The mixed venous-to-arterial carbon dioxide(CO_2)tension difference[P(v-a) CO_2]is the difference between carbon dioxide tension(PCO_2) in mixed venous blood(sampled from a pulmonary artery catheter) and the PCO_2 in arterial blood.P(v-a) CO_2 depends on the cardiac output and the global CO_2 production,and on the complex relationship between PCO_2 and CO_2 content.Experimental and clinical studies support the evidence that P(v-a) CO_2 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 CO_2generated by the peripheral tissues.P(v-a) CO_2 can be replaced by the central venous-to-arterial CO_2 difference(△PCO_2),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 △PCO_2 during the resuscitation of septic shock patients might be useful when deciding when to continue resuscitation despite a central venous oxygen saturation(SCVO_2) > 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 △PCO_2(> 6 mmHg)could be used to identify patients who still remain inadequately resuscitated.Monitoring the △PCO_2 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 CO_2 production,or to choose between hemoglobin correction or fluid/inotrope infusion in patients with a too low ScvO_2 related to metabolic demand.The combination of P(v-a) CO_2 or △PCO_2 with oxygen-derived parameters through the calculation of the P(v-a) CO_2 or △PCO_2/arteriovenous oxygen content difference ratio can detect the presence of global anaerobic metabolism. The mixed venous-to-arterial carbon dioxide (CO 2) tension difference [P (va) CO 2] is the difference between carbon dioxide tension (PCO 2) in mixed venous blood (sampled from a pulmonary artery catheter) and the PCO 2 in arterial blood. P (va) CO_2 depends on the cardiac output and the global CO 2 production, and on the complex relationship between PCO_2 and CO_2 content. Experimental and clinical studies support the evidence that P (va) CO_2 can not serve as an indicator of tissue hypoxia, and should be considered as an indicator of the adequacy of venous blood to wash out the total CO_2generated by the peripheral tissues. P (va) CO_2 can be replaced by the central venous-to-arterial CO_2 difference (ΔPCO_2), 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 ΔPCO_2 during the resuscitation of septic shock patients might be useful when deciding when to continue re re Suscitation despite a central venous oxygen saturation (SCVO_2)> 70% associated with elevated blood lactate levels. Notcauseatory factor in determining the source of that stress, an increased △ PCO_2 (> 6 mmHg) could be used to identify patients who still remain inadequately resuscitated. Monitor the PCO_2 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 CO_2 production, or to choose between hemoglobin correction or fluid / inotrope infusion in patients with a too low ScvO_2 related to metabolic demand. combination of P (va) CO_2 or ΔPCO_2 with oxygen -derived parameters through the calculation of the P (va) CO_2 or ΔPCO_2 / arteriovenous oxygen content difference ratio can detect the presence of global anaerobic metabolism.