SvO₂ to monitor resuscitation of septic patients: let's just understand the basic physiology

Real-time monitoring of mixed venous oxygen blood saturation (SvO₂) or of central venous oxygen blood saturation is often used during resuscitation of septic shock. However, the meaning of these parameters is far from straightforward. In the present commentary, we emphasize that SvO₂ - a global marker of tissue oxygen balance - can never be simplistically used as a marker of preload responsiveness, which is an intrinsic marker of cardiac performance. In some septic shock patients, because of profound hypovolemia or myocardial dysfunction, SvO₂ can be low but obviously cannot alone indicate whether a fluid challenge would increase cardiac output. In other patients, because of a profound impairment of oxygen extraction capacities, SvO₂ can be abnormally high even in patients who are still able to respond positively to fluid infusion. In any case, other reliable dynamic parameters can help to address the important question of fluid responsiveness/unresponsiveness. However, whether fluid administration in fluid responders and high SvO₂ would be efficacious to reduce tissue dysoxia in the most injured tissues is still uncertain.