood lactate levels is not a discriminatory factor in determining the source of that stress, an increased ΔPCO₂ (> 6 mmHg) could be used to identify patients who still remain inadequately resuscitated. Monitoring the ΔPCO₂ from the beginning of the reanimation of septic shock patients might be a valuable means to evaluate the adequacy of cardiac output in tissue perfusion and, thus, guiding the therapy. In this respect, it can aid to titrate inotropes to adjust oxygen delivery to CO₂ production, or to choose between hemoglobin correction or fluid/inotrope infusion in patients with a too low ScvO₂ related to metabolic demand. The combination of P (v-a) CO₂ or ΔPCO₂ with oxygen-derived parameters through the calculation of the P (v-a) CO₂ or ΔPCO₂/arteriovenous oxygen content difference ratio can detect the presence of global anaerobic metabolism.

Keywords: Venous-to-arterial carbon dioxide tension difference, Carbon dioxide production, Oxygen supply dependency, Cardiac output, tissue hypoxia, Anaerobic metabolism, Oxygen consumption, Resuscitation, Septic shock

Core tip: Early recognition and correction of tissue hypoperfusion are cornerstones in the management of septic shock patients. The venous-to-arterial carbon dioxide tension difference, which is a marker of the adequacy of cardiac output to global metabolic demand, is a helpful additional means to detect patients who stay under-resuscitated after optimization of O₂-derived parameters. In this regard, its monitoring should help the clinicians for the decision of giving therapy targeting at increasing cardiac output.