The duration and severity of tissue hypoxia have been related to increased mortality. Maintenance of adequate oxygen delivery (DO2) is essential to preserve organ function, as a sustained low DO2 is a path to organ failure and death. DO2 does not have influence on oxygen consumption (VO2) until it reaches critically low values (DO2crit), when VO2 starts to fall. Low cardiac output states (cardiogenic, hypovolemic and obstructive types of shock), anemic and hypoxic hypoxemia are characterized by a decreased DO2 but preserved oxygen extraction ratio (OER=the ratio of DO2 to VO2, VO2/ DO2) so that DO2crit remains normal. In distributive shock, the oxygen extraction capability is altered so that the critical oxygen extraction ratio is typically decreased. These situations are typically associated with an increased DO2crit, and VO2 can become dependant on DO2 even when the latter is normal or elevated. These observations help to characterize the four principal types of circulatory shock, however this classification is somewhat simplistic as several types of alternations may coexist, in particular in cardiogenic shock. Rhodes et al. reported that outcome was more favorable in septic patients whose VO2 increased after dobutamine administration. DO2 also increased in survivors. On the other hand DO2 did not increase in patients whose VO2 did not increase. In dobutamine test proposed by Rhodes et al. the hemodynamic response was influenced by cardiovascular reserve and the degree of stimulation of adrenoreceptors at baseline. Unfortunately global measurements of DO2 and VO2 may not be sensitive enough to be clinically relevant. They may fail to detect regional perfusion abnormalities as in splanchnic circulation.