Monitoring hypovolemia is an everyday challenge in critical care, with no consensus on the best indicator or what is the clinically relevant level of hypovolemia. The aim of this experiment was to determine how central venous oxygen saturation (ScvO2) and central venous-to-arterial carbon dioxide difference (CO2 gap) reflect hypovolemia-caused changes in the balance of oxygen delivery and consumption. Anesthetized, ventilated Vietnamese minipigs ( ) were given a bolus followed by a continuous infusion of furosemide. At baseline and then in five stages hemodynamic, microcirculatory measurements and blood gas analysis were performed. Oxygen extraction increased significantly, which was accompanied by a significant drop in ScvO2 and a significant increase in CO2 gap. There was a significant negative correlation between oxygen extraction and ScvO2 and significant positive correlation between oxygen extraction and CO2 gap. Taking % and CO2 gap >6？mmHg values together to predict an oxygen extraction >30%, the positive predictive value？is？100%; negative predicted value？is？72%. Microcirculatory parameters, capillary perfusion rate and red blood cell velocity, decreased significantly over time. Similar changes were not observed in the sham group. Our data suggest that % and CO2 gap >6？mmHg can be complementary tools in detecting hypovolemia-caused imbalance of oxygen extraction. 1. Introduction Diagnosing hypovolemia is an everyday challenge in critical care. Clinicians utilize a large array of tools from simple clinical signs to invasive hemodynamic measurements, but a universally accepted gold standard remains elusive [1]. Although diagnosis may prove difficult, early recognition of hypovolemia is of utmost importance. By the time macrohemodynamic changes manifest, the microcirculation may already be damaged [2]. Furthermore, fluid therapy is a double-edged sword: on the one hand fluid resuscitation can save lives, but on the other hand a cumulative positive fluid balance is an independent factor for mortality [3, 4]. Deciding on the level of monitoring (noninvasive, “less” invasive, invasive) and which parameter to monitor in order to keep the critically ill patient normovolemic remains uncertain. Central venous oxygen saturation (ScvO2), an easily obtained parameter via the central venous catheter already in situ in most critically ill patients, is often used as a marker of the balance between oxygen delivery (DO2) and consumption (VO2). The main factors, which influence ScvO2, are hemoglobin (Hb), arterial oxygen saturation (SaO2), cardiac output (CO), and
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