Principle 5: measurements of SvO2 can be helpfulSvO2 reflects the balance between oxygen consumption (VO2) third and DO2 and thus provides an indication of the adequacy of tissue oxygenation. If there is no PAC in situ, the oxygen saturation in the superior vena cava (ScvO2) can be measured using a central venous catheter and has been proposed as a surrogate for SvO2. Importantly, ScvO2 represents just an approximation of the SvO2 [13] and the absolute values of ScvO2 and SvO2 are not interchangeable. The difference between these two parameters is influenced by the sampling site of central venous blood, the presence of left-to-right shunts, incomplete mixing of venous blood, oxygen extraction in the renal and the splanchnic beds, redistribution of blood flow through the upper and lower body, level of consciousness (anesthesia) and myocardial VO2.

The reliability of ScvO2 is also dependent on the position of the tip of the catheter, with right atrial measurements closely approximating SvO2 and high vena cava measurements often deviating substantially from SvO2. In general, SvO2 is more useful when the value is below normal (see below), even though in these conditions it may not reflect a hemodynamic problem. Simultaneous measurements of blood lactate levels can be helpful. A diagnostic algorithm based on SvO2 and cardiac output is shown in Figure Figure33.Figure 3Diagnostic algorithm based on mixed venous oxygen saturation (SvO2) and cardiac output. VO2, oxygen consumption.

Principle 6: a high cardiac output and a high SvO2 are not always bestAlthough ICU physicians may like to increase cardiac output and SvO2 by giving more fluid and inotropic agents, is this always good? Excessive fluid administration to increase cardiac output may result in fluid overload with massive edema formation and this may be associated with worse outcomes [14]; some systems measure extravascular lung water, which can help document this. Similarly, excessive doses of dobutamine can be detrimental, compromising myocardial function, especially in patients with coronary artery disease; giving inotropic agents in the presence of coronary artery disease is like trying to stimulate a tired horse. Using vasoactive agents and fluids to increase DO2 to supranormal levels in all patients may result in excessive mortality rates and this strategy has been abandoned [15].

A high ScvO2 has been suggested as a target for some high risk patients or in shock resuscitation, with Rivers and colleagues [16] reporting that septic patients assigned to an early goal-directed therapy algorithm had higher ScvO2 values and reduced mortality rates. However, this was a strategy for early resuscitation of patients with severe sepsis in a single Carfilzomib institution, and needs further validation in multi-center studies, several of which are currently ongoing.